test/jdk/java/util/Locale/HashCodeTest.java

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/*
 * @test
 * @bug 4944561
 * @summary Test hashCode() to have less than 10% of hash code conflicts.
 * @modules jdk.localedata
 * @run junit HashCodeTest
 */

import java.util.HashMap;
import java.util.Locale;
import java.util.Map;

import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertFalse;

public class HashCodeTest {

    // Ensure Locale.hashCode() has less than 10% conflicts
    @Test
    public void hashConflictsTest() {
        Locale[] locales = Locale.getAvailableLocales();
        int min = Integer.MAX_VALUE;
        int max = Integer.MIN_VALUE;
        Map<Integer, Locale> map = new HashMap<>(locales.length);
        int conflicts = 0;

        for (Locale loc : locales) {
            int hc = loc.hashCode();
            min = Math.min(hc, min);
            max = Math.max(hc, max);
            Integer key = hc;
            if (map.containsKey(key)) {
                conflicts++;
                System.out.println("conflict: " + map.get(key) + ", " + loc);
            } else {
                map.put(key, loc);
            }
        }
        System.out.println(locales.length + " locales: conflicts=" + conflicts
                + ", min=" + min + ", max=" + max + ", diff=" + (max - min));
        assertFalse(conflicts >= (locales.length / 10),
                String.format("%s conflicts per %s locales", conflicts, locales.length));
    }
}