Java Immutable Object

The simple definition of immutable object is "An object called immutable whose states can't be changed after it constructed". It means every time we need to alter the state we actually end up having a new object. The best example is "java.lang.String".

Some body might be thinking its actually overhead, this practice ends up creating too many objects. Actually over the time with optimization of garbage collection and the cost of creating new objects decreases. So in a situation where you want to define constant, or say a multi threading case immutable objects work well. As the value doesn't change at any stage, we are assured of dirty or inconsistent value.

Lets understand how to create our own immutable object.

1) Make the class final, to stop any other class to extend.

2) All fields in the class should be private and final.

3) Since we don't want the state of the object to change,  there should not be any setter method.

4) Any method which require to change the state, must create a new object with updated state.

For Ex:

final public class MyCar

{

     private final String registrationNumber;

     public MyCar(String registrationNumber)

     {

        this.registrationNumber = registrationNumber;

     }

     public String getRegistrationNumber()

    {

        return registrationNumber;

    }

    public MyCar upgradeCar(String newCarRegNo)

    {

       return new MyCar(newCarRegNo);

    }

}