Comparable Interface : Problem while adding object to TreeSet

Posted by: Naveen Kapoor on: November 21, 2008

• In: Java Concepts : questions
• Comment!

public class X{
  public static void main(String [] arg){
     Set s=new TreeSet();
     s.add(new Person(20));
     s.add(new Person(10));
     System.out.println(s);
 }
}

class Person{
    int i;
    Person(int i){
         i = this.i; 
    }
}

Can anyone tell me why this code fragment shows me the “ClassCastException”, Using Java 2 1.5 compiler.

In the above code

Set s=new TreeSet();

Set S which is a TreeSet accepts an object that is very true when developer adds an object new Person(20) to code, code will not fail. Now when developer will add another object new Person(10) the code will fail.

Now replace the code with a fresh code

Set s=new TreeSet();
s.add(new Integer(10)); // replace person object with Integer object
s.add(new Integer(20)); // replace person object with Integer object
System.out.println(s);

The above code will not fail.

Now lets dig in to the concept.

TreeSet is a class belongs to Set interface, being a set it does not accept duplicates. First object insertion goes fine here,Now when we are trying to add the second object then there must be a criteria based on which the 2 object should be comparable. And in the above code there is no such criteria, for uniqueness the set internally use compareable interface to check object equality which is not implemented by the Person class,So in this case developer have to implement the comparable interface in his Person Class.

There are fourteen classes in the Java 2 SDK, version 1.2, that implements the Comparable interface.

BigDecimal, BigInteger, Byte, Double, Float, Integer, Long, Short, Character, CollationKey, Date, File, ObjectStreamField, String

So when developer will try t add the above objects they won’t give any problem, as developer is facing in above scenario.

Below code for person class will solve the developer’s problem.

public class Person implements Comparable{
    int i;
    Person(int i){
        i = this.i; 
    }

 public int compareTo(Object o1) {
        if (this.i == ((Person) o1).i)
               return 0;
        else if ((this.i) > ((Person) o1).i)
            return 1;
        else
            return -1;

    }
}