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why ClasscastException????  RSS feed

 
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why ClasscastException??? do explain me clearly

i didnot get it

import java.util.*;
public class test1{
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{
Person(int i){}
}
 
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TreeSet implements SortedSet. When elements are added to the TreeSet collection it has to compare the to-be-added object against the existing collection elements. So either the to-be-added object has to implement comparable or a comparator has to be provided during TreeSet creation. If u try to add only one element u wont get this exception.

Try this and hope this is of some use.

import java.util.*;
public class SetTest{
public static void main(String a[]){
Set s = new TreeSet();
s.add(new Person(20));
s.add(new Person(10));
s.add(new Person(20));
System.out.println(s);
}
}
class Person implements Comparable
{
int i=0;
Person(int i){this.i=i;}
public int compareTo(Object o)
{
if ((o instanceof Person) && (((Person)o).i==i))
return 0;
else
return 1; // not equal
}
}
 
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you wont see such questions in the 1.4 exam
 
It is sorta covered in the JavaRanch Style Guide.
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