Java 入门教程（六）：面向对象（下）——继承、多态与接口

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abstract class Shape {                      // abstract 类不能 new
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    protected String name;
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    protected Shape(String name) { this.name = name; }
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    public abstract double area();          // 抽象方法：无实现，子类必须实现
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    public void describe() {                // 普通方法：可复用
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        System.out.printf("%s 的面积 = %.2f%n", name, area());   // 多态调用
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    }
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}
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class Circle extends Shape {
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    private final double r;
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    public Circle(double r) { super("圆"); this.r = r; }
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    @Override
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    public double area() { return Math.PI * r * r; }
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}
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class Rect extends Shape {
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    private final double w, h;
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    public Rect(double w, double h) { super("矩形"); this.w = w; this.h = h; }
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    @Override
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    public double area() { return w * h; }
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}
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// Shape s = new Shape("x");   ❌ 抽象类不能实例化
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Shape s = new Circle(2);
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s.describe();

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class Money {
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    private final long cents;
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    Money(long cents) { this.cents = cents; }
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    // 1. equals：内容比较（== 才是引用比较）
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    @Override
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    public boolean equals(Object o) {
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        if (this == o) return true;                       // 同一对象
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        if (!(o instanceof Money)) return false;          // 类型检查
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        return cents == ((Money) o).cents;
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    }
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    // 2. hashCode：equals 相等 → hashCode 必须相等（HashMap 约定）
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    @Override
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    public int hashCode() { return Long.hashCode(cents); }
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    // 3. toString：打印友好
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    @Override
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    public String toString() { return "¥" + cents / 100.0; }
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}
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Money a = new Money(1050), b = new Money(1050);
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System.out.println(a.equals(b));    // true
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System.out.println(a);              // ¥10.5
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// IDEA：Alt+Insert 一键生成三者，日常用工具生成

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import java.util.List;
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interface Area {
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    double area();
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    // 接口 default 提供组合逻辑
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    default double areaDifference(Area other) {
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        return this.area() - other.area();
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    }
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}
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class Circle2 implements Area {
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    private final double r;
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    Circle2(double r) { this.r = r; }
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    @Override public double area() { return Math.PI * r * r; }
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    @Override public String toString() { return String.format("圆(r=%.1f)", r); }
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}
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class Square implements Area {
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    private final double side;
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    Square(double side) { this.side = side; }
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    @Override public double area() { return side * side; }
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    @Override public String toString() { return String.format("正方形(边%.1f)", side); }
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}
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public class Shapes {
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    public static void main(String[] args) {
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        List<Area> shapes = List.of(new Circle2(2), new Square(3));
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        double total = 0;
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        Area biggest = shapes.get(0);
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        for (Area s : shapes) {
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            System.out.printf("%s = %.2f%n", s, s.area());
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            total += s.area();
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            if (s.area() > biggest.area()) biggest = s;
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        }
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        System.out.printf("总面积 %.2f，最大的是 %s%n", total, biggest);
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        System.out.printf("圆比正方形大 %.2f%n",
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                shapes.get(0).areaDifference(shapes.get(1)));
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    }
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}