String comparison in Java: what is wrong with "=="?

Question

Possible Duplicate:
Java String.equals versus ==

public class S_eaqual {
    public static void main(String[] args) {
        String s1 = "one", s2 = "two";
        if (s1 + s2 == "onetwo") {
            System.out.println("Yes..equal");
        }
    }
}

This type of comparison shows errors. Is this not the right way of comparing strings? Two String objects can be compared using == operator. So why this is showing error?

Answer 1

String should be compared using equals method.

String s1 = "one", s2 = "two";
if("onetwo".equals(s1+s2)) {
  System.out.println("Yes..equal");
}

Answer 2

Try this...

String s3 = s1 + s2
if(s3.equals("onetwo")) {
...

== compares if they refer to the same object, and the result of s1+s2 isn't in this case, and the .equals() method on string compares that the values are the same. In general, you only use == for primitive value comparisons. Although you can do it for objects iff you intend to check to make sure that two references point to the same object.

Answer 3

use (s1+s2).equals("onetwo"); instead

Answer 4

Use equals:

if (s1.concat(s2).equals("onetwo")) System.out.println("Yes..equal");