When should you explicitly use a StringBuilder? [duplicate]

Question

As I understand it, when I do String baz = "foo" + "bar" + "123" the Java compiler internally replaces the expression with a StringBuilder. However our Java teacher told us that it is good practice to always use a StringBuilder explicitly...

Am I correct in assuming I will only need to explicitly use StringBuilder when concatenating inside loops as indicated in an answer to Stack Overflow question String builder vs string concatenation? Are there other cases where you should explicitly use a StringBuilder instead of + or +=?

Answer 1

It's more general than "inside loops" - it's any time you want to do concatenation over multiple statements, and don't need the intermediate result as a string. For example:

StringBuilder builder = new StringBuilder("Start"); if (someCondition) {     builder.append("Foo"); } if (someOtherCondition) {     builder.append("Bar"); } builder.append("End"); String result = builder.toString(); 

While you could write that as:

String result = "Start" + (someCondition ? "Foo" : "")     + (someOtherCondition ? "Bar" : "") + "End"; 

... that becomes hard to read. And if there are more statements within the if bodies, it may not even be feasible.

To correct something within your question though:

As I understand it, when I do String baz = "foo" + "bar" + "123" the java compiler internally replaces the expression with a StringBuilder.

No, when you write that expression the compiler recognizes that it's a compile-time constant, and replaces it with

String baz = "foobar123"; 

That's a very good reason not to explicitly use a StringBuilder - the code above is clearly more efficient at execution time than

String baz = new StringBuilder("foo").append("bar").append("123").toString(); 

When it isn't a compile-time constant, the Java compiler will perform the concatenation using a StringBuilder, usually leaving you with easier-to-understand code than with the explicit use of StringBuilder, but with no performance hit. I suspect your teacher either doesn't properly understand string concatenation, or simply read somewhere else that you should use StringBuilder without fully understanding when it's appropriate.