Where to use StringBuffer/StringBuilder than String

The main idea is that String is immutable. So when you are trying to modify it, new object created. StringBuffer and StringBuilder are mutable. And the difference is the first one is thread-safe.

The common approach to using StringBuilder is to parse something when you iteratively create a String object in a NON thread safe environment.

For example, you have a numbers array [1, 2, 3]. To create object "String1String2String3" you can use a StringBuilder

StringBuilder builder = new StringBuilder();
foreach(Integer num : array) {
    builder.append("String").append(num);
}
builder.toString();

It's better than just using String string = string + ("String" + num);. AFAIK, compiler will optimize using string concatenation in the loop to StringBuilder, but better to use it manually.

StringBuffer is used when you have shared states, that are modified by concurrent threads.