When is StringBuffer/StringBuilder not implicitly used by the compiler?

Question

I\'ve heard that the compiler (or was it the JVM?) will automatically use a StringBuilder for some string concatenation. When is the right time to explicitly declare one?

Answer 1

The compiler will use it automatically for any string concatenation using "+".

You'd usually use it explicitly if you wanted to concatenate in a loop. For example:

StringBuilder builder = new StringBuilder();
for (String name : names)
{
    builder.append(name);
    builder.append(", ");
}
if (builder.length() > 0)
{
    builder.setLength(builder.length() - 2);
}
System.out.println("Names: " + builder);

Another situation would be where you wanted to build up a string over multiple methods, or possibly conditionalise some bits of the building. Basically, if you're not building the string in a single statement (where the compiler can help you) you should at least consider using StringBuilder.