StringBuilder classes are used to store string values but the difference in
them is that String is immutable (read only) by nature, because a value once
assigned to a String object cannot be changed after its creation. When the
value in the String object is modified, a new object is created, in memory,
with a new value assigned to the String object. On the other hand, the
StringBuilder class is mutable, as it occupies the same space even if you
change the value. The StringBuilder class is more efficient where you have to perform
a large amount of string manipulation .