TreeSet

[Shravan Durvasula]

Why is it that the following code does not run?

  
public class TreeSetTest
{
public TreeSetTest()
{
Set s = new TreeSet();
 
for (int i=0; i<5; i++)
s.add(new Student(i));
} // END: TreeSetTest

public static void main(String[] args) 
{
new TreeSetTest();
}
 
class Student
{
int id;
Student(int id)
{
this.id = id;
} // END: Student
} // END: Student
} // END: TreeSetTest

Thanks in advance,
-skd

[Fletcher Estes]

Hi Shravan,

A TreeSet must keep all its elements in order, but unless you specify how to order the elements, it's going to run into problems! What this means is that either

Any object you add to the TreeSet implements the Comparable interface. To implement this interface, your objects must contain a compareTo(Object o) method.

Or, when you initialise your TreeSet, pass a Comparator object. This Comparator has a compare(Object o1, Object o2) method that you should override.

Read more about the TreeSet on Sun's website.

[Shravan Durvasula]

Thanks Fletcher. That helped.