References To Strings And ==

[Tom Scott]

Hi All,

I was surprised to get the output:
false
false
false

I expected to get...
true
false
false
... my reasoning being that s1 and s2 refer to the same string object.

Code below:

 
public class Main {
        String s1 = "I am a string";
        String s2 = s1; 
        String s3 = "I am a String";
    /** Creates a new instance of Main */
    public Main() {
    }
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
 
        Main m = new Main();
        System.out.println("s1 == s2 : " + m.s1==m.s2);
        System.out.println("s1 == s3 : " + m.s1==m.s3);
        System.out.println("s2 == s3 : " + m.s2==m.s3);
    }  
}

Can someone explain this please?
Thanks.
Tom

[Petrus Pelser]

Shouldn't it print out:

s1 == s2 : false
s1 == s3 : false
s2 == s3 : false

instead of:
false
false
false

???

add parenthesis around the statements as follows:

System.out.println("s1 == s2 : " + (m.s1==m.s2));
System.out.println("s1 == s3 : " + (m.s1==m.s3));
System.out.println("s2 == s3 : " + (m.s2==m.s3));

Then everything works as expected.

This is because the + operator's precedence is higher than the == operator's .
[ September 27, 2006: Message edited by: Petrus Pelser ]

[Tom Scott]

Ah ha - it does print out as I said however I think the precedence is at the heart of this.

Perhaps it evaluates as below...(check new parenthesis I added.)

System.out.println(("s1 == s2 : " + m.s1)==m.s2);

This would explain the output maybe...

Or am I just digging myself in deeper here

Cheers.
Tom.

[Tom Scott]

btw- thanks for the reply - the extra parentheis achieves the expected result.

[Joe Harry]

Hai Tom,

As per your code, the operator precedence plays a role. I accept. I'm not understanding to interpret the following from your code snippet,

System.out.println("s1 = s2 : " + m1.s1==m1.s2);

What happens above??How the + operator makes the output to be false? Please explain.

Regards,
Jothi Shankar Kumar. S

[wise owen]

System.out.println("s1 == s2 : " + m.s1==m.s2);

is the same as the following code:

String s4="s1 == s2 : " + m.s1;
System.out.println(s4==m.s2);

[Joe Harry]

Hi Wise Owen,

Thank you very much for the explanation and now I understood.

Thanks again,
Jothi Shankar Kumar. S

[Rajesh Kadle]

Should it not print

true
true
true

as all the three will be referring to the string constant pool? Can someone explain this.

Thanks in advance,
-Raj

[Tom Scott]

There is a difference in capitilisation of the letter s.

If I change them both to "I am a string" with a small s then all the references are '==' to each other.

Cheers,
Tom