String vs StringBuffer vs StringBuilder

The main difference between the three most commonly used String classes as follows.

• StringBuffer and StringBuilder objects are mutable whereas String class objects are immutable.
• StringBuffer class implementation is synchronized while StringBuilder class is not synchronized. 

Note: If you keep forgetting which one is synchronized which one is not remember StringBuilder is just implmentation of builder pattern so the other is Synchronized which is StringBuffer

• Concatenation operator "+" is internally implemented by Java using StringBuilder.

Note: For more details see this link

http://stackoverflow.com/questions/11408427/how-does-the-string-class-override-the-operator

Criteria to choose among String, StringBuffer and StringBuilder

• If the Object value will not change in a scenario use String Class because a String object is immutable.
• If the Object value can change and will only be modified from a single thread, use a StringBuilder because StringBuilder is unsynchronized(means faster).
• If the Object value may change, and can be modified by multiple threads, use a StringBuffer because StringBuffer is thread safe(synchronized).