Why use StringBuilder explicitly if the compiler converts string concatenation to a StringBuilder automatically? [duplicate]

Question

Possible Duplicate:
StringBuilder vs String concatenation in toString() in Java

I am wondering, since the compiler internally uses a StringBuilder to append Strings when performing String concatenation, then what's the point and why should I bother using StringBuilder if String concatenation already did the job for you? Are there any other specific reasons?

Answer 1

As you mention, you should not use StringBuilder instead of a simple string concatenation expression such as a + " = " + b. The latter is faster to type, easier to read, and the compiler will use a StringBuilder internally anyway so there is no performance advantage by rewriting it.

However StringBuilder is useful if you are concatenating a large number of strings in a loop. The following code is inefficient. It requires O(n²) time to run and creates many temporary strings.

String result = ""; for (int i = 0; i < foo.length; ++i) {     result += bar(foo[i]);  // Bad } 

Try this instead:

StringBuilder sb = new StringBuilder(); for (int i = 0; i < foo.length; ++i) {     sb.append(bar(foo[i])); } String result = sb.toString(); 

The compiler optimises only simple a + b + c expressions. It cannot optimize the above code automatically.

Answer 2

Where are you assuming that string concatination uses stringbuilder internally? Maybe a simple concat gets optimized away, but this will definitely not:

String s = "";  for (int i = 0; i < 1000; i++){   for (int j = 0; j < 1000; j++){     s+= "" + i + j } }