Java: What is the difference between String, StringBuilder, and StringBuffer?

1// String — immutable
2String s = "Hello";
3s += " World";  // New object
4
5// StringBuilder — mutable, not thread-safe
6StringBuilder sb = new StringBuilder();
7sb.append("Hello");
8
9// StringBuffer — mutable, thread-safe
10StringBuffer sbf = new StringBuffer();
11sbf.append("Hello");