Intellij warning "StringBuffer.toString() in concatenation"

Question

If you enable all inspections under the heading "Performance issues", you'll get a warning (as in the title) about the following code:

StringBuilder sb = new StringBuilder("Two");
String s = "One" + sb.toString();

I realise the toString() is unnecessary, but in what way is it a performance issue?

Answer 1

There is no point in using a StringBuilder if you're going to use the string concatenation operator (+), which creates a StringBuilder behind the scenes, leading to the creation of two StringBuilder objects instead of one. You can simply re-use the StringBuilder to concatenate strings:

StringBuilder sb = new StringBuilder("One");
sb.append("Two");
String s = sb.toString();

The original code is equivalent to:

StringBuilder sb = new StringBuilder("Two");
StringBuilder sb2 = new StringBuilder("One");
String s = sb2.append(sb.toString()).toString();

Another aspect related to the warning (which honestly has less to do with performance than code optimization) is that the toString() is redundant when used with the + operand. This is because the compiler will automatically call the toString(). This is what you already guessed in your question, and is explained in the Java Language Specification:

(from https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.18.1):

If only one operand expression is of type String, then string conversion (§5.1.11) is performed on the other operand to produce a string at run time.

where section 5.1.11 says:

• If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l).

• Otherwise, the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments; but if the result of invoking the toString method is null, then the string "null" is used instead.