Question

What does the return value inside the Comparator actually mean?

For example :

class TreeSetDemo
{
    public static void main(String arg[])
    {
        TreeSet t=new TreeSet(new MyComparator());
        t.add(new Integer(20));
        t.add(new Integer(10));
        t.add(new Integer(30));
        t.add(new Integer(100));
        System.out.println(t); 
    }    

    class MyComparator implements Comparator 
    {    
        public int compare(Object o1, Object o2) 
        {
            return 0;
        }
    }
}

If the return type is 1 then its actually returning

[20, 10, 30, 100]

If the return type is -1 then its actually returning

[100, 30, 10, 20]

If the return type is 0 then its actually returning

[20]

Please tell me what does this indicate?

Answer 1

The return value (not type, the type is int) tells the caller (the thing sorting the data):

-1 : o1 < o2
0 : o1 == o2
+1 : o1 > o2

If you always return the same value (o, 1, -1) for the comparator, regardless of it’s inputs, then you’re not using it correctly. You need to base the value returned on the values passed in. The idea is that the data structure (or sorter) calls the comparison function any time it needs to order two elements, to find out what order to put them in.

Its worth noting that the positive/negative integer values (-1, +1) don’t need to be 1, they can be any positive/negative numbers. It’s just common practice to return -1/+1.