如何在java中为两个类实现一个通用方法

Question

我有一个有一个普通方法和一个通用方法的接口。我已经为两个不同的类实现了普通的方法，但是现在不用如何用泛型方法来实现。这里是我的代码：

Sphere.java：

public class Sphere implements GeometricShape<Sphere> { 

    private double radius; 

    public Sphere (double radius) { 
     this.radius = radius; 
    } 

    public double volume() { 
     return (4.0/3.0) * Math.PI * radius * radius * radius; 
    } 

    public void describe() { 
     System.out.println("Sphere[radius=" + radius + "]"); 
    } 

    @Override 
    public Sphere supersize() 
    { 
     this.radius*=2; 
     return new Sphere(radius); 
    } 

} 

Rectangle.java

public class Rectangle implements TwoDShape { 

    private double width, height; 

    public Rectangle (double width, double height) { 
     this.width = width; 
     this.height = height; 
    } 

    public double area() 
    { 
     return width * height; 
    } 

    public double perimeter() 
    { 
     return 2.0 * (width + height); 
    } 

    public void describe() 
    { 
     System.out.println("Rectangle[width=" + width + ", height=" + height + "]"); 
    } 

    @Override 
    public Rectangle supersize() 
    { 

     this.width*=2; 
     this.height*=2; 
     return new Rectangle(width, height); 
    } 


} 

TwoDShape.java：

public interface TwoDShape extends GeometricShape 
{ 
    public double area(); 

} 

ThreeDShape.java：

public interface ThreeDShape extends GeometricShape<ThreeDShape> 
{ 
    public double volume(); 
} 

GeometricShape.java：

public interface GeometricShape<T extends GeometricShape<T>> 
{ 
    public void describe(); 
    public T supersize(); 

} 

最后主类ArrayListExample.java：

import java.util.ArrayList; 


public class ArrayListExample { 



    public static void describe_all(ArrayList<? extends GeometricShape> shapes) 

    { 
     for(int i=0;i<shapes.size();i++) 
     { 
      shapes.get(i).describe(); 

     } 
     System.out.println("Total number of shapes:"+ shapes.size()); 
    } 



    public static void main(String[] args) { 


     System.out.println("The describe() method:"); 

     System.out.println(); 
     System.out.println("Example rectangles"); 
     ArrayList<Rectangle> rects = new ArrayList<Rectangle>(); 
     rects.add(new Rectangle(2.0, 3.0)); 
     rects.add(new Rectangle(5.0, 5.0)); 
     describe_all(rects); 
     System.out.println(); 


     ArrayList<Sphere> spheres = new ArrayList<Sphere>(); 
     spheres.add(new Sphere(10.0)); 
     spheres.add(new Sphere(50.0)); 
     spheres.add(new Sphere(0.0)); 

     System.out.println("Example spheres"); 
     describe_all(spheres); 
     System.out.println(); 
     System.out.println("The supersize() method:"); 
     System.out.println(); 

     ArrayList<Rectangle> double_rects = supersize_list(rects); 
     describe_all(double_rects); 
     System.out.println(); 

     ArrayList<Sphere> double_spheres = supersize_list(spheres); 
     describe_all(double_spheres); 

    } 


} 

我如何能实现supersize_list方法，它需要超大方法从两个矩形和领域和输出，如

Rectangle[width=4.0, height=6.0] 
Rectangle[width=10.0, height=10.0] 
Total number of shapes: 2 

Sphere[radius=20.0] 
Sphere[radius=100.0] 
Sphere[radius=0.0] 
Total number of shapes: 3 

你能帮我解决这个问题吗？我非常感谢你的帮助！

Answer 1

的类层次看起来不协调。例如，您同时拥有ThreeDShape extends GeometricShape<ThreeDShape>和TwoDShape extends GeometricShape，原因不明。为这些类型编写泛型方法并不有趣。

这是一个不太令人困惑的版本。 （我希望）注意：我选择不在supersize方法中更改形状本身的大小，而是让它在保持原始大小不变的情况下返回更大的形状。

1. GeometricShape

/** 
* A geometric shape interface. You can do two things with it. 
* 1. Ask it to describe itself (to stdout); 
* 2. Ask it to return a bigger version of itself (double the size). 
*/ 
public interface GeometricShape<T extends GeometricShape<T>> { 
    /** 
    * Print a description to STDOUT 
    */ 
    void describe(); 

    /** 
    * Returns a bigger shape. 
    * @return Something that's a GeometricShape 
    */ 
    T supersize(); 
} 

2. Shape2D和矩形

/** 
* A 2-dimensional shape. 
* It has area. 
* Its supersize() method should return a Shape2D instance. 
*/ 
public interface Shape2D<T extends Shape2D<T>> extends GeometricShape<T> { 

    double area(); 
} 

/** 
* A rectangle. 
*/ 
public final class Rectangle implements Shape2D<Rectangle> { 

    private final double width; 
    private final double height; 

    public Rectangle(double width, double height) { 
     this.width = width; 
     this.height = height; 
    } 

    @Override 
    public String toString() { 
     return "Rectangle{" + 
       "width=" + width + 
       ", height=" + height + 
       '}'; 
    } 

    @Override 
    public void describe() { 
     System.out.println(this); 
    } 

    @Override 
    public Rectangle supersize() { 
     return new Rectangle(width*2, height*2); 
    } 

    @Override 
    public double area() { 
     return width * height; 
    } 
} 

3.了Shape3D和球

/** 
* A 3-dimensional shape. 
* It has volume. 
* Its supersize() method should return a Shape3D instance. 
*/ 
public interface Shape3D<T extends Shape3D<T>> extends GeometricShape<T> { 

    double volume(); 
} 

/** 
* A sphere 
*/ 
public final class Sphere implements Shape3D<Sphere> { 
    private final double radius; 

    public Sphere(double radius) { 
     this.radius = radius; 
    } 

    @Override 
    public String toString() { 
     return "Sphere{" + 
       "radius=" + radius + 
       '}'; 
    } 

    @Override 
    public void describe() { 
     System.out.println(this); 
    } 

    @Override 
    public Sphere supersize() { 
     return new Sphere(radius*2); 
    } 

    @Override 
    public double volume() { 
     return 4*Math.PI*Math.pow(radius, 3)/3; 
    } 
} 

现在转换列表

public static <T extends GeometricShape<T>> 
List<T> supersize_list(List<T> list) { 
    List<T> result = new ArrayList<>(); 
    for (T shape : list) { 
     result.add(shape.supersize()); 
    } 
    return result; 
} 

Answer 2

你不需要返回一个新的对象。对于Rectangle例如

@Override 
public void supersize() 
{ 

    this.width*=2; 
    this.height*=2; 
} 

足够