Question

Does this below class is immutable?

class Immutable {

  private int x;

  Immutable(int value) {
    this.x = value;
  }

  public int getX(){
    return x;
  }
}

1. Class is not final Since no member is visible to sub class

2. Instance Variable x is not final since there is no setter method.

Is there any possiblity this class will break the contract of Immutable functionality without adding any code in this class?

Answer 1

Joshua Bloch in his “Effective Java” states that “you should either design your classes for extensibility or make them non-extensible”.

If flexibility is your primary concern and you have a strong reason to design your immutable class for inheritance – leave this class non-final.

If security is your concern – make this class final and member field final. Because:

• Non-final classes can be extended to create a mutable child classes by overriding getter method.

• Java is reflective by design. As Belgther told it’s possible to set the value of member field using reflection. (no matter private or public) A long time ago I used this to integrate my specific debugger with another project.