Implementation of Immutable class in Java

Question

In my application, I need to make my class as immutable, but in my class I have an object of other class which is mutable, can anyone please suggest me Implementation.

Pseudo code:

Class A which I would like as immutable class

class A {
    /* ... */ 

    //class B is mutable
    B b = new B();
    /* ... */ 
}

Make b private inside the instance

vincentzhou
– vincentzhou

2016-09-06 19:58:44 +00:00
Commented Sep 6, 2016 at 19:58 — vincentzhou
– vincentzhou, Commented Sep 6, 2016 at 19:58
... and final

PM 77-1
– PM 77-1

2016-09-06 20:00:23 +00:00
Commented Sep 6, 2016 at 20:00 — PM 77-1
– PM 77-1, Commented Sep 6, 2016 at 20:00

sidgate · Accepted Answer · 2016-09-06 20:47:14Z

2

Assuming you need to access b from outside world, you will need to wrap all methods on B and declare those in A. Additionally, take b as constructor parameter, without getter/setter method.

class A{
   private final b;
   A(B b){
     this.b = b;    
   }
   public String getSomeValue(){
     return b.getSomeValue();
   }

}

Edit: Refer to Hoopje' comment regarding cloning b in constructor. If clone or copy constructor not available on B then you have to construct new B on your own in A

edited Sep 6, 2016 at 20:47

answered Sep 6, 2016 at 20:05

sidgate

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2 Comments

Hoopje Over a year ago

You should make a copy of b in the constructor. Now, you can save a reference to the B object elsewhere and change the object after creating the A object, thus changing the immutable A object.

Abacus Over a year ago

additionally, make your class final to prohibit inheritance

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