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Immutable class is a class which once created, it’s contents can not be changed and cannot be inherited. Immutable objects are the objects of immutable class whose state can not be changed once constructed. e.g. String class

To create an immutable class following steps should be followed:

  1. Create a final class.
  2. Set the values of properties using constructor only.
  3. Make the properties of the class final and private
  4. Do not provide any setters for these properties.
  5. If the instance fields include references to mutable objects, don’t allow those objects to be changed:
    1. Don’t provide methods that modify the mutable objects.
    2. Don’t share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.

E.g.
public final class FinalPersonClass {

      private final String name;
      private final int age;
     
      public FinalPersonClass(final String name, final int age) {
            super();
            this.name = name;
            this.age = age;
      }
      public int getAge() {
            return age;
      }
      public String getName() {
            return name;
      } 
}

All wrapper classes in java.lang are immutable –
String, Integer, Boolean, Character, Byte, Short, Long, Float, Double, BigDecimal, BigInteger

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