如果我的子类也有不同的方法怎么办？

Question

I have a question about polymorphism in Java. I have a abstract base class Shape and two derived classes Rectangle and Circle . I know that I can call overridden methods in Circle and Rectangle class with a Shape class reference but what if I have a method that Shape class doesn't have but Circle class has. How can I use polymorphism in this situation?

Answer 1

This happens all the time. The answer is that you need to cast your Shape to a Circle to access the method

Shape s = ...;
if (s instanceof Circle) {
  ((Circle)s).foo(); //method only on Circle
}

Answer 2

If you store a Circle in a Shape instance, you can invoke Circle methods on s by casting s to Circle:

Shape s = new Circle();
...
if ( s.instanceof(Circle) ) {
   ((Circle)s).circleMethod();
}

Answer 3

Apart from the casting solution suggested by other answers, if you think all subclasses of Shape should provide that method but you can't/don't want to implement it on Shape , you can declare the method as abstract and implement the method in Circle and Rectangle .

public abstract class Shape {
    public abstract void foo();
}


public class Circle extends Shape {
    public void foo() {
        // Implementation
    }
}

then you can do:

Shape s = new Circle();
s.foo();

Answer 4

You cannot use polymorphism in this situation. The very idea of polymorphism is that you can use an object in a specified way irrespective of its actual shape (specific object type). By requiring access to a method that is unique to (literally) a specific shape, you must forego polymorphism.

( The above argument would not apply if Circle actually had subclasses, in which case you could polymorphically call these methods, but this is just moving the argument somewhere else .)