equal() and ==

[Andrew Parker]

Hi,
Here is my code:

public class Test1
{
int x;
Test1(int y)
{
x = y;
        }
public static void main(String [] args)
{
Test1 t1 = new Test1(100);
Test1 t2 = new Test1(100);
if(t1.equals(t2)) // no matter it is equals() or ==, the result is the same. WHY?
{
System.out.println("=");
}
else
{
System.out.println("!=");
}
}
}

So far as I've learnt, == is used to check the memory address difference while equals() is used to check the object content. So, I don't understand why the result would print != as both object contents are 100.
Also, I would like to know is the wrapper : Interger x = new Interger(100) immutable?
Thanks for help.
Andrew
[ January 08, 2002: Message edited by: Cindy Glass ]

[David O'Meara]

Yes, Integer objects are immutable.
Since you haven't overridden equals() from java.lang.Object, you get the default implementation, which means you get this behaviour (quoted from the API):

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any reference values x and y, this method returns true if and only if x and y refer to the same object (x==y has the value true).

This is why == and .equals() are (currently) the same. You need to add an equals method specific to this object...

public boolean equals(Object o)
{
    if(!o instanceOf Test)
        return false;
    return ((Test)o.x == this.x);
}

(or similar, this is just an example )
Dave

[David O'Meara]

Oh, I almost forgot. Usually I make a reference to Effective Java by Joshua Bloch (link to the Javaranch Bunkhouse).
I find it an invaluable book to keep nearby but it also has great coverage of the whole .equals() thing. (this is what I was refering to when I said "or similar, this is just an example").
To see how great it is, follow the link and read Peter Tran's review (then buy the book! )
Dave.
[ January 03, 2002: Message edited by: David O'Meara ]

[Andrew Parker]

I think I will buy that 5-stars book. Thanks for your advice. Also, I want to learn the J2EE. Could you recommend one for beginner?
I have this one: http://www.amazon.com/exec/obidos/ASIN/0789725037/qid=1010120288/sr=1-6/ref=sr_1_11_6/002-1646055-8018453.
But, the chapter regarding RMI is hard to understand.
Thanks a lot.
Andrew

[Cindy Glass]

JavaRanch has a section in the BunkHouse just for J2EE and such stuff. Go read the reviews. If you use the link from there to Amazon JavaRanch gets a few pennies to pay for this "upgrade" .

[Andrew Parker]

Thanks for your kind reference and I will buy the core J2EE - the 1st one.
Andrew

[James Childers]

The following overrides equals() like you need:

 public class Test {
public int x;

Test(int y) {
this.x = y;
}
public boolean equals(Test o) {
if (!(o instanceof Test))
return false;
else
return (o.x == this.x);
}

public static void main (String args[]) {
Test t1 = new Test(100);
Test t2 = new Test(101);

System.out.println("t1.x = " + t1.x);
System.out.println("t2.x = " + t2.x);
if (t2.equals(t1))
System.out.println("They're equal.");
else
System.out.println("Not equal.");
}
}