数组与泛型容器区别

class Shape{
    void draw(){
        System.out.println(this+".draw()");
    }
}

class Circle extends Shape{
    @Override
    public String toString() {
        return "Circle";
    }
}
class Rect extends Shape{
    @Override
    public String toString() {
        return "Rect";
    }
}

class Triangle extends Shape{
    @Override
    public String toString() {
        return "Triangle";
    }
}

class Square extends Rect{
    @Override
    public String toString() {
        return "Square";
    }
}

 1         Shape[] shapes = new Rect[10];
 2         shapes[0] = new Rect();
 3         //Shape类型的引用当然可以放入Circle。但数组在运行时能保留它存储的对象类型的倣，运行时是Rect，因此运行出错
 4         //shapes[1] = new Circle(); // java.lang.ArrayStoreException
 5 
 6          //数组与泛型容器的区别：数组有协变类型，而泛型容器没有内建协变类型
 7         List<Shape> list = new ArrayList<Rect>();  //错误
 8         List<? extends Shape> list = new ArrayList<Rect>();
 9         //放不进任何元素。
10         //list.add(new Circle());
11         //list.add(new Rect());
12         
13         List<Rect> list = new ArrayList<Rect>();
14         //list.add(new Circle()); //泛型容器将运行时检查提前到编译期
15         list.add(new Rect());
16         list.add(new Square());
17         for(Rect rect : list){
18             rect.draw();
19         }