Comparing string references in Java [duplicate]

Question

The code in the following snippet just compares string references.

String str1 = "mystring9";
String str2 = "mystring"+String.valueOf(9);

System.out.println(str1==str2);

In this case, str1==str2 returns false.

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The following segment of code also returns false.

String str1 = "mystring9";
String str2="mystring"+str1.length();

System.out.println(str1==str2);

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The following code however, returns true.

String str1 = "mystring9";
String str2 = "mystring"+9;

System.out.println(str1==str2);

I think, the expression "mystring"+9 in this code should internally be evaluated to String.valueOf(9) though why do the first two examples return different output than the preceding example?

Answer 1

The Java compiler will pre-evaluate all constant-only operations. (this is called constant folding)

Therefore, "A" + "b" + 3 compiles to "Ab3".