代码

interface Frob{
    Object display();
}
class Inform implements Frob{
    public Frob display() {
        System.out.print("通知：");
        return null;
    }
}
class Car implements Frob{
    public Frob display() {
        System.out.print("汽车油量：");
        return null;
    }
}
class Ad implements Frob{
    public Frob display() {
        System.out.print("广告：");
        return null;
    }
}
public class test8_1 {
    public static void main(String[] args) {
        Inform T=new Inform();
        T.display();
    }
}

interface shape{
    float size();
}
class rectangle implements shape{
    int length;
    int wide;
    public rectangle(int x,int y){
        length=x;
        wide=y;
    }
    public float size() {
        return length*wide;
    }
}
class circle implements shape{
    int r;//半径
    public circle(int x){
        r=x;
    }
    public float size() {
        return (float) (3.14*r*r);
    }
}
class column implements shape{
    int r;//半径
    int hight;//高
    public column(int x,int y){
        r=x;
        hight =y;
    }
    public float size() {
        return (float) ((float) (2*3.14*r*r)+(2*3.14*r*hight));
    }
}
public class test8_2 {
    public static void main(String[] args) {
    shape[] S=new shape[3];
    S[0]=new rectangle(1,5);
    S[1]=new circle(5);
    S[2]=new column(5,2);
        for(int i=0;i<3;i++)
            System.out.println(S[i].size());
    }
}

abstract class fruit{
    abstract double getWeight();
}
class Apple extends fruit{
    double W;
    public Apple(int w){
        W=w;
    }
    public double getWeight() {
        return W;
    }
}
class Peach extends fruit{
    double W;
    public Peach(int w){
        W=w;
    }
    public double getWeight() {
        return W;
    }
}
class Orange extends fruit{
    double W;
    public Orange(int w){
        W=w;
    }
    public double getWeight() {
        return W;
    }
}
public class test8_3 {
    public static void main(String[] args) {
        fruit[] f=new fruit[3];
        f[0]=new Apple(100);
        f[1]=new Peach(200);
        f[2]=new Orange(300);
        for(int i=0;i<3;i++)
            System.out.println(f[i].getClass().getSimpleName()+":"+f[i].getWeight()+"KG");
    }
}