why this ClassCastExceptioni????

[chathura87]

In following code why it gives class cast exception??? If i comment insesrting second person object then it is ok... What is the reason ..!

Thank you..


import java.util.*;
public class Testy{
public static void main(String a[]){
Set s = new TreeSet();
s.add(new Person(20));
s.add(new Person(10)); //If i comment this then OK

System.out.println(s);
}
}
class Person{

int x;
Person(int i){



}
}

[masijade]

What does the compareTo method of your Person class look like?

[chathura87]

There is not compareTo method for this...
BUT i can still add element 1... but not 2..

[DarrylBurke]

First of all, post code in tags to make it more readable.

Read the API for Set
some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException or ClassCastException.

and for TreeSet
a TreeSet instance performs all element comparisons using its compareTo (or compare) method

You need to have your Person class implement Comparable.

import java.util.Set;
import java.util.TreeSet;

public class Testy {

  public static void main(String a[]) {
    Set s = new TreeSet();
    s.add(new Person(20));
    s.add(new Person(10));

    System.out.println(s);
  }
}

class Person implements Comparable {

  int x;

  Person(int i) {
    x = i;
  }

  public int compareTo(Object o) {
    return x - ((Person) o).x;
  }
}

Better than that, use Generics, that's what it's meant for.

import java.util.Set;
import java.util.TreeSet;

public class Testy {

  public static void main(String a[]) {
    Set s = new TreeSet<Person>();
    s.add(new Person(20));
    s.add(new Person(10));

    System.out.println(s);
  }
}

class Person implements Comparable<Person> {

  int x;

  Person(int i) {
    x = i;
  }

  public int compareTo(Person o) {
    return x - o.x;
  }
}

db

[masijade]

There is not compareTo method for this...
BUT i can still add element 1... but not 2..

That's what I thought. Read DBs comments above which are essentially saying, RTFM, and I will add to that, that you should know at least something about the structures you are using, at least enough to know how to use them.

[garyiskidding]

All sorted structures (like your TreeSet) would require a default sort behaviour. That's why you need the compareTo() method implemented.

[subith86]

TreeSet's job is to sort data. If you were adding some Integer or String type it'd have been fine and there is already a mechanism to sort. But this object is your own object and TreeSet doesn't know how to sort it. So you have to implement Comparable

[JosAH]

A TreeSet needs to compare objects a and b where a < b or a > b or a == b. It either does that by using a Comparator or the objects to be stored in the set have to implement the Comparable<T> interface where T is the type of the objects to be stored. Your Person class doesn't implement that interface nor did you supply a Comparator implementation.

kind regards,

Jos