Comparator and TreeSet

[Henry Zhi Lin]

Hi, guys

The following code is Devaka's Exam Lab practice1 question 55.

  13.
14. class A implements Comparator{
15.      public int compare(Object ob1, Object ob2){
16.           return 1;
17.      }
18.      public String toString(){
19.           return "W";
20.      }
21. }
22. 
23. public class TMap{
24.      public static void main(String argv[]){
25.           Set<A> cs=new TreeSet<A>();
26.           cs.add(new A());
27.           cs.add(new A());
28.           cs.add(new A());
29.           for(A a:cs)
30.                System.out.print(a+",");
31.      }
32. }
33.  

WHy this have an runtime exception, I thought the treeset has a comparator to compare the value that saved into the set?

Can anyone explain to me how does this work?

[Ankit Garg]

For inserting a class' element to a TreeSet, it must implement Comparable interface and override the compareTo method. This code is implementing Comparator interface and overriding compare method. Comparator is used when we want to use an external sorting logic which is different from the one in the class whose objects are being added to the TreeSet...