Strings in Java : equals vs ==

  String s1 = "andrei";
  String s2 = "andrei";

  String s3 = s2.toString();

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

Giving the following code why is the second comparison s2 == s3 true ?
What is actually s2.toString() returning ? Where is actually located (s2.toString()) ?

Answer

First of all String.toString is a no-op:

/**
 * This object (which is already a string!) is itself returned.
 *
 * @return  the string itself.
 */
public String toString() {
    return this;

}

Second of all, String constants are interned so s1 and s2 are behind the scenes changed to be the same String instance.