Confusion on equals() method

[dileep keely]

Hi All,
equals() method:It gnerally checks the conetnts are same or not.I am missing some basic scenario.Please comment on the above.
Thanks in advance.
class Test
{
String name;
Test(String name)
{
this.name=name;
}
public static void main(String[]args)
{
Test t=new Test("javaranch");
Test t1=new Test("javaranch");
System.out.println(t.equals(t1)+".-------->Test1");
Integer i=new Integer(10);
Integer i=new Integer(10);
System.out.println(i.equals(i1)+".-------->Integer");
}
}
output:
false.-------->Test1
true.-------->Integer

[Lester Burnham]

I take it you were expecting a different result? Which one, and why?

Class Integer overrides equals, whereas class Test does not - so it inherits an equals method from the Object class. Check the javadocs of both classes for a description of what each method does.

[Jelle Klap]

The Test class doesn't override the equals() implementation of java.lang.Object. The Integer class does. When you invoke equals() on an instance of Test the implementation will check if the reference you invoke the method on and the reference you supplied to the equals() method refer to the same object. They do not, so the method returns false. In case of the Integer instances the values are compared rather than the references, because that's what the overridden implementation does. So, if you want instances of Test to perform a similar comparison, you should override the equals() method and return a result based on a comparison of the "name" member.

Edit: Ugh, too late.

[Avishkar Nikale]

Dileep,

You will have to override the method & check the contents yourself.

The language will not do it for you.

[Campbell Ritchie]

Avishkar Nikale wrote:Dileep,

You will have to override the method & check the contents yourself.

The language will not do it for you.

You should also override hashCode.

[dileep keely]

import java.util.*;
public class ShowHash {
	  int a=0;
public ShowHash(int a) {
this.a=a;
} 
@Override
public boolean equals(Object obj) {
	if(obj==null)return false;
	if(obj instanceof ShowHash)	
		return true;
	return false;
	 
}
@Override
public   int hashCode() {
	// TODO Auto-generated method stub
	return 10;
}
	public static void main(String[] args) {
ShowHash s=new ShowHash(10);
ShowHash s1=new ShowHash(10);
 
System.out.println(s.equals(s1)+"\t"+ s.hashCode()+"\t"+System.identityHashCode(s));
 
System.out.println((s==s1)+"\t"+s1.hashCode()+"\t"+System.identityHashCode(s1));
 	}}

output:
true 10 17523401
false 10 8567361

Jelle klap:
Thanks for your comment!!
I have done according to the suggestion(overriding equals() and hashcode()).I have a query here.
you have commented that
"When we invoke equals() on an instance of Test the implementation will check if the reference you invoke the method on and the reference you supplied to the equals() method refer to the same object".
It implies that it should also satisfy == comparison.If so in the above output I got (false for == and true for equals() comparison
which shouldn't be the case then? because == checks if two references are to the same object)

Anyone can help me out in this?Apologies if my understanding is wrong.

[Campbell Ritchie]

dileep keely wrote: . . . Anyone can help me out in this? . . .

You are more likely to get help on code with the code tags on, so we can actually read it.

And I don't think your equals() method is anything like correctly implemented.

[dileep keely]

I have inserted the code tags can you please suggest

[Matthew Brown]

The main problem seems to be this:

if(obj instanceof ShowHash )return true; 

That means that you consider any two ShowHash objects to be equal, regardless of what data they contain. Is that what you intended?

[Campbell Ritchie]

Matthew Brown wrote: . . . you consider any two ShowHash objects to be equal, regardless of what data they contain. Is that what you intended?

Of course he didn't. That was th error I noticed earlier.

[dileep keely]

equals() method is now correctly implemented.
Now please clarify my doubt on equals and == (as described above as per my understanding equals compare the contents and not the reference ).
Thank you for your time

[Campbell Ritchie]

dileep keely wrote:

import java.util.*;
public class ShowHash {
	  int a=0;
public ShowHash(int a) {
this.a=a;
} 
@Override
public boolean equals(Object obj) {
	if(obj==null)return false;
	if(obj instanceof ShowHash)
		return true;
    return false;
	 
}
@Override
public   int hashCode() {
	// TODO Auto-generated method stub
	return 10;
}
	public static void main(String[] args) {
ShowHash s=new ShowHash(10);
ShowHash s1=new ShowHash(10);
 
System.out.println(s.equals(s1)+"\t"+ s.hashCode()+"\t"+System.identityHashCode(s));
 
System.out.println((s==s1)+"\t"+s1.hashCode()+"\t"+System.identityHashCode(s1));
 	}}

output:
true 10 17523401
false 10 8567361

Jelle klap:
Thanks for your comment!!
I have done according to the suggestion(overriding equals() and hashcode()).I have a query here.
you have commented that
"When we invoke equals() on an instance of Test the implementation will check if the reference you invoke the method on and the reference you supplied to the equals() method refer to the same object".
It implies that it should also satisfy == comparison.If so in the above output I got (false for == and true for equals() comparison
which shouldn't be the case then? because == checks if two references are to the same object)

Anyone can help me out in this?Apologies if my understanding is wrong.

Please don't try altering old posts. It would have made my post and Matthew Brown's look like nonsense. Please post any changes in a new posting.

I have edited your post to return it to its original state.

[Campbell Ritchie]

dileep keely wrote:equals() method is now correctly implemented. . . .

Since I have removed your emendation, please post the correct version again.

[Campbell Ritchie]

And you appear to be correct that an equals() method tests whether two objects are identical, whereas == on objects tests whether they are both the same object.

Now find out why using the instanceof operator is unreliable.

[dileep keely]

Because,
getClass() only gives equality if the two classes are identical-->when we are sure that we are comparing two objects.
whereas instanceof works for subtype(inheritance)-->when comparing two objects of different classes.

[Vinoth Kumar Kannan]

Dileep, Can we see the new implementation of the equals() method?

[dileep keely]

Campbell,I will take care going forward.
Here is my new implementation of equals() method.

public boolean equals(Object obj) {
ShowHash s=	(ShowHash)obj;
	if(obj==null)return false;
	if(obj instanceof ShowHash && (s.a==a) )
		return true;
	return false;
}

[Campbell Ritchie]

That will of course throw a ClassCastException if you pass an Object of any other type. Remember the equals() method must accept arguments of any reference type, or null.

[Vinoth Kumar Kannan]

Yeah....may be you want to do this..

if(obj instanceof ShowHash && ((ShowHash)obj).a==a )

[Campbell Ritchie]

Vinoth Kumar Kannan wrote:Yeah....may be you want to do this..

if(obj instanceof ShowHash && ((ShowHash)obj).a==a )

Actually you want to change the keyword if to the keywordreturn and you have the entire method in one line.

Now work out why. Also work out why the instanceof operator is not reliable in this instance.