Ambiguity in Vector

Question

Hi,
When I use contains() method of vector for build in type classes then in it gives different result than for the classes that I have built. Please tell me what is the problem. I am attacting my code below.
import java.util.*;
import java.awt.geom.*;
class num{
int a;
int b;
num(int x, int y){
a=x;
b=y;
public class test{
public static void main(String args[]){
Vector v=new Vector();
num n1=new num(5,3);
v.addElement(n1);
num n2=new num(2,2);
v.addElement(n2);
if (v.contains(n2)){
System.out.println("yes it contains n2");
if (v.contains(new num(5,3))){//=n1
System.out.println("yes it contains new num(5,3) ");
Vector r= new Vector();
Point2D.Double p1=new Point2D.Double(3.3,2.3);
Point2D.Double p2=new Point2D.Double(4.1,3.6);
r.addElement(p1);
r.addElement(p2);
if (r.contains(new Point2D.Double(4.1,3.6))){//=p2
System.out.println("yes it contains new Point2D.Double(4.1,3.6) ");
here it show that it contains p2, but on similar lines it should also show that it contains n1. I don't know why?

Answer 1

hi there,
the method contains() from the Vector returns true if and only if the specified object is the same as a component in this vector, as determined by the equals() method.
now you have a user-defined class called num which does not overwrite the equals() method inherited from the Object class. this equals method returns true only when both references are referring to the same object.
if you want to have a consistent behaviour for the contains() method, then overwrite the equals() method in num. this may take the following form:
public boolean equals(Object obj) {
if (obj instanceof num) {
num n = (num) obj;
if ((a == n.a) && (b == n.b))
return true;
return false;
hope this helps,
PSChan
Sun Microsystems