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• The String class objects are immutable whereas the StringBuffer and the StringBuilder objects are mutable.
• A StringBuffer is synchronized while a StringBuilder is not synchronized.
• A Concatenation operator "+" is internally implemented using either StringBuffer or StringBuilder.
• If the Object value is not going to change use String Class because a String object is immutable.
• If the Object value can change and will only be accessed from a single thread, use a StringBuilder because StringBuilder is unsynchronized.
• If the Object value can change and will be modified by multiple threads, use a StringBuffer because StringBuffer is synchronized.