Clarification request for == on Strings

Question

I've tested the following code, and it prints true.
String s1 = "dog";
String s2 = "dog";
System.out.println(s1 == s2);But the following code prints false.
String s1 = new String("dog");
String s2 = new String("dog");
System.out.println(s1 == s2);And I don't really understand why. I was under the impression that the latter two snippets were the same, leaving the second code snippet redundant, unless a copy of a String needed to be obtained. Could someone please explain to me why the two println statements result in different outputs?
Thanks,
sca.tj

Answer 1

Ignoring the fact that "abc" wasn't mentioned at all, you're close but not quite on.
In the first snippet, s1 is a new string, and so a new String object is created. s2 is created as a reference to the string object "abc".s1 isn't assigned a new String per se. As I understand it, Strings which appear as literals in a Class are added to the String constant pool (see String.intern) when the Class is first initialized (or loaded, not sure which; but either way before the Class can be used). By doing this:
String s1 = "dog";you're just assigning the instance of String that already exists in the constant pool to a reference variable s1. There is no way to say "which line" created the String because the String object predates all of those assignments.
You might be interested in running this:
public class StringInternTest {
   public static void main(String[] args) {
      System.out.println(new String("abc") == new String("abc"));
      System.out.println("abc" == "abc");
      System.out.println(new String("abc").intern() == "abc");
      System.out.println(new String(new char[]{'a', 'b', 'c'}).intern() == "abc");
      System.out.println("abc" == StringInternTestClass.STR);
class StringInternTestClass {
   public static String STR = "abc";
}