How to sort ArrayLists using booleans in java?

Question

I have an ArrayList with custom objects. They contain a checkbox object that I want to sort on. I am using this comparator function to sort it:

I am using the XOR operator to check if they are equal to each other, then negate it.

However this is not working, and the list is staying in the same order.

Does anyone know whats wrong?

public class CustomSelectSort implements Comparator<ObjPerson> {
    @Override
    public int compare(ObjPerson o1, ObjPerson o2) {
        return !(o1.select.isChecked() ^ o2.select.isChecked()) ? 1 : -1;
    }
}

Solution

You return only -1 (less than) or +1 (greater than), never 0 (equals to).

See the java.util.Comparator definition :

Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

In the foregoing description, the notation sgn(expression) designates the mathematical signum function, which is defined to return one of -1, 0, or 1 according to whether the value of expression is negative, zero or positive.

The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y, x)) for all x and y. (This implies that compare(x, y) must throw an exception if and only if compare(y, x) throws an exception.)

The implementor must also ensure that the relation is transitive: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.

Finally, the implementor must ensure that compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z.

It is generally the case, but not strictly required that (compare(x, y)==0) == (x.equals(y)). Generally speaking, any comparator that violates this condition should clearly indicate this fact. The recommended language is "Note: this comparator imposes orderings that are inconsistent with equals."

Proposal before Java 1.7 :

public int compare(ObjPerson o1, ObjPerson o2) {
   boolean b1 = o1.select.isChecked();
   boolean b2 = o2.select.isChecked();
   if( b1 && ! b2 ) {
      return +1;
   }
   if( ! b1 && b2 ) {
      return -1;
   }
   return 0;
}

Proposal since Java 1.7 :

public int compare(ObjPerson o1, ObjPerson o2) {
   boolean b1 = o1.select.isChecked();
   boolean b2 = o2.select.isChecked();
   return Boolean.compare( b1, b2 );
}

Comments

[Aubin]

Post edited because Java 1.7 adds a static boolean comparator to class Boolean

[AkashG]

have changed boolean b1 & b2 to Boolean b1 & b2, since boolean does not invokes compare function its been provided by Boolean.

[Aubin]

@AkashG: Explicit call is used with Boolean.compare, no inference used here. See docs.oracle.com/javase/8/docs/api/java/lang/…