不可变的非最终课程

Question

Is a class still considered as immutable if it can be subclassed(but other rules are kept)? For example:

abstract class Figure {
abstract double area();
}

class Rectangle extends Figure {
private final double length;
private final double width;

Rectangle(double length, double width) {
    this.length = length;
    this.width = width;
} 

double area() { return length * width; }
}

Is it immutable?

Answer 1

The fields length and width are still immutable for all subclasses (if they are not shadowed by the subclasses fields with the same name).

But a subclass can define it's own mutable fields.

So the answer is: it depends.

If you do not inherit other classes of Rectangle with mutable fields, than the answer is yes. Otherwise no.

Answer 2

Immutable non-final classes

do not exist.

If your class is not final , it can be subclassed.

By definition, an immutable class only has invariants. But if it is not final , you can subclass it and introduce instance fields which are NOT invariants. So:

public class A 
{
    private final int a;

    public A(final int a)
    {
        this.a = a;
    }
}

is NOT immutable, since you can:

public class B
    extends A
{
    // NOT AN INVARIANT
    private int b;

    public B(final int a)
    {
        super(a);
    }

    public void setNonInvariant(final int b)
    {
        this.b = b;
    }
}

HOWEVER: class A is thread safe . But it is not immutable. And class B is not thread safe...

Answer 3

If the base class itself is immutable in your example then yes, this is immutable. In a language such as Java that doesn't support compiler enforced immutability it falls on to you as the developer to handle it- and what means basically is that any object that cannot have its state changed is considered immutable.

In your example above you don't even need to make your variables final - the fact that nothing can change them means it's immutable.