chapter 4

src/chapter4/Example01.java

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+package chapter4;
+
+class Animal {
+    private String name;
+    private int age;
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public int getAge() {
+        return age;
+    }
+
+    public void setAge(int age) {
+        this.age = age;
+    }
+}
+
+// 定义 Dog 类继承 Animal 类
+class Dog extends Animal {
+    private String color;
+
+    public String getColor() {
+        return color;
+    }
+
+    public void setColor(String color) {
+        this.color = color;
+    }
+}
+
+public class Example01 {
+    public static void main(String[] args) {
+        Dog dog = new Dog();
+        dog.setName("牧羊犬");
+        dog.setAge(3);
+        System.out.println("名称：" + dog.getName() + "，年龄：" + dog.getAge());
+    }
+}

src/chapter4/Example02.java

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+package chapter4;
+
+
+public class Example02 {
+}

src/chapter4/Example11/Example11.java

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+package chapter4.Example11;
+
+interface Animal {
+    int ID = 1;             // 接口默认为常量添加 public static final
+    String NAME = "牧羊犬";    // 接口默认为常量添加 public static final
+
+    // 定义抽象方法shout()，接口默认添加 public abstract
+    void shout();
+
+    // 静态方法，默认添加 public
+    static int getID() {
+        return Animal.ID;
+    }
+
+    // 抽象方法
+    public void info();
+}
+
+interface Action {
+    public void eat();
+}
+
+class Dog implements Animal, Action {
+    public void eat() {
+        System.out.println("喜欢吃骨头");
+    }
+
+    // 重写Animal接口中的抽象方法shout()
+    public void shout() {
+        System.out.println("汪汪...");
+    }
+
+    public void info() {
+        System.out.println("名称：" + NAME);
+    }
+}
+
+public class Example11 {
+    public static void main(String[] args) {
+        System.out.println("编号：" + Animal.getID());
+        Dog dog = new Dog();
+        dog.info();
+        dog.shout();
+        dog.eat();
+    }
+}

src/chapter4/demo45/Circle.java

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+package chapter4.demo45;
+
+/**
+ * 圆形类
+ * 必需实现 Shape 接口中的 getArea 和 getPerimeter 方法
+ */
+public class Circle implements Shape {
+    // 半径
+    private double radius = 0;
+    private final static double PI = 3.14;
+
+    // 有参构造
+    public Circle(double radius) {
+        this.radius = radius;
+    }
+
+    // 返回面积
+    public double getArea() {
+        return PI * radius * radius;
+    }
+
+    // 返回周长
+    public double getPerimeter() {
+        return 2 * PI * radius;
+    }
+
+}

src/chapter4/demo45/Main.java

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+package chapter4.demo45;
+
+import java.util.Scanner;
+
+public class Main {
+    public static void main(String[] args) {
+        // 处理用户输入
+        Scanner sc = new Scanner(System.in);
+
+        while(true) {
+            // 打印菜单
+            ShapeCalculate.printMenu();
+            int select = sc.nextInt();
+            // 判断用户选项
+            switch (select) {
+                case 1: // 圆形
+                    System.out.print("请输入圆形的半径：");
+                    double radius = sc.nextDouble();
+                    Shape circle = new Circle(radius);
+                    ShapeCalculate.printArea(circle);
+                    ShapeCalculate.printPerimeter(circle);
+                    break;
+                case 2: // 长方形
+                    System.out.print("请输入长方形的长和宽：");
+                    double length = sc.nextDouble();
+                    double width = sc.nextDouble();
+                    Shape rectangle = new Rectangle(length, width);
+                    ShapeCalculate.printArea(rectangle);
+                    ShapeCalculate.printPerimeter(rectangle);
+                    break;
+                default:
+                    System.out.println("输入错误，请重新选择");
+                    ShapeCalculate.printMenu();
+                    break;
+            }
+            // 询问用户是否继续
+            System.out.println("继续计算？（Y / N）");
+            String answer = sc.next();
+            if ("N".equalsIgnoreCase(answer)) { // 无论大小写，都识别
+                break;
+            }
+        }
+        System.out.println("感谢您的使用，再见！");
+        sc.close();
+    }
+
+
+
+}

src/chapter4/demo45/Rectangle.java

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+package chapter4.demo45;
+
+/**
+ * 长方形类
+ * 必需实现 Shape 接口中的 getArea 和 getPerimeter 方法
+ */
+
+public class Rectangle implements Shape {
+    // 长度
+    private double length = 0;
+    // 宽度
+    private double width = 0;
+
+    // 有参构造
+    public Rectangle(double length, double width) {
+        this.length = length;
+        this.width = width;
+    }
+
+    // 获取面积
+    public double getArea() {
+        return length * width;
+    }
+
+    // 获取周长
+    public double getPerimeter() {
+        return 2 * length + 2 * width;
+    }
+}

src/chapter4/demo45/Shape.java

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+package chapter4.demo45;
+
+/**
+ * 定义“图形”接口，只拥有抽象方法
+ * 后续的圆形和长方形都实现该接口，即圆形和长方形都需要实现 getArea 和 getPerimeter 方法
+ * 通过接口，可以制订“规范、规范”
+ */
+public interface Shape {
+    // 获取面积，省略 public abstract
+    double getArea();
+    // 获取周长，省略 public abstract
+    double getPerimeter();
+}
+
+

src/chapter4/demo45/ShapeCalculate.java

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+package chapter4.demo45;
+
+/**
+ * 图形计算器类
+ * 1. final 类：防止被继承
+ * 2. 私有构造函数（不可被实例化）
+ * 只有静态方法，不需要实例化，所有方法都可以通过类名调用
+ */
+public final class ShapeCalculate {
+    private ShapeCalculate() {
+        throw new AssertionError("不应该被实例化");
+    }
+
+    // 打印图形面积
+    public static void printArea(Shape shape) {
+        System.out.println("面积："+shape.getArea());
+    }
+
+    // 打印图形周长
+    public static void printPerimeter(Shape shape) {
+        System.out.println("周长："+shape.getPerimeter());
+    }
+
+    // 打印计算器菜单
+    public static void printMenu() {
+        System.out.println("==========图形计算器==========");
+        System.out.println("1.圆形");
+        System.out.println("2.长方形");
+        System.out.print("请输入序号选择功能：");
+    }
+}

src/chapter4/example07/Example07.java

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+package chapter4.example07;
+
+final class Animal {
+    // 空
+}
+
+//class Dog extends Animal {
+//
+//}
+
+public class Example07 {
+    public static void main(String[] args) {
+//        Dog dog = new Dog();
+    }
+}

src/chapter4/example10/Example10.java

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+package chapter4.example10;
+
+abstract class Animal {
+    abstract void shout();
+
+}
+
+class Dog extends Animal {
+    void shout() {
+        System.out.println("汪汪...");
+    }
+}
+
+public class Example10 {
+    public static void main(String[] args) {
+        Animal dog = new Dog();
+        dog.shout();
+    }
+}