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Common reusable components

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package org.openl.util;

import java.util.Comparator;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Set;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.TreeSet;
import java.util.function.Function;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

/**
 * Set of utilities for Java Streams.
 *
 * @author Yury Molchan
 */
public class StreamUtils {

    /**
     * To use with Java Streams to collect in in a map preserving the order of elements.
     */
    public static  Collector> toLinkedMap(Function keyMapper,
                                                                   Function valueMapper) {
        return Collectors.toMap(keyMapper, valueMapper, (u, v) -> {
            throw new IllegalStateException(String.format("Duplicate key %s", u));
        }, LinkedHashMap::new);
    }

    /**
     * To use with Java Streams to collect in tree set with custom order
     */
    public static  Collector> toTreeSet(Comparator comparator) {
        return Collectors.toCollection(() -> new TreeSet<>(comparator));
    }

    /**
     * Transform iterator to stream.
     *
     * @param iterator iterator to transform
     * @param       type of elements
     * @return stream
     */
    public static  Stream fromIterator(Iterator iterator) {
        return StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED), false);
    }
}