Question

Possible Duplicate:
How do I compare strings in Java?
Demonstrating string comparison with Java

I am getting confused by the very basics here. Since I’ve started using java I was always told “do not use == for string comparison; always use equals”. Now I’ve started learning for the OCJP and came across some confusing points.

1)

String s1 = "Monday"; 
String s2 = "Monday";

s1 == s2 returns true;

2) Let’s say a have a class defined as follows:

class Friend {
    String name;

     Friend(String n){
        this.name = n;
    }

     public boolean equals(Object o){
         return ((Friend) o).name == this.name;
     }
}

and I create instances of two objects:

Friend f1 = new Friend("1");
Friend f2 = new Friend("1");

f1.equals(f2) will return true

3) Finally, I have MyClass with main method defined:

public static void main(String[] args) {
        System.out.println(args[0] == "x");
    }

After the invocation java MyClass x false will be printed.

What is the difference between 1, 2 and 3? Why isn’t false returned in case 1 and 2, when two strings obviously refer to different objects?

Answer 1

The short is answer is: String pool. You can find a brief explanation here:

What is String pool in Java?

and a longer here:

http://www.dreamincode.net/forums/topic/187631-the-string-pool/