Java Compiler replacing StringBuilder with + concatenation

Question

Here's some simple Java code:

String s = new StringBuilder().append("a").append("b").append("c").toString();

I compile it with JRE 1.6, and I observe the following in the decompiled class file:

String s = "a" + "b" + "c";

I had the following questions:

1. Why does the compiler choose '+' over StringBuilder?
2. Do we have any official Java specification that justifies this behavior?
3. Does it really make sense to use StringBuilder in such cases, where we know compiler is going to change it anyway?

Answer 1

It's the other way round. + for String is implemented using StringBuilder (or StringBuffer) behind the scenes (see http://docs.oracle.com/javase/7/docs/api/java/lang/String.html or http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.18.1).

Thus, once they're compiled, your two code snippets are indistinguishable. The decompiler has to make a guess at the original form.