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A set of some common useful libraries.
There is a newer version: 3.9.1
package com.github.fmjsjx.libcommon.collection;

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.ToIntFunction;
import java.util.stream.Collector;
import java.util.stream.Collectors;

import io.netty.util.collection.IntObjectHashMap;
import io.netty.util.collection.IntObjectMap;

/**
 * Utility class for {@link Collector}s.
 */
public class CollectorUtil {

    /**
     * Returns a {@code Collector} that accumulates elements into a
     * {@link LinkedHashMap} whose keys and values are the result of applying the
     * provided mapping functions to the input elements.
     * 
     * @param        the type of the values
     * @param        the output type of the key mapping function
     * @param keyMapper a mapping function to produce keys
     * @return a {@code Collector} that accumulates elements into a
     *         {@link LinkedHashMap} whose keys and values are the result of
     *         applying the provided mapping functions to the input elements
     */
    public static final  Collector> toLinkedHashMap(Function keyMapper) {
        return Collectors.toMap(keyMapper, Function.identity(), throwingMerger(), LinkedHashMap::new);
    }

    /**
     * Returns a {@code Collector} that accumulates elements into a
     * {@link LinkedHashMap} whose keys and values are the result of applying the
     * provided mapping functions to the input elements.
     * 
     * @param          the type of the input elements
     * @param          the output type of the key mapping function
     * @param          the output type of the value mapping function
     * @param keyMapper   a mapping function to produce keys
     * @param valueMapper a mapping function to produce values
     * @return a {@code Collector} that accumulates elements into a
     *         {@link LinkedHashMap} whose keys and values are the result of
     *         applying the provided mapping functions to the input elements
     */
    public static final  Collector> toLinkedHashMap(Function keyMapper,
            Function valueMapper) {
        return Collectors.toMap(keyMapper, valueMapper, throwingMerger(), LinkedHashMap::new);
    }

    /**
     * Returns a {@code Collector} that accumulates elements into a
     * {@link IntObjectMap} whose keys and values are the result of applying the
     * provided mapping functions to the input elements.
     * 
     * @param        the type of the input elements
     * @param keyMapper a mapping function to produce keys
     * @return a {@code Collector} that accumulates elements into a
     *         {@link IntObjectMap} whose keys and values are the result of applying
     *         the provided mapping functions to the input elements.
     */
    public static final  Collector> toIntObjectMap(ToIntFunction keyMapper) {
        return Collectors.toMap(keyMapper::applyAsInt, Function.identity(), throwingMerger(), IntObjectHashMap::new);
    }

    /**
     * Returns a {@code Collector} that accumulates elements into a
     * {@link IntObjectMap} whose keys and values are the result of applying the
     * provided mapping functions to the input elements.
     * 
     * @param          the type of the input elements
     * @param          the output type of the value mapping function
     * @param keyMapper   a mapping function to produce keys
     * @param valueMapper a mapping function to produce values
     * @return a {@code Collector} that accumulates elements into a
     *         {@link IntObjectMap} whose keys and values are the result of applying
     *         the provided mapping functions to the input elements.
     */
    public static final  Collector> toIntObjectMap(ToIntFunction keyMapper,
            Function valueMapper) {
        return Collectors.toMap(keyMapper::applyAsInt, valueMapper, throwingMerger(), IntObjectHashMap::new);
    }

    /**
     * Returns the merger that always throwing {@code IllegalStateException}.
     * 
     * @param  the type of the elements
     * @return the merger that always throwing {@code IllegalStateException}
     */
    public static final  BinaryOperator throwingMerger() {
        return (u, v) -> {
            throw new IllegalStateException(String.format("Duplicate key %s", u));
        };
    }

    private CollectorUtil() {
    }

}