== vs .equals - why different behaviour?

Question

Possible Duplicate:
How do I compare strings in Java?

I'm sorry for this rather simple question. I have this very simple java program:

public class ArgIt {
    public static void main(String[] args){
            if(args[0].equals("x")) System.out.print("x");
            if(args[0] == "x") System.out.println("x2 ");
    }
}

If I call the program >java ArgIt x it only prints a single x. Why will the program not acknowledge the == on the string when in any other circumstances it does?

Check about string interning - http://en.wikipedia.org/wiki/String_intern_pool. The second conditional fails because the two string objects are not the same, therefor the references are not the same and == returns false. — , May 10 '12 at 14:14
http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.21 — hmjd, May 10 '12 at 14:14
FYI, the link I posted above is part of the Java FAQ at the bottom of [that page](http://stackoverflow.com/tags/java/info). — assylias, May 10 '12 at 14:14

score 6 · Accepted Answer · answered May 10 '12 at 14:11

6

In Java, you must use equals() for comparing equality between Strings. == tests for identity, a different concept.

Two objects can be equal but not identical; on the other hand if two objects are identical it's implied that they're equal.

Two objects are identical if they physically point to the same address in memory, whereas two objects are equal if they have the same value, as defined by the programmer in the equals() method. In general, you're more interested in finding out if two objects are equal.

answered May 10 '12 at 14:11

Óscar López

232,561
37
312
386

then for the given example which option is suitable – Shahzad Imam May 10 '12 at 14:12
Strings should compare equal with equality operator? String x = "X"; x == "X" should evaluate to true. – maress May 10 '12 at 14:14
@maress Your example happens to work as you expect. – assylias May 10 '12 at 14:16
2

You should use `equals()` in the general case. Only use `==` if you're interested in comparing references (memory addresses, in the end). – Óscar López May 10 '12 at 14:17

score 3 · Answer 2 · answered May 10 '12 at 14:11

3

== tests for pointer equality;.equals exists to test for value equality.

answered May 10 '12 at 14:11

Marcin

48,559
18
128
201

score 0 · Answer 3 · answered May 10 '12 at 14:14

In Java, comparing with the == operator checks for identity equality, as in the references (in the case of objects) point to the same memory location. Because of this, only primitives should be compared using the == operator, as primitives (int, long, boolean, etc) are stored by value rather than by reference.

In short, use the equals method to compare objects, and the == operator to compare primitives.

== vs .equals - why different behaviour?

3 Answers3

Linked