TreeSet Ques

[Nikhil Bansal]

Hi All,

Why does the foll. ques throws ClassCastException

import java.util.*;


class CollectionTest {

public static void main(String args) throws NullPointerException
{

}
public static void main(String args[])
{


try
{
TreeSet s = new TreeSet();
s.add(new Person(10));
s.add(new Person(20));
System.out.println(s);
}
catch(Exception e)
{
System.out.println(e.toString());
}
}

}

class Person
{
Person(int i)
{

}
}

Regards

Nikhil

[Keith Lynn]

The problem is that the TreeSet when created the way you have will only accept objects whose class definitions implement the Comparable interface. You need to implement that interface in Person. It would be a good idea to also override equals() and hashCode() methods in Person also.

[Edisandro Bessa]

Hi Nikhil,

What Keith said is correct !

TreeSet is a kind of Set where its elements are sorted by their natural order.

In order to perform the sort, TreeSet uses the Comparator interface which must be implemented by your Person class.

When using Strings for example, there's no problem because String object implements all needed methods properly.

The code below is the changed version for your Person class. Please check it out :

import java.util.*;
 
class CollectionTest {
 
    public static void main(String args[]){
        Set s = new TreeSet([B]new PersonSort()[/B]);
 
        s.add(new Person(20));
        s.add(new Person(10)); 
        s.add(new Person(15));
        s.add(new Person(40));
        s.add(new Person(30));
        System.out.println(s);
    }
}
 
class Person{
    private Integer age;
           
    Person(Integer age){
        this.age = age;
    }
    
    public Integer getAge(){
        return age;
    }
    
    public String toString(){
        return "This person is " + age + " years old";
    }
}
 
[B]class PersonSort implements Comparator<Person> {
    public int compare(Person one, Person two) {
        return one.getAge().compareTo(two.getAge());        
    }
}[/B]

When checking the output you can see that all elements are sorted by age.

Hope that helps.