Question 10 Chapter 7 : K and B

[jose chiramal]

 

public static void main(String args [])
	{
		
		Object o = new Object();
		
		//Set s = new HashSet();
		TreeSet s = new TreeSet();
		//LinkedHashSet s = new LinkedHashSet();
		
		
		s.add("o");
		s.add(o); // what happens here ?
		
		System.out.println(s.size());
	}
   

Exception in thread "main" java.lang.ClassCastException: java.lang.Object cannot be cast to java.lang.Comparable


I tried compiling the above prog but got this exception :

Exception in thread "main" java.lang.ClassCastException: java.lang.Object cannot be cast to java.lang.Comparable

[Harpreet Singh janda]

You need to add object which implements the comparable interface of. This is the default implementation of the add method.
Below is the excerpt from API

   add

public boolean add(Object o)

    Adds the specified element to this set if it is not already present.

    Specified by:
        add in interface Set
    Overrides:
        add in class AbstractCollection

    Parameters:
        o - element to be added to this set. 
    Returns:
        true if the set did not already contain the specified element. 
    Throws:
        ClassCastException - if the specified object cannot be compared with the elements currently in the set.

 

[Rahul Manikuntla]

Yes, TreeSet is sorted collection and must have elements that implement compareTo() from interface Comparable. That is how TreeSet will know where to add() the new elements.

[Rajeev Rnair]

Rahul Manikuntla wrote:Yes, TreeSet is sorted collection and must have elements that implement compareTo() from interface Comparable. That is how TreeSet will know where to add() the new elements.

Exactly! In

 s.add("o");   
 s.add(o); // what happens here ?   

you are adding a String and an Object o. String, Wrappers, Date classes implement Comparable by default. But cannot add a plain "Object o" which hasn't implemented Comparable.

[Ankit Garg]

It is not necessary that the objects that we add to a TreeSet implement Comparable. If we use a Comparator, then we can add elements to the TreeSet which don't implement Comparable. Eg:-

import java.util.*;
class PersonComparator implements Comparator<Person> {
	public int compare(Person p1, Person p2) {
		return 1;
	}
}
class Person {
	public static void main(String[] args) {
		TreeSet<Person> persons = new TreeSet<Person>(new PersonComparator());
		persons.add(new Person());
		persons.add(new Person());
		System.out.println(persons);
	}
	public String toString() {
		return "I'm a human";
	}
}

The rule is, the elements that we add to the TreeSet must be comparable to each other...