HashMapTest.java

package test;

import java.util.HashSet;
import java.util.Set;

public class HashMapTest {

    public static void main(String[] args) {

        Set<TestClass> set = new HashSet<>();

        // Insert 10 unequal objects to ensure treeified bin
        for (int i = 0; i < 10; i++)
            set.add(new TestClass(i, 0));

        // The next two objects inserted into the set are equal, and the second should replace the first,
        // totalling 11 objects in the set.

        // Next object is going on the far left of the tree
        set.add(new TestClass(-1, -1));
        // Final object goes on the far right of the tree
        set.add(new TestClass(-1, 1));

        System.out.println(set.size()); // If everything goes well, this should print 11. But due to JDK bug it prints 12.

    }

    /**
     * Class designed to demonstrate the bug. A class with inconsistent equals and compareTo is rare in practice,
     * and when it does occur, it tends to be messier than this to exploit. Also, unless the hash function is bad
     * (as in this case for ease of demonstration), you have to deliberately find pairs of unequal objects whose
     * hashcode collide.
     */
    private static class TestClass implements Comparable<TestClass> {
        private final int accordingToEquals;
        private final int accordingToComparator;

        public TestClass(int accordingToEquals, int accordingToComparator) {
            this.accordingToEquals = accordingToEquals;
            this.accordingToComparator = accordingToComparator;
        }

        @Override
        public int hashCode() {
            // put everything in the same bin, and force comparison to use the comparator rather than the hash
            // this is just for demonstration, but any hashcode which is equal when equals returns true (as per the contract) should work
            return 0;
        }

        @Override
        public boolean equals(Object other) {
            if (other == this) return true;
            if (!(other instanceof TestClass)) return false;
            TestClass that = (TestClass) other;
            return accordingToEquals == that.accordingToEquals;
        }

        @Override
        public int compareTo(TestClass other) {
            return Integer.compare(accordingToComparator, other.accordingToComparator);
        }
    }

}