JAVA中的对象引用和类型转换

Question

Do I still work on the same object data after a typecast of the data?

A pseudo code sample could be:

MyClass car = new MyClass();
car.setColor(BLUE);

VW vw = (VW)car; //Expecting to get a blue VW.
vw.topSpeed = 220;

//Will car have the top speed set now? If topSpeed is a part of the Car object.

Answer 1

Do I still work on the same object data after a typecast of the data?

Yes. Casting changes the type of the reference you have to the object. It has no effect on the object itself at all.

Note that in your example, for the cast to be successful, VW would have to be a superclass of MyClass or an interface MyClass implements, eg:

class MyClass extends VW // or extends something that extends VW

or

class MyClass implements VW

Concrete example:

class Base {
    private int value;

    Base(int v) {
        this.value = v;
    }

    public int getValue() {
        return this.value;
    }

    public void setValue(int v) {
        this.value = v;
    }
}

class Derived extends Base {
    Derived(int v) {
        super(v);
    }
}

Then:

Derived d = new Derived(1);
System.out.println(d.getValue());  // 1
Base b = d;                        // We don't need an explicit cast
b.setValue(2);                     // Set the value via `b`
System.out.println(d.getValue());  // 2 -- note we're getting via `d`
Derived d2 = (Derived)b;           // Explicit cast since we're going more specific;
                                   // would fail if `b` didn't refer to a Derived
                                   // instance (or an instance of something
                                   // deriving (extending) Derived)
d2.setValue(3);
System.out.println(d.getValue());  // 3 it's the same object
System.out.println(b.getValue());  // 3 we're just getting the value from
System.out.println(d2.getValue()); // 3 differently-typed references to it