Scjp test preparation-9

The execution result of the following code is as follows:

abstract class Vehicle{    public int speed(){        return 0;    }}class Car extends Vehicle{    public int speed(){        return 60;    }}class RacerCar extends Car{    public int speed(){        return 150;    }        public void run(){        System.out.println("I‘m running...");    }}public class Test{    public static void main(String[] args){        RacerCar racer = new RacerCar();        Car car = new RacerCar();         Vehicle vehicle = new RacerCar();         System.out.println(racer.speed() + ", " + car.speed()  + ", " + vehicle.speed());     }}

 

Analyze the question. Java polymorphism is examined in the question. For example, in the "car = new racercar ();" clause, the variable car is of the car type during compilation and is of the racercar type during runtime. In this case, when calling a method that references a variable at runtime, its method behavior always shows the characteristics of the subclass method, rather than the behavior characteristics of the parent class method, which may occur: variables of the same type call the same method and present different behavior characteristics. This is polymorphism.

It should be noted that the referenced variable can only call the method of its type during compilation, while the method of its runtime type is executed during execution. In the code above, if the car variable executes the run () method, it cannot be compiled because the car type does not have the run () method during compilation. Although the car variable points to an object, it does have the run () method (visible during reflection ).

Let's take a look at the execution result:

Scjp test preparation-9