1.定义一个基类Shape，在此基础上派生出Rectangle和Circle，二者都有getArea()函数计算对象的面积。使用Rectangle类创建一个派生类Square。

 1 //基类Shape
 2 class Shape {
 3     public double getArea() {
 4         return 0;
 5     }
 6 }
 7 //Rectangle类继承Shape类
 8 class Rectangle extends Shape {
 9     private double len;
10     private double width;
11     public Rectangle(double len, double width) {
12         super();
13         this.len = len;
14         this.width = width;
15     }
16     public double getArea() {
17         return len * width;
18     }
19 }
20 //Circle类继承Shape类
21 class Circle extends Shape {
22     private double r;
23     public Circle(double r) {
24         super();
25         this.r = r;
26     }
27     public double getArea() {
28         return 3.14 * r * r;
29     }
30 }
31 //Square类继承Rectangle类
32 public class Square extends Rectangle {
33     public Square(double len, double width) {
34         super(len, width);
35     }
36     public static void main(String[] args) {
37         Shape p = new Rectangle(3, 6);
38         System.out.println("The area of the rectangle is " + p.getArea());
39         p = new Circle(3);
40         System.out.printf("The area of the circle is %.2f\n", p.getArea());
41         p = new Square(4, 4);
42         System.out.println("The area of the square is " + p.getArea());
43     }
44 }

 

 

 

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2.定义一个哺乳动物类Mammal，再由此派生出狗类Dog，定义一个Dog类的对象，观察基类和派生类的构造函数的调用顺序。

 

 1 //父类Mammal类
 2 class Mammal{
 3     private String name;
 4     private int age;
 5     public Mammal(String name, int age) {
 6         this.name=name;
 7         this.age=age;
 8         System.out.println("构造哺乳动物类");
 9     }
10     public String getName() {
11         return name;
12     }
13     public int getAge() {
14         return age;
15     }
16 }
17 //Dog类继承Mammal类
18 public class Dog extends Mammal{
19     private int leg; 
20     public Dog(String name, int age, int leg) {
21         super(name, age);
22         this.leg=leg;
23         System.out.println("构造狗类");
24     }
25     public void display() {
26         System.out.println("Name："+getName());
27         System.out.println("Age："+getAge());
28         System.out.println("Leg："+leg);
29     }
30     public static void main(String[] args) {
31         Dog p=new Dog("Sera",5,4);
32         p.display();
33     }
34 }