Java string interning, what is guaranteed?

Question

The question boils down to this code:

// setup
String str1 = "some string";
String str2 = new String(str1);
assert str1.equals(str2);
assert str1 != str2;
String str3 = str2.intern();

// question cases
boolean case1 = str1 == "some string";
boolean case2 = str1 == str3;

Does Java standard give any guarantees about values of case1 and case2? Link to relevant part of Java spec would be nice, of course.

Yes, I looked at all the "Similar Questions" found by SO, and found no duplicates, as none I found answered the question this way. And no, this is not about the misguided idea of "optimizing" string comparisons by replacing equals with ==.

Answer 1

Here is your JLS quote, Section 3.10.5:

Each string literal is a reference (§4.3) to an instance (§4.3.1, §12.5) of class String (§4.3.3). String objects have a constant value. String literals-or, more generally, strings that are the values of constant expressions (§15.28)-are "interned" so as to share unique instances, using the method String.intern.

Thus, the test program consisting of the compilation unit (§7.3):

package testPackage;
class Test {
        public static void main(String[] args) {
                String hello = "Hello", lo = "lo";
                System.out.print((hello == "Hello") + " ");
                System.out.print((Other.hello == hello) + " ");
                System.out.print((other.Other.hello == hello) + " ");
                System.out.print((hello == ("Hel"+"lo")) + " ");
                System.out.print((hello == ("Hel"+lo)) + " ");
                System.out.println(hello == ("Hel"+lo).intern());
        }
}

class Other { static String hello = "Hello"; }

and the compilation unit:

package other;

public class Other { static String hello = "Hello"; }

produces the output: true true true true false true

This example illustrates six points:

Literal strings within the same class (§8) in the same package (§7) represent references to the same String object (§4.3.1).

Literal strings within different classes in the same package represent references to the same String object.

Literal strings within different classes in different packages likewise represent references to the same String object.

Strings computed by constant expressions (§15.28) are computed at compile time and then treated as if they were literals.

Strings computed by concatenation at run time are newly created and therefore distinct. The result of explicitly interning a computed string is the same string as any pre-existing literal string with the same contents.

Combined with the JavaDoc for intern, and you have enough information to deduce that both of your cases will return true.