它是不可变的类吗？

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

The class is practically immutable.

But you can still use reflection to set the value of the variable x since the variable is not final.

Answer 2

The Immutable class is immutable.

However, it is possible to declare a subclass of Immutable whose instances are mutable. Indeed, the only way to prevent this is to make this class final ... or make its constructor private.

You could argue (and I would) that a mutable subclass of Immutable is violating the contract of Immutable .

---

The approach of changing the value of x using reflection is breaking the rules. The Java specs say that the behavior of an application that does this is unspecified. (Certainly, it breaks all sorts of assumptions that the JLS says that the JIT compiler is allowed to make.)

A sensible developer won't do that kind of thing, will discount the possibility of someone else doing it, and will say that Immutable is immutable despite the theoretical possibility that some total lunatic could change it.

Answer 3

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.

Answer 4

Yes, it seems to me that it's immutable.

Since you have a private field that can only be set in the constructor and the only method in that class is a getter, then that field cannot be modified anymore, and the class can be said immutable.