Question

I want to sort objects based on Boolean values and I want to sort true values before false values.

Which of these implementations of compareTo is more readable?

Using -1 to change default behavior

public class Example implements Comparable<Example>{

  Boolean isOk;

  public int compareTo(Example o) {
      return -1 * this.isOk.compareTo(o.isOk);
  }

}

or swap sides of Boolean#compareTo method?

public class ExampleTwo implements Comparable<ExampleTwo>{

  Boolean isOk;

  public int compareTo(ExampleTwo o) {
      return o.isOk.compareTo(this.isOk);
  }

}

Answer 1

The first form is simply wrong – because if compareTo returns Integer.MIN_VALUE, it will try to negate that – and result in Integer.MIN_VALUE again.

The easiest way to fix that is just to use the code from the second snippet.

On the other hand:

• Both could fail if isOk is null
• If you’re really only using Booleans, a simple truth table may be simpler
• It’s possible that Boolean.compareTo will never return Integer.MIN_VALUE. I wouldn’t rely on that though.