Adding to Treeset

[Mary John]

The following code for adding elements to Treeset Compiles but gives the exception at runtime

Exception is thread main java.lang.classCastException
at TreeMap.compare
at TreeMap.put
at TreeSet.add


Can you please explain why???

Thanks in Advance


import java.util.*;

class Dog{
       String name;
    
 }
class TestCollection{
    
    public static void main(String [] args)
    
    {
    
        
        Dog d = new Dog();
        
        Dog a =new Dog();
        d.name = "clover";
        
        a.name ="arthur";
       Set dset = new TreeSet();
        
        dset.add(d);
        dset.add(a);
        
       // d.name ="magnolia";
       // a = (Dog)dlist.get(0);
        System.out.println(dset);
        
     
        
        
        
        
        
    }
    
    
}

[ahmed yehia]

This is because you are using TreeSet, which requires that its elements should implement the Comparable interface in order to sort them.

[Anonymous]

Hi ,

As Ahmed suggested, it should implement Comparable interface for Treeset to work and also I guess, you need to check your equals method, as Set does not allow duplicate values. You should override equals method, and that way you can determine whether the newly being added object is already existing or not. Please some one let me know, if this is not a requirement.

[ahmed yehia]

Hi Chandra
Implementing Comparable and overriding compareTo() is enough to get elements in the TreeSet so it can sort them. And as equals() is not overriden so the default inherited implementation will be used which will check refrences.

[ahmed yehia]

Here is the code again with identation and added compareTo, toString()

    
import java.util.*;
class Dog implements Comparable<Dog> {
   String name;
   public int compareTo(Dog o) {
   return o.name.compareTo(this.name);
   }
   public String toString() {
       return this.name;
   }
 }
 
class TestCollection {
    public static void main(String [] args) {
  Dog d = new Dog();
          d.name = "clover";
          Dog a =new Dog();
          a.name ="arthur";
 
          Set<Dog> dset = new TreeSet<Dog>(); 
 
          dset.add(d);
          dset.add(a);
          System.out.println(dset);
    }
}

[ October 11, 2007: Message edited by: Ahmed Yehia ]