设置扩展抽象类的子类的实例变量

Question

I have a class called Shape that is abstract, and a class named Circle that extends Shape

Shape shapeCircle = new Circle();

I can set and get colors of shapeCircle fine because the color getters and setters are in Shape , but the dimensions of Circle is only for the Circle class (radius).

If Circle class has an instance variable private int radius and a method called getRadius() , how can I get/set the radius of shapeCircle ? I tried shapeCircle.getRadius(); , but no luck.

Answer 1

Only methods of Shape are accessible with object shapeCircle. shapeCircle is type Shape, methods in Circle are not visible.

By using this,

 Circle shapeCircle = new Circle();

you can call both method of Circle and Shape. The above case is take advantage of only Inheritance .

Edit

But if you add public abstract int getDimension(); in Shape class and Circle class implements getDimension() method

@Override
    public int getDimension() {
        return radius;
    }

All classes that extends Shape needs to implements getDimension() method. But each subclass has its own dimension.

You can use

    Shape shapeCircle = new Circle();
    shapeCircle.getDimension();

This takes advantage of both polymorphism and abstract class .

Answer 2

Shape shapeCircle = new Circle();

Here the reference is of Shape and shape class doesn't defines radius variable.So you cannot use shapeCircle.getRadius();

To invoke get/set radious method type cast the shapecircle variable like this

Circle shapeCircle = (Circle)shapeCircle;

Now you can invoke the get/set radious methods. Please note that typecasting code should be in some different method to benefit using polymorphism.

EDIT:

You should design your abstract classes or interfaces to be having all important/common operations/methods which will help you in writing polymorphic code. For example: Radius field is specific to a Circle class.But consider you eventually want to calculate the area. So you should define CalculateArea() method in your abstract class and let every shape implement this method.This way, you achieve polymorphism.

Shape ShapeCicle = new Circle(Radius);
Float area = ShapeCircle.area();

Point is don't confine yourself with the radius example.Try to define classes in a way so that you can benefit from polymorphism.

Answer 3

You have to use instanceof keyword to check for the original type here . If the type is Circle , then you can type cast and call the getRadius method. Something like

if (shapeCircle instanceof Circle) {
 Circle circle = (Circle)shapeCircle;
 int radius = circle.getRadius();
}

That should do.

Answer 4

“形状不是圆形” =圆形方法不适用于“形状”。