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

The compilier optimize the + concatenation.

So

int a = 1;
String s = "Hello " + a;

is transformed into

new StringBuilder().append("Hello ").append(1).toString();

There an excellent topic here explaining why you should use the + operator.

Answer 2

I personally prefer the Strings.format(), simple easy to read one-line string formatter.

String b = "B value";
String d = "D value";
String fullString = String.format("A %s C %s E F", b, d);
// Output: A B value C D value E F

Answer 3

You should always use append.

concat create a new String so it's pretty like + I think.

If you concat or use + with 2 final String the JVM can make optimisation so it's the same as doing append in this case.