IntValue(int value) {
      this.value = value;
    }

    @Override
    public String toString() {
      return "value = " + value;
    }
  }

  public void testOnResultOf() {
    EquivalenceTester.of(Equivalence.equals().onResultOf(Functions.toStringFunction()))
        .addEquivalenceGroup(new IntValue(1), new IntValue(1))
        .addEquivalenceGroup(new IntValue(2))
        .test();