If == compares references in Java, why does it evaluate to true with these Strings?

Question

As it is stated the == operator compares object references to check if they are referring to the same object on a heap. If so why am I getting the "Equal" for this piece of code?

public class Salmon {
    public static void main(String[] args) {

        String str1 = "Str1";
        String str2 = "Str1";

        if (str1 == str2) {
            System.out.println("Equal");
        } else {
            System.out.println("Not equal");
        }
    }
}

Answer 1

The program will print Equal. (At least using the Sun Hotspot and suns Javac.) Here it is demonstrated on http://ideone.com/8UrRrk

This is due to the fact that string-literal constants are stored in a string pool and string references may be reused.

Further reading:

• What is String literal pool?
• String interning

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This however:

public class Salmon {
    public static void main(String[] args) {

        String str1 = "Str1";
        String str2 = new String("Str1");

        if (str1 == str2) {
            System.out.println("Equal");
        } else {
            System.out.println("Not equal");
        }
    }
}

Will print Not equal since new is guaranteed to introduce a fresh reference.

So, rule of thumb: Always compare strings using the equals method.

Answer 2

Java stores all Strings in a string table internally during a run. The references to the two strings are identical because in memory they're stored in the same place. Hence, Equal.

Your statement is right, that == compares object references. Try the same thing with any other class but Strings and you won't get the same result.

Answer 3

This code won't print Equal.
But if the two strings were the same, this case would be special.

Now that you've updated your code, it is the case :

A simple (but not totally exact) explanation is that the compiler see that the two strings are the same and do something like :

String str1 = "Str1";
String str2 = str1;

---

What really happens here is that the compiler will see the literal string and put it in the "String literal pool".

As a String can't be modified (it's immutable) the literal values of Strings (those found during compilation) are put in a "pool".
This way, if two different literal strings which have the same content (like in this particular case), the memory isn't wasted to store "Str1" and "Str1" two times.