Java入门学习笔记（五）Static

package review.learn;

public class TestStatic2 {
    public static void main(String[] args) {
        System.out.println(Teacher.id);
        Teacher.ready();
    }
}
class Teacher{
    String name;
    static int id;

    public void teach(){
        System.out.println("teaching!");
    }
    public static void ready(){
        System.out.println("preparing!");
    }
}

上述name和teach函数没有用static修饰，所以要先定义对象才能调用

static修饰过的可以直接用类调用，不需要再定义对象

1. 多态的前提1：是继承
2. 多态的前提2：要有方法的重写
3. 父类引用指向子类对象,如：Animal a = new Cat();
4. 多态中，编译看左边，运行看右边

package practise;

public class SelfTest {
    public static void main(String[] args) {
        Car c1=new Car();
        System.out.println(c1.getBrand());

        c1.stop();
        c1.start();

        BMW b=new BMW();
        System.out.println(b.color+ b.color);
        System.out.println(b.getName());//子类继承，可以查看父类成员变量
        b.start();//子类里面有，所以执行子类的
        b.stop();//子类没有，执行父类的

        
        Car c2=new Tesla();//多态，左边父类，右边子类，
        c2.start();//子类没有，执行父类的
        c2.stop();//执行tesla
        System.out.println(c2.getBrand());//子类没有 执行父类的
    }
}
class Car{
    private String brand;
    private String name;
    private double price;
    private int id;
    public void start(){
        System.out.println("let's get started!");
    }
    public void stop(){
        System.out.println("let's stop!");
    }

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
}
class BMW extends  Car{
    String color="莫名其妙的黑";
    @Override
    public void start(){
        System.out.println("get away i am bmw, i am gonna take off!");
    }
//    public void stop(){
//        System.out.println("oh my god!i am too fast!i cannot stop!");
//    }
}
class Tesla extends Car{
    @Override
    public void stop(){
        System.out.println("oh my god!i am too fast!i cannot stop!");
    }
    public void swim(){
        System.out.println("i can also swim how cool am i!");
    }
}