args[] and ==

[Michel ten Voorde]

This following code is based on Practice Exam 2, question 6 from K&B's new book OCP Java SE 6 Programmer Practice Exams:

public class Test
{
   public static void main(String[] args) {
	   System.out.println("args[0] is " + args[0]);
	   System.out.println("However, args[0]==\"x\" is " + (args[0]=="x"));
	   args[0]="x";
	   System.out.println("We do: args[0]=\"x\"...");
	   System.out.println("...and strangely, now args[0]==\"x\" is " + (args[0]=="x"));
   }
}

Giving:

args[0] is x
However, args[0]=="x" is false
We do: args[0]="x"...
...and strangely, now args[0]=="x" is true

Can anyone explain?

[Sandra Bachan]

Line # 6 will make args[0] point to the exact same address as x, hense args[0] == x becomes true

Can someone elaborate....

[Michel ten Voorde]

But then, where did args[0] point to before? I would think that it already pointed to an 'x', since that's what the println says.

Btw, I forgot to mention that the program is invoked with 'java Test x', but perhaps that was obvious.

[Piotr Nowicki]

AFAIK, the String Literal Pool is created when the class is loaded. At this point the JVM doesn't know what String will you type when you will execute the class. Every value you type is treated as a new object, therefore it acts just like you would type new String("test").

Edit: Oh, and try changing

System.out.println("However, args[0]==\"x\" is " + (args[0]=="x"));  

to

System.out.println("However, args[0]==\"x\" is " + (args[0].intern()=="x"));  

Just for fun :-)

[Michel ten Voorde]

Thanks for the explanation. Kinda tough concepts for OCPJP, I'd say... but then again, the questions in the book are supposed to be a little harder than the real exam, if I've read correctly.

[Piotr Nowicki]

Michel, yea, the questions in Kathy and Bert's book are a bit tougher than the real exam ones.

By the way, try running this as java Test xx (or better - try to guess what will be the output before running). I think this example will give you the better feeling of what is happening in this weird String Literal Pool thing ;-)

 public class Test
 {
    public static void main(String[] args) {

        String x1 = "x";
        String x2 = "x";
        final String x3 = "x";
        final String x4 = "x";
        String x5 = new String("xx");

        System.out.println("t1=" + (args[0].intern()=="xx"));
        System.out.println("t2=" + (x1+x2=="xx"));                                       
        System.out.println("t3=" + (x3+x4=="xx"));
        System.out.println("t4=" + (x5=="xx"));   
    }
 }

Cheers!