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    /*
 * Copyright (c) 2017, Haiyang Li.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.landawn.abacus.util;

import java.util.AbstractMap;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Set;
import java.util.TreeMap;
import java.util.TreeSet;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.regex.Pattern;

import com.landawn.abacus.util.Tuple.Tuple2;
import com.landawn.abacus.util.Tuple.Tuple3;
import com.landawn.abacus.util.function.BiConsumer;
import com.landawn.abacus.util.function.BiFunction;
import com.landawn.abacus.util.function.BiPredicate;
import com.landawn.abacus.util.function.BinaryOperator;
import com.landawn.abacus.util.function.Consumer;
import com.landawn.abacus.util.function.Function;
import com.landawn.abacus.util.function.IndexedBiConsumer;
import com.landawn.abacus.util.function.IndexedBiFunction;
import com.landawn.abacus.util.function.IndexedBiPredicate;
import com.landawn.abacus.util.function.IndexedConsumer;
import com.landawn.abacus.util.function.IndexedFunction;
import com.landawn.abacus.util.function.IndexedPredicate;
import com.landawn.abacus.util.function.IntFunction;
import com.landawn.abacus.util.function.Predicate;
import com.landawn.abacus.util.function.Supplier;
import com.landawn.abacus.util.function.ToByteFunction;
import com.landawn.abacus.util.function.ToCharFunction;
import com.landawn.abacus.util.function.ToDoubleFunction;
import com.landawn.abacus.util.function.ToFloatFunction;
import com.landawn.abacus.util.function.ToIntFunction;
import com.landawn.abacus.util.function.ToLongFunction;
import com.landawn.abacus.util.function.ToShortFunction;
import com.landawn.abacus.util.function.TriConsumer;
import com.landawn.abacus.util.function.TriFunction;
import com.landawn.abacus.util.function.TriPredicate;
import com.landawn.abacus.util.function.UnaryOperator;
import com.landawn.abacus.util.stream.Collector;
import com.landawn.abacus.util.stream.Collectors;

/**
 * Factory utility class for functional interfaces.
 * 
 * 
     * 
 * 
 * Map map = N.asMap("a", 1, "b", 2, "c", 3);
 * // Instead of
 * Stream.of(map).filter(e -> e.getKey().equals("a") || e.getKey().equals("b")).toMap(e -> e.getKey(), e -> e.getValue());
 * // Using Fn
 * Stream.of(map).filter(Fn.testByKey(k -> k.equals("a") || k.equals("b"))).collect(Collectors.toMap());
 * 
 * 
 * 
    
 * 
 * 
 * @author haiyang li
 *
 */
public final class Fn {

    @SuppressWarnings("rawtypes")
    public static final IntFunction> FACTORY_OF_MAP = (IntFunction) Factory.MAP_FACTORY;
    @SuppressWarnings("rawtypes")
    public static final IntFunction> FACTORY_OF_LINKED_HASH_MAP = (IntFunction) Factory.LINKED_HASH_MAP_FACTORY;
    @SuppressWarnings("rawtypes")
    public static final Supplier> SUPPLIER_OF_MAP = (Supplier) Suppliers.MAP;
    @SuppressWarnings("rawtypes")
    public static final Supplier> SUPPLIER_OF_LINKED_HASH_MAP = (Supplier) Suppliers.LINKED_HASH_MAP;

    private static final Runnable EMPTY_ACTION = new Runnable() {
        @Override
        public void run() {
        }
    };

    @SuppressWarnings("rawtypes")
    private static final Consumer DO_NOTHING = new Consumer() {
        @Override
        public void accept(Object value) {
            // do nothing.
        }
    };

    @SuppressWarnings("rawtypes")
    private static final Consumer PRINTLN = new Consumer() {
        @Override
        public void accept(Object value) {
            N.println(value);
        }
    };

    @SuppressWarnings("rawtypes")
    private static final Function IDENTITY = new Function() {
        @Override
        public Object apply(Object t) {
            return t;
        }
    };

    private static final Function, Object> KEY = new Function, Object>() {
        @Override
        public Object apply(Map.Entry t) {
            return t.getKey();
        }
    };

    private static final Function, Object> VALUE = new Function, Object>() {
        @Override
        public Object apply(Map.Entry t) {
            return t.getValue();
        }
    };

    private static final BiFunction> ENTRY = new BiFunction>() {
        @Override
        public Map.Entry apply(Object key, Object value) {
            return new AbstractMap.SimpleImmutableEntry<>(key, value);
        }
    };

    private static final Function TRIM = new Function() {
        @Override
        public String apply(String t) {
            return t == null ? null : t.trim();
        }
    };

    private static final Function TRIM_TO_EMPTY = new Function() {
        @Override
        public String apply(String t) {
            return t == null ? "" : t.trim();
        }
    };

    private static final Function TRIM_TO_NULL = new Function() {
        @Override
        public String apply(String t) {
            if (t == null || (t = t.trim()).length() == 0) {
                return null;
            }

            return t;
        }
    };

    private static final BiFunction> PAIR = new BiFunction>() {
        @Override
        public Pair apply(Object key, Object value) {
            return Pair.of(key, value);
        }
    };

    private static final TriFunction> TRIPLE = new TriFunction>() {
        @Override
        public Triple apply(Object a, Object b, Object c) {
            return Triple.of(a, b, c);
        }
    };

    @SuppressWarnings("rawtypes")
    private static final Predicate ALWAYS_TRUE = new Predicate() {
        @Override
        public boolean test(Object value) {
            return true;
        }
    };

    @SuppressWarnings("rawtypes")
    private static final Predicate ALWAYS_FALSE = new Predicate() {
        @Override
        public boolean test(Object value) {
            return false;
        }
    };

    @SuppressWarnings("rawtypes")
    private static final Predicate IS_NULL = new Predicate() {
        @Override
        public boolean test(Object value) {
            return value == null;
        }
    };

    @SuppressWarnings("rawtypes")
    private static final Predicate NOT_NULL = new Predicate() {
        @Override
        public boolean test(Object value) {
            return value != null;
        }
    };

    private static final Object NULL = new Object();

    private Fn() {
        // Singleton.
    }

    public static  T get(final Supplier supplier) {
        return supplier.get();
    }

    public static  Comparator naturalOrder() {
        return Comparators.naturalOrder();
    }

    public static  Comparator reversedOrder() {
        return Comparators.reversedOrder();
    }

    public static  Comparator reversedOrder(final Comparator cmp) {
        return Comparators.reversedOrder(cmp);
    }

    @SuppressWarnings("rawtypes")
    public static  Comparator comparingBy(final Function keyExtractor) {
        return Comparators.comparingBy(keyExtractor);
    }

    @SuppressWarnings("rawtypes")
    public static  Comparator reversedComparingBy(final Function keyExtractor) {
        return Comparators.reversedComparingBy(keyExtractor);
    }

    public static Runnable emptyAction() {
        return EMPTY_ACTION;
    }

    public static  Consumer doNothing() {
        return DO_NOTHING;
    }

    public static  Consumer println() {
        return PRINTLN;
    }

    public static  BiConsumer println(final String separator) {
        return new BiConsumer() {
            @Override
            public void accept(T t, U u) {
                N.println(t + separator + u);
            }
        };
    }

    public static  Function identity() {
        return IDENTITY;
    }

    @SuppressWarnings("rawtypes")
    public static  Function, K> key() {
        return (Function) KEY;
    }

    @SuppressWarnings("rawtypes")
    public static  Function, V> value() {
        return (Function) VALUE;
    }

    @SuppressWarnings("rawtypes")
    public static  BiFunction> entry() {
        return (BiFunction) ENTRY;
    }

    @SuppressWarnings("rawtypes")
    public static  BiFunction> pair() {
        return (BiFunction) PAIR;
    }

    @SuppressWarnings("rawtypes")
    public static  TriFunction> triple() {
        return (TriFunction) TRIPLE;
    }

    public static Function trim() {
        return TRIM;
    }

    public static Function trimToEmpty() {
        return TRIM_TO_EMPTY;
    }

    public static Function trimToNull() {
        return TRIM_TO_NULL;
    }

    public static  Function cast(final Class clazz) {
        return new Function() {
            @Override
            public U apply(T t) {
                return (U) t;
            }
        };
    }

    public static  Predicate alwaysTrue() {
        return ALWAYS_TRUE;
    }

    public static  Predicate alwaysFalse() {
        return ALWAYS_FALSE;
    }

    public static  Predicate isNull() {
        return IS_NULL;
    }

    public static  Predicate notNull() {
        return NOT_NULL;
    }

    public static  Predicate equal(final Object target) {
        return new Predicate() {
            @Override
            public boolean test(T value) {
                return N.equals(value, target);
            }
        };
    }

    public static  Predicate notEqual(final Object target) {
        return new Predicate() {
            @Override
            public boolean test(T value) {
                return !N.equals(value, target);
            }
        };
    }

    @SuppressWarnings("rawtypes")
    public static  Predicate greaterThan(final T target) {
        return new Predicate() {
            @Override
            public boolean test(T value) {
                return N.compare(value, target) > 0;
            }
        };
    }

    @SuppressWarnings("rawtypes")
    public static  Predicate greaterEqual(final T target) {
        return new Predicate() {
            @Override
            public boolean test(T value) {
                return N.compare(value, target) >= 0;
            }
        };
    }

    @SuppressWarnings("rawtypes")
    public static  Predicate lessThan(final T target) {
        return new Predicate() {
            @Override
            public boolean test(T value) {
                return N.compare(value, target) < 0;
            }
        };
    }

    @SuppressWarnings("rawtypes")
    public static  Predicate lessEqual(final T target) {
        return new Predicate() {
            @Override
            public boolean test(T value) {
                return N.compare(value, target) <= 0;
            }
        };
    }

    public static  Predicate in(final Collection c) {
        return new Predicate() {
            @Override
            public boolean test(T value) {
                return c.contains(value);
            }
        };
    }

    public static  Predicate notIn(final Collection c) {
        return new Predicate() {
            @Override
            public boolean test(T value) {
                return !c.contains(value);
            }
        };
    }

    public static  Predicate instanceOf(final Class clazz) {
        return new Predicate() {
            @Override
            public boolean test(T value) {
                return clazz.isInstance(value);
            }
        };
    }

    @SuppressWarnings("rawtypes")
    public static Predicate subtypeOf(final Class clazz) {
        return new Predicate() {
            @Override
            public boolean test(Class value) {
                return clazz.isAssignableFrom(value);
            }
        };
    }

    public static Predicate startsWith(final String prefix) {
        return new Predicate() {
            @Override
            public boolean test(String value) {
                return value.startsWith(prefix);
            }
        };
    }

    public static Predicate endsWith(final String suffix) {
        return new Predicate() {
            @Override
            public boolean test(String value) {
                return value.endsWith(suffix);
            }
        };
    }

    public static Predicate matches(final Pattern pattern) {
        return new Predicate() {
            @Override
            public boolean test(CharSequence value) {
                return pattern.matcher(value).find();
            }
        };
    }

    public static  BiPredicate equal() {
        return BiPredicates.EQUAL;
    }

    public static  BiPredicate notEqual() {
        return BiPredicates.NOT_EQUAL;
    }

    @SuppressWarnings("rawtypes")
    public static  BiPredicate greaterThan() {
        return (BiPredicate) BiPredicates.GREATER_THAN;
    }

    @SuppressWarnings("rawtypes")
    public static  BiPredicate greaterEqual() {
        return (BiPredicate) BiPredicates.GREATER_EQUAL;
    }

    @SuppressWarnings("rawtypes")
    public static  BiPredicate lessThan() {
        return (BiPredicate) BiPredicates.LESS_THAN;
    }

    @SuppressWarnings("rawtypes")
    public static  BiPredicate lessEqual() {
        return (BiPredicate) BiPredicates.LESS_EQUAL;
    }

    public static  Predicate> testByKey(final Predicate predicate) {
        return new Predicate>() {
            @Override
            public boolean test(Entry entry) {
                return predicate.test(entry.getKey());
            }
        };
    }

    public static  Predicate> testByValue(final Predicate predicate) {
        return new Predicate>() {
            @Override
            public boolean test(Entry entry) {
                return predicate.test(entry.getValue());
            }
        };
    }

    public static  Consumer> acceptByKey(final Consumer consumer) {
        return new Consumer>() {
            @Override
            public void accept(Entry entry) {
                consumer.accept(entry.getKey());
            }
        };
    }

    public static  Consumer> acceptByValue(final Consumer consumer) {
        return new Consumer>() {
            @Override
            public void accept(Entry entry) {
                consumer.accept(entry.getValue());
            }
        };
    }

    public static  Function, R> applyByKey(final Function func) {
        return new Function, R>() {
            @Override
            public R apply(Entry entry) {
                return func.apply(entry.getKey());
            }
        };
    }

    public static  Function, R> applyByValue(final Function func) {
        return new Function, R>() {
            @Override
            public R apply(Entry entry) {
                return func.apply(entry.getValue());
            }
        };
    }

    public static  Function, Map.Entry> mapKey(final Function func) {
        return new Function, Map.Entry>() {
            @Override
            public Map.Entry apply(Entry entry) {
                return Pair.of(func.apply(entry.getKey()), entry.getValue());
            }
        };
    }

    public static  Function, Map.Entry> mapValue(final Function func) {
        return new Function, Map.Entry>() {
            @Override
            public Map.Entry apply(Entry entry) {
                return Pair.of(entry.getKey(), func.apply(entry.getValue()));
            }
        };
    }

    public static  Predicate> testKeyVal(final BiPredicate predicate) {
        return new Predicate>() {
            @Override
            public boolean test(Entry entry) {
                return predicate.test(entry.getKey(), entry.getValue());
            }
        };
    }

    public static  Consumer> acceptKeyVal(final BiConsumer consumer) {
        return new Consumer>() {
            @Override
            public void accept(Entry entry) {
                consumer.accept(entry.getKey(), entry.getValue());
            }
        };
    }

    public static  Function, R> applyKeyVal(final BiFunction func) {
        return new Function, R>() {
            @Override
            public R apply(Entry entry) {
                return func.apply(entry.getKey(), entry.getValue());
            }
        };
    }

    public static  BinaryOperator throwingMerger() {
        return BinaryOperators.THROWING_MERGER;
    }

    public static  BinaryOperator ignoringMerger() {
        return BinaryOperators.IGNORING_MERGER;
    }

    public static  BinaryOperator replacingMerger() {
        return BinaryOperators.REPLACING_MERGER;
    }

    public static ToByteFunction unboxB() {
        return ToByteFunction.UNBOX;
    }

    public static ToCharFunction unboxC() {
        return ToCharFunction.UNBOX;
    }

    public static ToShortFunction unboxS() {
        return ToShortFunction.UNBOX;
    }

    public static ToIntFunction unboxI() {
        return ToIntFunction.UNBOX;
    }

    public static ToLongFunction unboxL() {
        return ToLongFunction.UNBOX;
    }

    public static ToFloatFunction unboxF() {
        return ToFloatFunction.UNBOX;
    }

    public static ToDoubleFunction unboxD() {
        return ToDoubleFunction.UNBOX;
    }

    /**
     * 
     * @param predicate
     * @param limit
     * @return
     */
    public static  Predicate limited(final Predicate predicate, final int limit) {
        N.requireNonNull(predicate);

        return new Predicate() {
            private final AtomicInteger counter = new AtomicInteger(limit);

            @Override
            public boolean test(T t) {
                return predicate.test(t) && counter.decrementAndGet() >= 0;
            }
        };
    }

    /**
     * 
     * @param predicate
     * @param limit
     * @return
     */
    public static  Predicate limited(final Predicate predicate, final long limit) {
        N.requireNonNull(predicate);

        return new Predicate() {
            private final AtomicLong counter = new AtomicLong(limit);

            @Override
            public boolean test(T t) {
                return predicate.test(t) && counter.decrementAndGet() >= 0;
            }
        };
    }

    /** 
     * 
     * @param predicate
     * @param limit
     * @return
     */
    public static  BiPredicate limited(final BiPredicate predicate, final int limit) {
        N.requireNonNull(predicate);

        return new BiPredicate() {
            private final AtomicInteger counter = new AtomicInteger(limit);

            @Override
            public boolean test(T t, U u) {
                return predicate.test(t, u) && counter.decrementAndGet() >= 0;
            }
        };
    }

    /**
     * 
     * @param predicate
     * @param limit
     * @return
     */
    public static  BiPredicate limited(final BiPredicate predicate, final long limit) {
        N.requireNonNull(predicate);

        return new BiPredicate() {
            private final AtomicLong counter = new AtomicLong(limit);

            @Override
            public boolean test(T t, U u) {
                return predicate.test(t, u) && counter.decrementAndGet() >= 0;
            }
        };
    }

    /**
     * 
     * @param predicate
     * @param limit
     * @return
     */
    public static  TriPredicate limited(final TriPredicate predicate, final int limit) {
        N.requireNonNull(predicate);

        return new TriPredicate() {
            private final AtomicInteger counter = new AtomicInteger(limit);

            @Override
            public boolean test(A a, B b, C c) {
                return predicate.test(a, b, c) && counter.decrementAndGet() >= 0;
            }
        };
    }

    /**
     * 
     * @param predicate
     * @param limit
     * @return
     */
    public static  TriPredicate limited(final TriPredicate predicate, final long limit) {
        N.requireNonNull(predicate);

        return new TriPredicate() {
            private final AtomicLong counter = new AtomicLong(limit);

            @Override
            public boolean test(A a, B b, C c) {
                return predicate.test(a, b, c) && counter.decrementAndGet() >= 0;
            }
        };
    }

    /**
     * Returns a stateful Predicate which should not be used in parallel stream.
     * 
     * @param predicate
     * @return
     */
    public static  Predicate indexed(final IndexedPredicate predicate) {
        N.requireNonNull(predicate);

        return new Predicate() {
            private final AtomicInteger idx = new AtomicInteger(0);

            @Override
            public boolean test(T t) {
                return predicate.test(idx.getAndIncrement(), t);
            }
        };
    }

    /**
     * Returns a stateful BiPredicate which should not be used in parallel stream.
     * 
     * @param predicate
     * @return
     */
    public static  BiPredicate indexed(final IndexedBiPredicate predicate) {
        N.requireNonNull(predicate);

        return new BiPredicate() {
            private final AtomicInteger idx = new AtomicInteger(0);

            @Override
            public boolean test(U u, T t) {
                return predicate.test(u, idx.getAndIncrement(), t);
            }
        };
    }

    /**
     * Returns a stateful Function which should not be used in parallel stream.
     * 
     * @param func
     * @return
     */
    public static  Function indexeD(final IndexedFunction func) {
        N.requireNonNull(func);

        return new Function() {
            private final AtomicInteger idx = new AtomicInteger(0);

            @Override
            public R apply(T t) {
                return func.apply(idx.getAndIncrement(), t);
            }
        };
    }

    /**
     * Returns a stateful BiFunction which should not be used in parallel stream.
     * 
     * @param func
     * @return
     */
    public static  BiFunction indexeD(final IndexedBiFunction func) {
        N.requireNonNull(func);

        return new BiFunction() {
            private final AtomicInteger idx = new AtomicInteger(0);

            @Override
            public R apply(U u, T t) {
                return func.apply(u, idx.getAndIncrement(), t);
            }
        };
    }

    /**
     * Returns a stateful Consumer which should not be used in parallel stream.
     * 
     * @param action
     * @return
     */
    public static  Consumer indexeed(final IndexedConsumer action) {
        N.requireNonNull(action);

        return new Consumer() {
            private final AtomicInteger idx = new AtomicInteger(0);

            @Override
            public void accept(T t) {
                action.accept(idx.getAndIncrement(), t);
            }
        };
    }

    /**
     * Returns a stateful BiConsumer which should not be used in parallel stream.
     * 
     * @param action
     * @return
     */
    public static  BiConsumer indexeed(final IndexedBiConsumer action) {
        N.requireNonNull(action);

        return new BiConsumer() {
            private final AtomicInteger idx = new AtomicInteger(0);

            @Override
            public void accept(U u, T t) {
                action.accept(u, idx.getAndIncrement(), t);
            }
        };
    }

    public static  Collector> toList() {
        return Collectors.toList();
    }

    public static  Collector> toSet() {
        return Collectors.toSet();
    }

    public static  Collector counting() {
        return Collectors.counting();
    }

    public static  Collector countingInt() {
        return Collectors.countingInt();
    }

    public static  Collector summingInt(final ToIntFunction mapper) {
        return Collectors.summingInt(mapper);
    }

    public static  Collector summingLong(final ToLongFunction mapper) {
        return Collectors.summingLong(mapper);
    }

    public static  Collector summingDouble(final ToDoubleFunction mapper) {
        return Collectors.summingDouble(mapper);
    }

    public static  Collector averagingInt(final ToIntFunction mapper) {
        return Collectors.averagingInt(mapper);
    }

    public static  Collector averagingLong(final ToLongFunction mapper) {
        return Collectors.averagingLong(mapper);
    }

    public static  Collector averagingDouble(final ToDoubleFunction mapper) {
        return Collectors.averagingDouble(mapper);
    }

    public static final class Factory {

        private static final IntFunction BOOLEAN_ARRAY = new IntFunction() {
            @Override
            public boolean[] apply(int len) {
                return new boolean[len];
            }
        };

        private static final IntFunction CHAR_ARRAY = new IntFunction() {
            @Override
            public char[] apply(int len) {
                return new char[len];
            }
        };

        private static final IntFunction BYTE_ARRAY = new IntFunction() {
            @Override
            public byte[] apply(int len) {
                return new byte[len];
            }
        };

        private static final IntFunction SHORT_ARRAY = new IntFunction() {
            @Override
            public short[] apply(int len) {
                return new short[len];
            }
        };

        private static final IntFunction INT_ARRAY = new IntFunction() {
            @Override
            public int[] apply(int len) {
                return new int[len];
            }
        };

        private static final IntFunction LONG_ARRAY = new IntFunction() {
            @Override
            public long[] apply(int len) {
                return new long[len];
            }
        };

        private static final IntFunction FLOAT_ARRAY = new IntFunction() {
            @Override
            public float[] apply(int len) {
                return new float[len];
            }
        };

        private static final IntFunction DOUBLE_ARRAY = new IntFunction() {
            @Override
            public double[] apply(int len) {
                return new double[len];
            }
        };

        private static final IntFunction STRING_ARRAY = new IntFunction() {
            @Override
            public String[] apply(int len) {
                return new String[len];
            }
        };

        private static final IntFunction OBJECT_ARRAY = new IntFunction() {
            @Override
            public Object[] apply(int len) {
                return new Object[len];
            }
        };

        private static final IntFunction BOOLEAN_LIST = new IntFunction() {
            @Override
            public BooleanList apply(int len) {
                return new BooleanList(len);
            }
        };

        private static final IntFunction CHAR_LIST = new IntFunction() {
            @Override
            public CharList apply(int len) {
                return new CharList(len);
            }
        };

        private static final IntFunction BYTE_LIST = new IntFunction() {
            @Override
            public ByteList apply(int len) {
                return new ByteList(len);
            }
        };

        private static final IntFunction SHORT_LIST = new IntFunction() {
            @Override
            public ShortList apply(int len) {
                return new ShortList(len);
            }
        };

        private static final IntFunction INT_LIST = new IntFunction() {
            @Override
            public IntList apply(int len) {
                return new IntList(len);
            }
        };

        private static final IntFunction LONG_LIST = new IntFunction() {
            @Override
            public LongList apply(int len) {
                return new LongList(len);
            }
        };

        private static final IntFunction FLOAT_LIST = new IntFunction() {
            @Override
            public FloatList apply(int len) {
                return new FloatList(len);
            }
        };

        private static final IntFunction DOUBLE_LIST = new IntFunction() {
            @Override
            public DoubleList apply(int len) {
                return new DoubleList(len);
            }
        };
        @SuppressWarnings("rawtypes")
        private static final IntFunction LIST_FACTORY = new IntFunction() {
            @Override
            public List apply(int len) {
                return new ArrayList<>(len);
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction LINKED_LIST_FACTORY = new IntFunction() {
            @Override
            public LinkedList apply(int len) {
                return new LinkedList<>();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction SET_FACTORY = new IntFunction() {
            @Override
            public Set apply(int len) {
                return new HashSet<>(N.initHashCapacity(len));
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction LINKED_HASH_SET_FACTORY = new IntFunction() {
            @Override
            public LinkedHashSet apply(int len) {
                return new LinkedHashSet<>(N.initHashCapacity(len));
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction TREE_SET_FACTORY = new IntFunction() {
            @Override
            public TreeSet apply(int len) {
                return new TreeSet<>();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction MAP_FACTORY = new IntFunction() {
            @Override
            public Map apply(int len) {
                return new HashMap<>(N.initHashCapacity(len));
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction LINKED_HASH_MAP_FACTORY = new IntFunction() {
            @Override
            public LinkedHashMap apply(int len) {
                return new LinkedHashMap<>(N.initHashCapacity(len));
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction TREE_MAP_FACTORY = new IntFunction() {
            @Override
            public TreeMap apply(int len) {
                return new TreeMap<>();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction CONCURRENT_HASH_MAP_FACTORY = new IntFunction() {
            @Override
            public ConcurrentHashMap apply(int len) {
                return new ConcurrentHashMap(N.initHashCapacity(len));
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction QUEUE_FACTORY = new IntFunction() {
            @Override
            public Queue apply(int len) {
                return new LinkedList();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction ARRAY_DEQUE_FACTORY = new IntFunction() {
            @Override
            public ArrayDeque apply(int len) {
                return new ArrayDeque(len);
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction LINKED_BLOCKING_QUEUE_FACTORY = new IntFunction() {
            @Override
            public LinkedBlockingQueue apply(int len) {
                return new LinkedBlockingQueue(len);
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction CONCURRENT_LINKED_QUEUE_FACTORY = new IntFunction() {
            @Override
            public ConcurrentLinkedQueue apply(int len) {
                return new ConcurrentLinkedQueue();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final IntFunction PRIORITY_QUEUE_FACTORY = new IntFunction() {
            @Override
            public PriorityQueue apply(int len) {
                return new PriorityQueue(len);
            }
        };

        private Factory() {
            // singleton.
        }

        public static IntFunction ofBooleanArray() {
            return BOOLEAN_ARRAY;
        }

        public static IntFunction ofCharArray() {
            return CHAR_ARRAY;
        }

        public static IntFunction ofByteArray() {
            return BYTE_ARRAY;
        }

        public static IntFunction ofShortArray() {
            return SHORT_ARRAY;
        }

        public static IntFunction ofIntArray() {
            return INT_ARRAY;
        }

        public static IntFunction ofLongArray() {
            return LONG_ARRAY;
        }

        public static IntFunction ofFloatArray() {
            return FLOAT_ARRAY;
        }

        public static IntFunction ofDoubleArray() {
            return DOUBLE_ARRAY;
        }

        public static IntFunction ofStringArray() {
            return STRING_ARRAY;
        }

        public static IntFunction ofObjectArray() {
            return OBJECT_ARRAY;
        }

        public static IntFunction ofBooleanList() {
            return BOOLEAN_LIST;
        }

        public static IntFunction ofCharList() {
            return CHAR_LIST;
        }

        public static IntFunction ofByteList() {
            return BYTE_LIST;
        }

        public static IntFunction ofShortList() {
            return SHORT_LIST;
        }

        public static IntFunction ofIntList() {
            return INT_LIST;
        }

        public static IntFunction ofLongList() {
            return LONG_LIST;
        }

        public static IntFunction ofFloatList() {
            return FLOAT_LIST;
        }

        public static IntFunction ofDoubleList() {
            return DOUBLE_LIST;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofList() {
            return (IntFunction) LIST_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofLinkedList() {
            return (IntFunction) LINKED_LIST_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofSet() {
            return (IntFunction) SET_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofLinkedHashSet() {
            return (IntFunction) LINKED_HASH_SET_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofTreeSet() {
            return (IntFunction) TREE_SET_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofMap() {
            return (IntFunction) MAP_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofLinkedHashMap() {
            return (IntFunction) LINKED_HASH_MAP_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofTreeMap() {
            return (IntFunction) TREE_MAP_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofConcurrentHashMap() {
            return (IntFunction) CONCURRENT_HASH_MAP_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofQueue() {
            return (IntFunction) QUEUE_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofArrayDeque() {
            return (IntFunction) ARRAY_DEQUE_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofLinkedBlockingQueue() {
            return (IntFunction) LINKED_BLOCKING_QUEUE_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofConcurrentLinkedQueue() {
            return (IntFunction) CONCURRENT_LINKED_QUEUE_FACTORY;
        }

        @SuppressWarnings("rawtypes")
        public static  IntFunction> ofPriorityQueue() {
            return (IntFunction) PRIORITY_QUEUE_FACTORY;
        }
    }

    public static final class Suppliers {
        private static final Supplier UUID = new Supplier() {
            @Override
            public String get() {
                return N.uuid();
            }
        };

        private static final Supplier GUID = new Supplier() {
            @Override
            public String get() {
                return N.guid();
            }
        };

        private static final Supplier EMPTY_BOOLEAN_ARRAY = new Supplier() {
            @Override
            public boolean[] get() {
                return N.EMPTY_BOOLEAN_ARRAY;
            }
        };

        private static final Supplier EMPTY_CHAR_ARRAY = new Supplier() {
            @Override
            public char[] get() {
                return N.EMPTY_CHAR_ARRAY;
            }
        };

        private static final Supplier EMPTY_BYTE_ARRAY = new Supplier() {
            @Override
            public byte[] get() {
                return N.EMPTY_BYTE_ARRAY;
            }
        };

        private static final Supplier EMPTY_SHORT_ARRAY = new Supplier() {
            @Override
            public short[] get() {
                return N.EMPTY_SHORT_ARRAY;
            }
        };

        private static final Supplier EMPTY_INT_ARRAY = new Supplier() {
            @Override
            public int[] get() {
                return N.EMPTY_INT_ARRAY;
            }
        };

        private static final Supplier EMPTY_LONG_ARRAY = new Supplier() {
            @Override
            public long[] get() {
                return N.EMPTY_LONG_ARRAY;
            }
        };

        private static final Supplier EMPTY_FLOAT_ARRAY = new Supplier() {
            @Override
            public float[] get() {
                return N.EMPTY_FLOAT_ARRAY;
            }
        };

        private static final Supplier EMPTY_DOUBLE_ARRAY = new Supplier() {
            @Override
            public double[] get() {
                return N.EMPTY_DOUBLE_ARRAY;
            }
        };

        private static final Supplier EMPTY_STRING_ARRAY = new Supplier() {
            @Override
            public String[] get() {
                return N.EMPTY_STRING_ARRAY;
            }
        };

        private static final Supplier EMPTY_OBJECT_ARRAY = new Supplier() {
            @Override
            public Object[] get() {
                return N.EMPTY_OBJECT_ARRAY;
            }
        };

        private static final Supplier BOOLEAN_LIST = new Supplier() {
            @Override
            public BooleanList get() {
                return new BooleanList();
            }
        };

        private static final Supplier CHAR_LIST = new Supplier() {
            @Override
            public CharList get() {
                return new CharList();
            }
        };

        private static final Supplier BYTE_LIST = new Supplier() {
            @Override
            public ByteList get() {
                return new ByteList();
            }
        };

        private static final Supplier SHORT_LIST = new Supplier() {
            @Override
            public ShortList get() {
                return new ShortList();
            }
        };

        private static final Supplier INT_LIST = new Supplier() {
            @Override
            public IntList get() {
                return new IntList();
            }
        };

        private static final Supplier LONG_LIST = new Supplier() {
            @Override
            public LongList get() {
                return new LongList();
            }
        };

        private static final Supplier FLOAT_LIST = new Supplier() {
            @Override
            public FloatList get() {
                return new FloatList();
            }
        };

        private static final Supplier DOUBLE_LIST = new Supplier() {
            @Override
            public DoubleList get() {
                return new DoubleList();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier LIST = new Supplier() {
            @Override
            public List get() {
                return new ArrayList();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier LINKED_LIST = new Supplier() {
            @Override
            public LinkedList get() {
                return new LinkedList();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier SET = new Supplier() {
            @Override
            public Set get() {
                return new HashSet();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier LINKED_HASH_SET = new Supplier() {
            @Override
            public LinkedHashSet get() {
                return new LinkedHashSet();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier TREE_SET = new Supplier() {
            @Override
            public TreeSet get() {
                return new TreeSet();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier MAP = new Supplier() {
            @Override
            public Map get() {
                return new HashMap();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier LINKED_HASH_MAP = new Supplier() {
            @Override
            public LinkedHashMap get() {
                return new LinkedHashMap();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier TREE_MAP = new Supplier() {
            @Override
            public TreeMap get() {
                return new TreeMap();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier CONCURRENT_HASH_MAP = new Supplier() {
            @Override
            public ConcurrentHashMap get() {
                return new ConcurrentHashMap();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier QUEUE = new Supplier() {
            @Override
            public Queue get() {
                return new LinkedList();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier ARRAY_DEQUE = new Supplier() {
            @Override
            public ArrayDeque get() {
                return new ArrayDeque();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier LINKED_BLOCKING_QUEUE = new Supplier() {
            @Override
            public LinkedBlockingQueue get() {
                return new LinkedBlockingQueue();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier CONCURRENT_LINKED_QUEUE = new Supplier() {
            @Override
            public ConcurrentLinkedQueue get() {
                return new ConcurrentLinkedQueue();
            }
        };

        @SuppressWarnings("rawtypes")
        private static final Supplier PRIORITY_QUEUE = new Supplier() {
            @Override
            public PriorityQueue get() {
                return new PriorityQueue();
            }
        };

        private Suppliers() {
            // singleton.
        }

        public static Supplier ofUUID() {
            return UUID;
        }

        public static Supplier ofGUID() {
            return GUID;
        }

        public static Supplier ofEmptyBooleanArray() {
            return EMPTY_BOOLEAN_ARRAY;
        }

        public static Supplier ofEmptyCharArray() {
            return EMPTY_CHAR_ARRAY;
        }

        public static Supplier ofEmptyByteArray() {
            return EMPTY_BYTE_ARRAY;
        }

        public static Supplier ofEmptyShortArray() {
            return EMPTY_SHORT_ARRAY;
        }

        public static Supplier ofEmptyIntArray() {
            return EMPTY_INT_ARRAY;
        }

        public static Supplier ofEmptyLongArray() {
            return EMPTY_LONG_ARRAY;
        }

        public static Supplier ofEmptyFloatArray() {
            return EMPTY_FLOAT_ARRAY;
        }

        public static Supplier ofEmptyDoubleArray() {
            return EMPTY_DOUBLE_ARRAY;
        }

        public static Supplier ofEmptyStringArray() {
            return EMPTY_STRING_ARRAY;
        }

        public static Supplier ofEmptyObjectArray() {
            return EMPTY_OBJECT_ARRAY;
        }

        public static Supplier ofBooleanList() {
            return BOOLEAN_LIST;
        }

        public static Supplier ofCharList() {
            return CHAR_LIST;
        }

        public static Supplier ofByteList() {
            return BYTE_LIST;
        }

        public static Supplier ofShortList() {
            return SHORT_LIST;
        }

        public static Supplier ofIntList() {
            return INT_LIST;
        }

        public static Supplier ofLongList() {
            return LONG_LIST;
        }

        public static Supplier ofFloatList() {
            return FLOAT_LIST;
        }

        public static Supplier ofDoubleList() {
            return DOUBLE_LIST;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofList() {
            return (Supplier) LIST;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofLinkedList() {
            return (Supplier) LINKED_LIST;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofSet() {
            return (Supplier) SET;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofLinkedHashSet() {
            return (Supplier) LINKED_HASH_SET;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofTreeSet() {
            return (Supplier) TREE_SET;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofMap() {
            return (Supplier) MAP;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofLinkedHashMap() {
            return (Supplier) LINKED_HASH_MAP;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofTreeMap() {
            return (Supplier) TREE_MAP;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofConcurrentHashMap() {
            return (Supplier) CONCURRENT_HASH_MAP;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofQueue() {
            return (Supplier) QUEUE;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofArrayDeque() {
            return (Supplier) ARRAY_DEQUE;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofLinkedBlockingQueue() {
            return (Supplier) LINKED_BLOCKING_QUEUE;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofConcurrentLinkedQueue() {
            return (Supplier) CONCURRENT_LINKED_QUEUE;
        }

        @SuppressWarnings("rawtypes")
        public static  Supplier> ofPriorityQueue() {
            return (Supplier) PRIORITY_QUEUE;
        }
    }

    public static final class Predicates {

        private Predicates() {
            // singleton.
        }

        public static  Predicate of(final U u, final BiPredicate predicate) {
            N.requireNonNull(predicate);

            return new Predicate() {
                @Override
                public boolean test(T t) {
                    return predicate.test(t, u);
                }
            };
        }

        /**
         * Remove the continuous repeat elements.
         * Returns a stateful predicate which should not be used in parallel stream.
         * 
         * @return
         */
        public static  Predicate skipRepeats() {
            return new Predicate() {
                private T pre = (T) NULL;

                @Override
                public boolean test(T value) {
                    boolean res = pre == NULL || N.equals(value, pre) == false;
                    pre = value;
                    return res;
                }
            };
        }

        public static  Predicate indexed(final IndexedPredicate predicate) {
            return Fn.indexed(predicate);
        }
    }

    public static final class BiPredicates {

        @SuppressWarnings("rawtypes")
        private static final BiPredicate ALWAYS_TRUE = new BiPredicate() {
            @Override
            public boolean test(Object t, Object u) {
                return true;
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiPredicate ALWAYS_FALSE = new BiPredicate() {
            @Override
            public boolean test(Object t, Object u) {
                return false;
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiPredicate EQUAL = new BiPredicate() {
            @Override
            public boolean test(Object t, Object u) {
                return N.equals(t, u);
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiPredicate NOT_EQUAL = new BiPredicate() {
            @Override
            public boolean test(Object t, Object u) {
                return !N.equals(t, u);
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiPredicate GREATER_THAN = new BiPredicate() {
            @Override
            public boolean test(Comparable t, Comparable u) {
                return N.compare(t, u) > 0;
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiPredicate GREATER_EQUAL = new BiPredicate() {
            @Override
            public boolean test(Comparable t, Comparable u) {
                return N.compare(t, u) >= 0;
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiPredicate LESS_THAN = new BiPredicate() {
            @Override
            public boolean test(Comparable t, Comparable u) {
                return N.compare(t, u) < 0;
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiPredicate LESS_EQUAL = new BiPredicate() {
            @Override
            public boolean test(Comparable t, Comparable u) {
                return N.compare(t, u) <= 0;
            }
        };

        private BiPredicates() {
            // singleton.
        }

        public static  BiPredicate alwaysTrue() {
            return ALWAYS_TRUE;
        }

        public static  BiPredicate alwaysFalse() {
            return ALWAYS_FALSE;
        }

        public static  BiPredicate indexed(final IndexedBiPredicate predicate) {
            return Fn.indexed(predicate);
        }
    }

    public static final class TriPredicates {

        @SuppressWarnings("rawtypes")
        private static final TriPredicate ALWAYS_TRUE = new TriPredicate() {
            @Override
            public boolean test(Object a, Object b, Object c) {
                return true;
            }
        };

        @SuppressWarnings("rawtypes")
        private static final TriPredicate ALWAYS_FALSE = new TriPredicate() {
            @Override
            public boolean test(Object a, Object b, Object c) {
                return false;
            }
        };

        private TriPredicates() {
            // singleton.
        }

        public static  TriPredicate alwaysTrue() {
            return ALWAYS_TRUE;
        }

        public static  TriPredicate alwaysFalse() {
            return ALWAYS_FALSE;
        }

    }

    public static final class Consumers {

        private Consumers() {
            // singleton.
        }

        public static  Consumer of(final U u, final BiConsumer action) {
            N.requireNonNull(action);

            return new Consumer() {
                @Override
                public void accept(T t) {
                    action.accept(t, u);
                }
            };
        }

        /**
         * Returns a Consumer which calls the specified func.
         * 
         * @param func
         * @return
         */
        public static  Consumer of(final Function func) {
            return new Consumer() {
                @Override
                public void accept(T t) {
                    func.apply(t);
                }
            };
        }

        public static  Consumer indexed(final IndexedConsumer action) {
            return Fn.indexeed(action);
        }
    }

    public static final class BiConsumers {

        @SuppressWarnings("rawtypes")
        private static final BiConsumer DO_NOTHING = new BiConsumer() {
            @Override
            public void accept(Object t, Object u) {
                // do nothing.
            }
        };

        private static final BiConsumer, Object> ADD = new BiConsumer, Object>() {
            @Override
            public void accept(Collection t, Object u) {
                t.add(u);
            }
        };

        private static final BiConsumer, Collection> ADD_ALL = new BiConsumer, Collection>() {
            @Override
            public void accept(Collection t, Collection u) {
                t.addAll(u);
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiConsumer ADD_ALL_2 = new BiConsumer() {
            @Override
            public void accept(PrimitiveList t, PrimitiveList u) {
                t.addAll(u);
            }
        };

        private static final BiConsumer, Object> REMOVE = new BiConsumer, Object>() {
            @Override
            public void accept(Collection t, Object u) {
                t.remove(u);
            }
        };

        private static final BiConsumer, Collection> REMOVE_ALL = new BiConsumer, Collection>() {
            @Override
            public void accept(Collection t, Collection u) {
                t.removeAll(u);
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiConsumer REMOVE_ALL_2 = new BiConsumer() {
            @Override
            public void accept(PrimitiveList t, PrimitiveList u) {
                t.removeAll(u);
            }
        };

        private static final BiConsumer, Map.Entry> PUT = new BiConsumer, Map.Entry>() {
            @Override
            public void accept(Map t, Map.Entry u) {
                t.put(u.getKey(), u.getValue());
            }
        };

        private static final BiConsumer, Map> PUT_ALL = new BiConsumer, Map>() {
            @Override
            public void accept(Map t, Map u) {
                t.putAll(u);
            }
        };

        private static final BiConsumer, Object> REMOVE_BY_KEY = new BiConsumer, Object>() {
            @Override
            public void accept(Map t, Object u) {
                t.remove(u);
            }
        };

        private static final BiConsumer MERGE = new BiConsumer() {
            @Override
            public void accept(Joiner t, Joiner u) {
                t.merge(u);
            }
        };

        private static final BiConsumer APPEND = new BiConsumer() {
            @Override
            public void accept(StringBuilder t, Object u) {
                t.append(u);
            }
        };

        private BiConsumers() {
            // singleton.
        }

        public static  BiConsumer doNothing() {
            return DO_NOTHING;
        }

        public static > BiConsumer ofAdd() {
            return (BiConsumer) ADD;
        }

        public static > BiConsumer ofAddAll() {
            return (BiConsumer) ADD_ALL;
        }

        @SuppressWarnings("rawtypes")
        public static  BiConsumer ofAddAll2() {
            return (BiConsumer) ADD_ALL_2;
        }

        public static > BiConsumer ofRemove() {
            return (BiConsumer) REMOVE;
        }

        public static > BiConsumer ofRemoveAll() {
            return (BiConsumer) REMOVE_ALL;
        }

        @SuppressWarnings("rawtypes")
        public static  BiConsumer ofRemoveAll2() {
            return (BiConsumer) REMOVE_ALL_2;
        }

        public static , E extends Map.Entry> BiConsumer ofPut() {
            return (BiConsumer) PUT;
        }

        public static > BiConsumer ofPutAll() {
            return (BiConsumer) PUT_ALL;
        }

        public static > BiConsumer ofRemoveByKey() {
            return (BiConsumer) REMOVE_BY_KEY;
        }

        public static BiConsumer ofMerge() {
            return MERGE;
        }

        public static  BiConsumer ofAppend() {
            return (BiConsumer) APPEND;
        }

        /**
         * Returns a BiConsumer which calls the specified func.
         * 
         * @param func
         * @return
         */
        public static  BiConsumer of(final BiFunction func) {
            return new BiConsumer() {
                @Override
                public void accept(T t, U u) {
                    func.apply(t, u);
                }
            };
        }

        public static  BiConsumer indexed(final IndexedBiConsumer action) {
            return Fn.indexeed(action);
        }
    }

    public static final class TriConsumers {
        private TriConsumers() {
            // singleton.
        }

        /**
         * Returns a TriConsumer which calls the specified func.
         * 
         * @param func
         * @return
         */
        public static  TriConsumer of(final TriFunction func) {
            return new TriConsumer() {
                @Override
                public void accept(A a, B b, C c) {
                    func.apply(a, b, c);
                }
            };
        }
    }

    public static final class Functions {

        private Functions() {
            // singleton.
        }

        public static  Function of(final U u, final BiFunction func) {
            N.requireNonNull(func);

            return new Function() {
                @Override
                public R apply(T t) {
                    return func.apply(t, u);
                }
            };
        }

        /**
         * Returns a Function which calls the specified action and always return an emptyOptional if action is executed successfully.
         * 
         * @param action
         * @return
         */
        public static  Function> of(final Consumer action) {
            return new Function>() {
                @Override
                public Optional apply(T t) {
                    action.accept(t);
                    return Optional.empty();
                }
            };
        }

        public static  Function indexed(final IndexedFunction func) {
            return Fn.indexeD(func);
        }
    }

    public static final class BiFunctions {

        private static final BiFunction RETURN_FIRST = new BiFunction() {
            @Override
            public Object apply(Object t, Object u) {
                return t;
            }
        };

        private static final BiFunction RETURN_SECOND = new BiFunction() {
            @Override
            public Object apply(Object t, Object u) {
                return u;
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiFunction TUPLE = new BiFunction() {
            @Override
            public Tuple2 apply(Object t, Object u) {
                return Tuple.of(t, u);
            }
        };

        private static final BiFunction, Object, Collection> ADD = new BiFunction, Object, Collection>() {
            @Override
            public Collection apply(Collection t, Object u) {
                t.add(u);
                return t;
            }
        };

        private static final BiFunction, Collection, Collection> ADD_ALL = new BiFunction, Collection, Collection>() {
            @Override
            public Collection apply(Collection t, Collection u) {
                t.addAll(u);
                return t;
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiFunction ADD_ALL_2 = new BiFunction() {
            @Override
            public PrimitiveList apply(PrimitiveList t, PrimitiveList u) {
                t.addAll(u);
                return t;
            }
        };

        private static final BiFunction, Object, Collection> REMOVE = new BiFunction, Object, Collection>() {
            @Override
            public Collection apply(Collection t, Object u) {
                t.remove(u);
                return t;
            }
        };

        private static final BiFunction, Collection, Collection> REMOVE_ALL = new BiFunction, Collection, Collection>() {
            @Override
            public Collection apply(Collection t, Collection u) {
                t.removeAll(u);
                return t;
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BiFunction REMOVE_ALL_2 = new BiFunction() {
            @Override
            public PrimitiveList apply(PrimitiveList t, PrimitiveList u) {
                t.removeAll(u);
                return t;
            }
        };

        private static final BiFunction, Map.Entry, Map> PUT = new BiFunction, Map.Entry, Map>() {
            @Override
            public Map apply(Map t, Map.Entry u) {
                t.put(u.getKey(), u.getValue());
                return t;
            }
        };

        private static final BiFunction, Map, Map> PUT_ALL = new BiFunction, Map, Map>() {
            @Override
            public Map apply(Map t, Map u) {
                t.putAll(u);
                return t;
            }
        };

        private static final BiFunction, Object, Map> REMOVE_BY_KEY = new BiFunction, Object, Map>() {
            @Override
            public Map apply(Map t, Object u) {
                t.remove(u);
                return t;
            }
        };

        private static final BiFunction MERGE = new BiFunction() {
            @Override
            public Joiner apply(Joiner t, Joiner u) {
                return t.merge(u);
            }
        };

        private static final BiFunction APPEND = new BiFunction() {
            @Override
            public StringBuilder apply(StringBuilder t, Object u) {
                return t.append(u);
            }
        };

        private BiFunctions() {
            // singleton.
        }

        public static  BiFunction returnFirst() {
            return (BiFunction) RETURN_FIRST;
        }

        public static  BiFunction returnSecond() {
            return (BiFunction) RETURN_SECOND;
        }

        @SuppressWarnings("rawtypes")
        public static  BiFunction> ofTuple() {
            return (BiFunction) TUPLE;
        }

        public static > BiFunction ofAdd() {
            return (BiFunction) ADD;
        }

        public static > BiFunction ofAddAll() {
            return (BiFunction) ADD_ALL;
        }

        @SuppressWarnings("rawtypes")
        public static  BiFunction ofAddAll2() {
            return (BiFunction) ADD_ALL_2;
        }

        public static > BiFunction ofRemove() {
            return (BiFunction) REMOVE;
        }

        public static > BiFunction ofRemoveAll() {
            return (BiFunction) REMOVE_ALL;
        }

        @SuppressWarnings("rawtypes")
        public static  BiFunction ofRemoveAll2() {
            return (BiFunction) REMOVE_ALL_2;
        }

        public static , E extends Map.Entry> BiFunction ofPut() {
            return (BiFunction) PUT;
        }

        public static > BiFunction ofPutAll() {
            return (BiFunction) PUT_ALL;
        }

        public static , U> BiFunction ofRemoveByKey() {
            return (BiFunction) REMOVE_BY_KEY;
        }

        public static BiFunction ofMerge() {
            return MERGE;
        }

        public static  BiFunction ofAppend() {
            return (BiFunction) APPEND;
        }

        /**
         * Returns a BiFunction which calls the specified action and always return an emptyOptional if action is executed successfully.
         * 
         * @param action
         * @return
         */
        public static  BiFunction> of(final BiConsumer action) {
            return new BiFunction>() {
                @Override
                public Optional apply(T t, U u) {
                    action.accept(t, u);
                    return Optional.empty();
                }
            };
        }

        public static  BiFunction indexed(final IndexedBiFunction func) {
            return Fn.indexeD(func);
        }
    }

    public static final class TriFunctions {

        @SuppressWarnings("rawtypes")
        private static final TriFunction TUPLE = new TriFunction() {
            @Override
            public Tuple3 apply(Object a, Object b, Object c) {
                return Tuple.of(a, b, c);
            }
        };

        private TriFunctions() {
            // singleton.
        }

        @SuppressWarnings("rawtypes")
        public static  TriFunction> ofTuple() {
            return (TriFunction) TUPLE;
        }

        /**
         * Returns a TriFunction which calls the specified action and always return an emptyOptional if action is executed successfully.
         * 
         * @param action
         * @return
         */
        public static  TriFunction> of(final TriConsumer action) {
            return new TriFunction>() {
                @Override
                public Optional apply(A a, B b, C c) {
                    action.accept(a, b, c);
                    return Optional.empty();
                }
            };
        }
    }

    public static final class BinaryOperators {
        @SuppressWarnings("rawtypes")
        private static final BinaryOperator THROWING_MERGER = new BinaryOperator() {
            @Override
            public Object apply(Object t, Object u) {
                throw new IllegalStateException(String.format("Duplicate key (attempted merging values %s and %s)", t, u));
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BinaryOperator IGNORING_MERGER = new BinaryOperator() {
            @Override
            public Object apply(Object t, Object u) {
                return t;
            }
        };

        @SuppressWarnings("rawtypes")
        private static final BinaryOperator REPLACING_MERGER = new BinaryOperator() {
            @Override
            public Object apply(Object t, Object u) {
                return u;
            }
        };

        private static final BinaryOperator> ADD_ALL = new BinaryOperator>() {
            @Override
            public Collection apply(Collection t, Collection u) {
                t.addAll(u);
                return t;
            }
        };

        private static final BinaryOperator> REMOVE_ALL = new BinaryOperator>() {
            @Override
            public Collection apply(Collection t, Collection u) {
                t.removeAll(u);
                return t;
            }
        };

        private static final BinaryOperator> PUT_ALL = new BinaryOperator>() {
            @Override
            public Map apply(Map t, Map u) {
                t.putAll(u);
                return t;
            }
        };

        private BinaryOperators() {
            // singleton.
        }

        public static > BinaryOperator ofAddAll() {
            return (BinaryOperator) ADD_ALL;
        }

        public static > BinaryOperator ofRemoveAll() {
            return (BinaryOperator) REMOVE_ALL;
        }

        public static > BinaryOperator ofPutAll() {
            return (BinaryOperator) PUT_ALL;
        }

        public static BinaryOperator ofMerge() {
            return new BinaryOperator() {
                @Override
                public Joiner apply(Joiner t, Joiner u) {
                    return t.merge(u);
                }
            };
        }

        public static BinaryOperator ofAppend() {
            return new BinaryOperator() {
                @Override
                public StringBuilder apply(StringBuilder t, StringBuilder u) {
                    return t.append(u);
                }
            };
        }

        public static BinaryOperator ofJoin(final String seperator) {
            return new BinaryOperator() {
                @Override
                public String apply(String t, String u) {
                    return t + seperator + u;
                }
            };
        }

        public static  BinaryOperator minBy(final Comparator comparator) {
            N.requireNonNull(comparator);

            return new BinaryOperator() {
                @Override
                public T apply(T t, T u) {
                    return comparator.compare(t, u) <= 0 ? t : u;
                }
            };
        }

        public static  BinaryOperator maxBy(final Comparator comparator) {
            N.requireNonNull(comparator);

            return new BinaryOperator() {
                @Override
                public T apply(T t, T u) {
                    return comparator.compare(t, u) >= 0 ? t : u;
                }
            };
        }
    }

    public static final class UnaryOperators {
        @SuppressWarnings("rawtypes")
        private static final UnaryOperator IDENTITY = new UnaryOperator() {
            @Override
            public Object apply(Object t) {
                return t;
            }
        };

        private UnaryOperators() {
            // singleton.
        }

        public static  UnaryOperator identity() {
            return IDENTITY;
        }
    }
}
    
    
    
    

    




    
    
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