StringBuilder java :-
1) StringBuilder class is like same as String.
2) StringBuilder class is not thread-safe.
3) StringBuilder class is mutable sequence of characters.
4) Main method in StringBuilder class are append and insert method.
5) Through the insert method you can insert string after any index of the string.
6) There are four constructors in StringBuilder class.
public StringBuilder(int capacity)
public StringBuilder(String str)
public StringBuilder(CharSequence seq)
Note: majaor different between StringBuilder and StringBuffer is
StringBuffer is thread-safe and StringBuilder is not thread-safe