Question

I am writing a immutable class.One of the instance is an object of another class which is mutable.How can i prevent the modification of the state of that instance.

Answer 1

If you provide delegates to all the getters, but don’t provide a getter for the field itself. For example:

public final class Foo {
   private Bar bar;

   public String getBarField1() { 
       return bar.getBarField1();
   }
   public String getBarField2() {
       return bar.getBarField2();
   }
}

But there is a thing to consider – how is the field created. If it is passed in constructor, then you have to create a copy of it rather than get the passed object, otherwise the creating code will have reference to the Bar instance and be able to modify it.