Java多态: How to indicate comparison should be made using subclass?

Question

介绍性问题来了!以下代码...

public class HelloWorld
{
  public static void main(String[] args)
  {
    ArrayList<Object> A = new ArrayList<Object>();
    Integer I = new Integer(3);
    Double D = new Double(3.14);

    A.add(I);
    A.add(D);

    if (A.get(0) > 2)
    {
        System.out.print("Hello World");      
    }  
  }
}

...编译失败

bad operand types for binary operator '>'
        if (A.get(0) > 2)
                     ^
  first type:  Object
  second type: int

如何表明我希望与子类 Integer 进行比较，而不是与 Object 进行比较？

Answer 1

由于Integer和Double都扩展了Number，然后将ArrayList重新定义为

ArrayList<Number> A = new ArrayList<Number>();

然后

if (A.get(0).intValue () > 2)