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

Use + operator is best practice, it is also simple and readable.

The Java language provides special support for the string concatenation operator ( + ), and for conversion of other objects to strings. String concatenation is implemented through the StringBuilder(or StringBuffer) class and its append method.

Offical document: https://docs.oracle.com/javase/8/docs/api/java/lang/String.html