When to use StringBuilder in Java [duplicate]

Question

It is supposed to be generally preferable to use a StringBuilder for string concatenation in Java. Is this always the case?

What I mean is this: Is the overhead of creating a StringBuilder object, calling the append() method and finally toString() already smaller then concatenating existing strings with the + operator for two strings, or is it only advisable for more (than two) strings?

If there is such a threshold, what does it depend on (perhaps the string length, but in which way)?

And finally, would you trade the readability and conciseness of the + concatenation for the performance of the StringBuilder in smaller cases like two, three or four strings?

Explicit use of StringBuilder for regular concatenations is being mentioned as obsolete at obsolete Java optimization tips as well as at Java urban myths.

Answer 1

If you use String concatenation in a loop, something like this,

String s = ""; for (int i = 0; i < 100; i++) {     s += ", " + i; } 

then you should use a StringBuilder (not StringBuffer) instead of a String, because it is much faster and consumes less memory.

If you have a single statement,

String s = "1, " + "2, " + "3, " + "4, " ...; 

then you can use Strings, because the compiler will use StringBuilder automatically.

Answer 2

Ralph's answer is fabulous. I would rather use StringBuilder class to build/decorate the String because the usage of it is more look like Builder pattern.

public String decorateTheString(String orgStr){             StringBuilder builder = new StringBuilder();             builder.append(orgStr);             builder.deleteCharAt(orgStr.length()-1);             builder.insert(0,builder.hashCode());             return builder.toString(); } 

It can be use as a helper/builder to build the String, not the String itself.