public boolean equals(@Nullable Object obj) {
      if (obj instanceof SameBooleanInstance) {
        SameBooleanInstance that = (SameBooleanInstance) obj;
        return i == that.i;
      }
      return false;
    }
  }

  static class SameStringInstance {
    private final String s;

    public SameStringInstance(String s) {
      this.s = checkNotNull(s);
    }