Confused about comparing string in this example

Question

I know that \"==\" compare reference and the a java String is immutable and use a string pool cache but I\'m still confusing on this example :

 final String         


        

Answer 1

Since fName is final and initialized with a literal String, it's a constant expression.

So the instruction

String name2 = fName + "Gosling"

is compiled to

String name2 = "James" + "Gosling"

which is compiled to

String name2 = "JamesGosling"

So, in the bytecode, you have the equivalent of

String name2 = "JamesGosling";
String name3 = "JamesGosling";

So both name2 and name3 reference the same literal String, which is interned.

On the other hand, lName is not final, and is thus not a constant expression, and the concatenation happens at runtime rather than at compile time. So the concatenation creates a new, non-interned String.

Answer 2

Creating two strings with same string values does not guarantee they are both from the String Pool. You need to call s.intern() to ensure you get a String that has both same value and memory address from String Pool.

There is a good chance the concatenation is causing the String to be read as String Object instead of a String literal. This is due to the compiler not knowing if fName + lName will result to a literal or not. If it's a String literal, it goes to the String Pool. If it's a String Object, it is is not guaranteed to go to the String Pool and be treated like any other Object.