Best practices/performance: mixing StringBuilder.append with String.concat

Question

I\'m trying to understand what the best practice is and why for concatenating string literals and variables for different cases. For instance, if I have code like this

Answer 1

Optimization is done automatically by the compiler.

The Java2 compiler will automatically convert the following:

String s = s1 + s2; 

to

String s = (new StringBuffer()).append(s1).append(s2).toString();

Taken straight from the Java Best Practices on Oracles website.