publicclass Tree
implements Comparator<Tree>
{
publicstaticvoid main( String args[] )
{
TreeSet<Tree> l =new TreeSet<Tree>();
for(int i = 0; i < 5; i++ )
{
l.add(new Tree());
}
}
publicint compare( Tree t, Tree t1 )
{
return 3;
}
}
When I run the above program I am ClassCastException. Why?
First of all, the program above does not compile, because you're missing import statements.
But to answer the question: Did you look at the error message? It says:
Exception in thread "main" java.lang.ClassCastException: Tree cannot be cast to java.lang.Comparable
at java.util.TreeMap.put(TreeMap.java:542)
at java.util.TreeSet.add(TreeSet.java:238)
at Tree.main(Tree.java:13)
Your class Tree must either (1) implement Comparable (not Comparator), or (2) you must pass an instance of a Comparator<Tree> to the constructor of TreeSet.
In case (1), the TreeSet will compare the objects by calling compareTo(...) on the objects; in case (2), the TreeSet will use the Comparator object that you passed to the constructor to compare the objects.
You can't mix those two approaches, which is what you're doing in your code above.
ok I got it.
But just 1 query,
say I use TreeSet without implementing Comparable as well as by not using any Comparator.
So while adding elements in the TreeSet it calls the compareTo method on the object that is added and results in ClassCastException.
But my question is where is the compareTo method now since the class has not implemented Comparable.
