test/jdk/java/util/Collections/Shuffle.java

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/*
 * @test
 * @bug     8294693
 * @summary Basic test for Collections.shuffle
 * @key     randomness
 */

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Random;
import java.util.function.Consumer;
import java.util.random.RandomGenerator;

public class Shuffle {
    static final int N = 100;

    public static void main(String[] args) {
        test(new ArrayList<>());
        test(new LinkedList<>());
    }

    static void test(List<Integer> list) {
        for (int i = 0; i < N; i++) {
            list.add(i);
        }
        Collections.shuffle(list);
        if (list.size() != N) {
            throw new RuntimeException(list.getClass() + ": size " + list.size() + " != " + N);
        }
        for (int i = 0; i < N; i++) {
            if (!list.contains(i)) {
                throw new RuntimeException(list.getClass() + ": does not contain " + i);
            }
        }
        checkRandom(list, l -> Collections.shuffle(l, new Random(1)));
        RandomGenerator.JumpableGenerator generator = RandomGenerator.JumpableGenerator.of("Xoshiro256PlusPlus");
        checkRandom(list, l -> Collections.shuffle(l, generator.copy()));
    }

    private static void checkRandom(List<Integer> list, Consumer<List<?>> randomizer) {
        list.sort(null);
        randomizer.accept(list);
        ArrayList<Integer> copy = new ArrayList<>(list);
        list.sort(null);
        if (list.equals(copy)) {
            // Assume that at least one pair of elements must be reordered during shuffle
            throw new RuntimeException(list.getClass() + ": list is not shuffled");
        }
        randomizer.accept(list);
        if (!list.equals(copy)) {
            throw new RuntimeException(list.getClass() + ": " + list + " != " + copy);
        }
    }
}