hashCode()-> reference,Object

[dileep keely]

I am not able to figure out when I put an object as a key into a map and retrieve it I will get value as null,but
when I put a reference pointing to an Object as a key into a map and retrieve I get value.
Intentionally I didn't add equals()..

import java.util.*;
class Test
{
	public int hashCode() {
	System.out.println("hashCode():::"+super.hashCode());
	return super.hashCode();
	}}
public class HashTest {
	public static void main(String[] args) {
	java.util.Map map=new java.util.HashMap();
		
		[color=orange]map.put(new Test(),"java");[/color]
		System.out.println(map.get(new Test()));
		
		Test testobj=new Test();
		[color=orange]map.put(testobj,"java");[/color]
		System.out.println(map.get(testobj));
	}}

output:
hashCode():::8567361
hashCode():::9584176
null

hashCode():::19972507
hashCode():::19972507
java
Any suggestion on this?

Edit1: to paste output
Edit2:In my comment I changed null to value and viceversa..

[Henry Wong]

dileep keely wrote:I am not able to figure out when I put an object as a key into a map and retrieve it I will get corresponding value,but
when I put a reference pointing to an Object as a key into a map and retrieve I get null as value.
Intentionally I didn't add equals()..

There is no such distinction as "put an object as key" and "put a reference pointing to an Object as a key". In both cases, you are passing a reference to an object to the map, but in one case, you are using a declared reference, and in the other, the compiler is creating an anonymous reference for you.... ie. there is little difference between...

java.util.Map map=new java.util.HashMap();
			
map.put(new Test(),"java");
System.out.println(map.get(new Test()));

and...

java.util.Map map=new java.util.HashMap();
		
Test temp = new Test();
map.put(temp,"java");
Test temp2 = new Test();
System.out.println(map.get(temp2));

Henry

[dileep keely]

Thanks Henry!!
But, why do I get null in the first case.I was expecting the value("java").
Sorry for my understanding..

[Henry Wong]

dileep keely wrote:Thanks Henry!!
But, why do I get null in the first case.I was expecting the value("java").
Sorry for my understanding..

It's returning null because you are using the equal() / hashCode() methods of the Object class -- which by default considers objects equal only when they are the same object.

Henry

[dileep keely]

yep henry cleared..
Correct.I must have used the

if(new Test()==testobj)

to check.

[dileep keely]

A last doubt..

 map.put(new Test(),"java");       map.get(new Test()) //null
Test testobj=new Test(); map.put(testobj,"java");    map.get(testobj) //java

In both the cases above, I believe the hashCode() of Object class is called,I am right isn't it?. If that is the case I should return null ..

[Henry Wong]

dileep keely wrote:
I must have used the

if(new Test()==testobj)

to check.

which would not serve any purpose, as there is no way any newly instantiate object would be the same exact object as a previously created one.

Henry

[Henry Wong]

dileep keely wrote:
In both the cases above, I believe the hashCode() of Object class is called,I am right isn't it?. If that is the case I should return null ..

In the first case, the hashCode() method of the two Test objects will be called. And in the second case, the one created Test object's hashCode() method will be called twice.

Henry