String is immutable ( once created can not be changed )object . The object created as a String is stored in the Constant String Pool. Every immutable object in Java is thread safe ,that implies String is also thread safe . String can not be used by two threads simultaneously. String once assigned can not be changed. StringBuilder is same as the StringBuffer , that is it stores the object in heap and it can also be modified . The main difference between the StringBuffer and StringBuilder is that StringBuilder is not thread safe. StringBuilder is fast as it is not thread safe .