Q3. Why String class has been made immutable in Java? A4. For performance & thread-safety. 1. Performance: Immutable objects are ideal for representing values of abstract data (i.e. value objects) types like numbers, enumerated types, etc. If you need a different value, create a different object. In Java, Integer, Long, Float, Character, BigInteger and BigDecimal are all immutable objects. Optimisation strategies like caching of hashcode, caching of objects, object pooling, etc can be easily applied to improve performance. If Strings were made mutable, string pooling would not be possible as changing the string with one reference will lead to the wrong value for the other references.