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Apache Isis Commons is a library with utilities, that are shared with the entire Apache Isis ecosystem.
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/*
 *  Licensed to the Apache Software Foundation (ASF) under one
 *  or more contributor license agreements.  See the NOTICE file
 *  distributed with this work for additional information
 *  regarding copyright ownership.  The ASF licenses this file
 *  to you 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 org.apache.isis.commons.collections;

import java.lang.reflect.Array;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Optional;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.stream.Stream;

import org.springframework.lang.Nullable;

import org.apache.isis.commons.internal.base._Casts;
import org.apache.isis.commons.internal.exceptions._Exceptions;

import lombok.AccessLevel;
import lombok.NoArgsConstructor;
import lombok.NonNull;
import lombok.Value;
import lombok.val;

@Value @NoArgsConstructor(access = AccessLevel.PRIVATE)
final class Can_Empty implements Can {

    private static final long serialVersionUID = 1L;

    static final Can_Empty INSTANCE = new Can_Empty<>();

    @Override
    public Cardinality getCardinality() {
        return Cardinality.ZERO;
    }

    @Override
    public Stream stream() {
        return Stream.empty();
    }

    @Override
    public Stream parallelStream() {
        return Stream.empty();
    }

    @Override
    public Optional getSingleton() {
        return Optional.empty();
    }

    @Override
    public Optional getFirst() {
        return Optional.empty();
    }

    @Override
    public Optional getLast() {
        return Optional.empty();
    }

    @Override
    public Optional get(final int elementIndex) {
        return Optional.empty();
    }

    @Override
    public boolean contains(final @Nullable T element) {
        return false;
    }

    @Override
    public int size() {
        return 0;
    }

    @Override
    public Iterator iterator() {
        return Collections.emptyList().iterator();
    }

    @Override
    public Can unique() {
        return this;
    }

    @Override
    public Can reverse() {
        return this;
    }

    @Override
    public Iterator reverseIterator() {
        return iterator();
    }

    @Override
    public void forEach(final Consumer action) {
    }

    @Override
    public Can filter(final @Nullable Predicate predicate) {
        return this; // identity
    }

    @Override
    public  void zip(final Iterable zippedIn, final BiConsumer action) {
        // no-op
    }

    @Override
    public  Can zipMap(final Iterable zippedIn, final BiFunction mapper) {
        return Can.empty();
    }

    @Override
    public Can add(final @Nullable T element) {
        return element != null
                ? Can.ofSingleton(element)
                : this;
    }

    @Override
    public Can addUnique(final @Nullable T element) {
        return add(element);
    }

    @Override
    public Can addAll(final @Nullable Can other) {
        return other != null
                ? other
                : this;
    }

    @Override
    public Can add(final int index, final @Nullable T element) {
        if(index!=0) {
            throw new IndexOutOfBoundsException(
                    "cannot add to empty can with index other than 0; got " + index);
        }
        return add(element);
    }

    @Override
    public Can replace(final int index, final @Nullable T element) {
        throw _Exceptions.unsupportedOperation("cannot replace an element in an empty Can");
    }

    @Override
    public Can remove(final int index) {
        throw new IndexOutOfBoundsException("cannot remove anything from an empty Can");
    }

    @Override
    public Can remove(final @Nullable T element) {
        return this; // on an empty can this is a no-op
    }

    @Override
    public Can pickByIndex(final @Nullable int... indices) {
        return Can.empty();
    }

    @Override
    public int indexOf(final @Nullable T element) {
        return -1;
    }

    @Override
    public String toString() {
        return "Can[]";
    }

    @Override
    public boolean equals(final @Nullable Object obj) {
        if(INSTANCE == obj) {
            return true; // optimization not strictly necessary
        }
        return (obj instanceof Can)
                ? ((Can)obj).isEmpty()
                : false;
    }

    @Override
    public int hashCode() {
        return 0;
    }

    @Override
    public int compareTo(final @Nullable Can other) {
        if(other==null) {
            return 0;
        }
        // when returning
        // -1 ... this is before other
        // +1 ... this is after other
        return Integer.compare(0, other.size()); // all empty Cans are same and come first
    }

    @Override
    public List toList() {
        return Collections.emptyList(); // serializable and immutable
    }

    @Override
    public Set toSet() {
        return Collections.emptySet(); // serializable and immutable
    }

    @Override
    public Set toSet(final @NonNull Consumer onDuplicated) {
        return Collections.emptySet(); // serializable and immutable
    }

    @Override
    public > C toCollection(final @NonNull Supplier collectionFactory) {
        return collectionFactory.get();
    }

    @Override
    public T[] toArray(final @NonNull Class elementType) {
        val array = _Casts.uncheckedCast(Array.newInstance(elementType, 0));
        return array;
    }



}