Inquisition Question

[Neha Daga]

   
import java.util.*;
class test implements Comparator < test >
{
   int testNumber;
   public static void main( String args[] )
   {
      Set < test > s1 = new TreeSet < test > ();
      
      s1.add(new test());
      s1.add(new test());

   }
   public int compare( test t1 , test t2 )
   {
      return t1.testNumber - t2.testNumber;
   }

} 

Choose all that apply:
A) Compiles with warning
B) A runtime exception occurs
C) Compiles without warning
D) Only one test object is added, Other one is considered duplicate
E) TreeSet can only accept objects of type test.

Answer is : B,C & E

my question is why will there be a runtime exception.

[Ankit Garg]

Hint: The class test implements Comparator and not Comparable...

[Neha Daga]

so that means, Comparator interface can only be implemented by a class separate from the class whose instances are to be sort.

am I right?

[G Venu]

TreeSet collection object created with default constructor takes elements that implement Comparable interface only.
When the code tries to insert elements that violate this constraint, ClassCastException will be thrown.

To me the code should not even compile.

http://java.sun.com/javase/7/docs/api/java/util/TreeSet.html#TreeSet()

[Neha Daga]

G Venu wrote:TreeSet collection takes elements that implement Comparable interface only. When the code tries to insert elements that violate this constraint, ClassCastException will be thrown.

Got this point.

To me the code should not even compile.

the code will compile because objects added are mutually comparable i.e they both are Test objects.
It would throw ClassCastException if you add a Test and say a Integer.

[G Venu]

Agreed. exception should be seen at runtime only

replacing

 Set<test>  s1 = new TreeSet <test> ();    

with

 Set<test>  s1 = new TreeSet <test> (new test());    

should fix the issue. (the code does not make much sense doing this though)

[Phungsuk Wangdu]

thanks venu and neha you just cleared a lot of my doubts