如何填写列表<？延伸形状&gt;与派生的圆形和矩形对象？ [英] How to fill List&lt;? extends Shape&gt; with derived Circle and Rectangle objects?

问题描述

  public abstract class Shape {
 public abstract void draw（Canvas c） ; 
} 
 
 public class Circle扩展形状{
 private int x，y，radius; 
 public void draw（Canvas c）{...} 
} 
 
 public class Rectangle extends Shape {
 private int x，y，width，height; 
 public void draw（Canvas c）{...} 
} 
  

这些类可以绘制在画布上：

  public class Canvas {
 public void draw（Shape s）{
 s.draw（this）; 
} 
} 
  

我想将所有的Circle和Rectangle对象放入一个List，并在单个迭代语句中将它们全部绘制，如下所示：

  public void drawAll（List< ;? extends Shape> ;形状）{
（形状s：形状）{
 s.draw（this）; 
} 
} 
  

一切都在编译。

如果我创建一个测试类并尝试创建如下列表，则问题开始：

  public class Main {
 
 public static void main（String [] args）{
 
 Canvas canvas = new Canvas（）; 
 
列表< ;?延伸形状> list = new ArrayList<>（）; 
 Circle circle = new Circle（）; 
 Rectangle rectangle = new Rectangle（）; 
 list.add（circle）; //类型List< capture＃1-of？中的方法add（捕获＃1 of？extends Shape）延伸形状>不适用于参数（Circle）
 canvas.drawAll（list）; 
 
 
 
 
 
 
 $ b 
正如代码中所记录的那样，我不能将Circle和Rectangle对象添加到List中。
 
 
 
问题：我应该如何修改代码，以便构建一个
 列表与LT ;?延伸形状> 
  
然后遍历该列表绘制形状。 
 
 
在此先感谢。
 
 
 
 发布编辑：谢谢！这是一个很大的突破。应该给SpaceTrucker和他的关键PECS链接的信贷，但这是不可能的。由于某种原因，我告诉自己我应该使用extends，并且从此不会对此作出任何质疑。尝试指定<$ c    $ c> List  type 
  List< Shape> list = new ArrayList< Shape>（）; 
 Shape circle = new Circle（）; 
 Rectangle rectangle = new Rectangle（）; 
 list.add（circle）; 
 list.add（矩形）; 
 canvas.drawAll（list）; 
  
 
I have these simple classes:
public abstract class Shape {
    public abstract void draw(Canvas c);
}

public class Circle extends Shape {
    private int x, y, radius;
    public void draw(Canvas c) { ... }
}

public class Rectangle extends Shape {
    private int x, y, width, height;
    public void draw(Canvas c) { ... }
}
These classes can be drawn on a canvas:
public class Canvas {
    public void draw(Shape s) {
        s.draw(this);
    }
}
I would like to put all Circle and Rectangle objects in a single List and draw all of them in a single iteration statement, like this:
public void drawAll(List<? extends Shape> shapes) {
    for (Shape s : shapes) {
        s.draw(this);
    }
}
Everything compiles.


The problem begins if I make a test class and try to create a list like this:
public class Main {

    public static void main(String[] args) {

        Canvas canvas = new Canvas();

        List<? extends Shape> list = new ArrayList<>();
        Circle circle = new Circle();
        Rectangle rectangle = new Rectangle();
        list.add(circle); // The method add(capture#1-of ? extends Shape) in the type List<capture#1-of ? extends Shape> is not applicable for the arguments (Circle)
        canvas.drawAll(list);
    }
}
As is documented in the code, I cannot add Circle and Rectangle objects to the List.

Question: how should I modify the code such, that I can construct a 
List<? extends Shape> 
and next iterate over that List to draw the shapes.

thanks in advance.

Post Edit: Thanks! That was quite a breakthrough. Should have given the credits to SpaceTrucker and his crucial PECS link, but that wasn't possible. For some reason I had told myself that I should use extends and never questioned that assumption since.
解决方案
Try to specify List type
List<Shape> list = new ArrayList<Shape>();
Shape circle = new Circle();
Rectangle rectangle = new Rectangle();
list.add(circle);
list.add(rectangle);
canvas.drawAll(list);


                        
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