When to use StringBuilder in Java

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.

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.

As a general rule, always use the more readable code and only refactor if performance is an issue. In this specific case, most recent JDK's will actually optimize the code into the StringBuilder version in any case.

You usually only really need to do it manually if you are doing string concatenation in a loop or in some complex code that the compiler can't easily optimize.