BrokenHashDemo.java

import javax.annotation.Nonnull;
import java.util.HashSet;

class BrokenHashDemo
{
	public static void main(String[] args)
	{
		HashSet<Dumb> veryDumb = new HashSet<>();
		for (int i = 0;i<1000000;++i) {
			veryDumb.add(new Dumb(i, i));
		}
		/*
		Output should be true, but is false. Java version:
		java version "1.8.0_201"
		Java(TM) SE Runtime Environment (build 1.8.0_201-b09)
		Java HotSpot(TM) 64-Bit Server VM (build 25.201-b09, mixed mode)
		*/
		System.out.println(veryDumb.contains(new Dumb(0, Integer.MAX_VALUE)));
	}

	static class Dumb implements Comparable<Dumb> {
		int valEqual, valComp;
		public Dumb(int valEqual, int valComp) {
			this.valEqual = valEqual;
			this.valComp = valComp;
		}

		@Override
		public boolean equals(Object other) {
			if (!(other instanceof Dumb)) return false;
			return valEqual==((Dumb)other).valEqual;
		}

		@Override
		public int hashCode()
		{
			return 0;
		}

		@Override
		public int compareTo(@Nonnull Dumb o)
		{
			return Integer.compare(valComp, o.valComp);
		}
	}
}