具有接口成员变量和成员方法的 Java 类的行为不同。请解释

Question

接口和一个类Number1，后来的Number2等。 当我尝试在方法add() 中使用成员数据时，它要求我首先将其转换为类类型。 但是，如果我在方法add() 中使用成员方法，则不需要我强制转换为类类型。 任何解释将不胜感激。

附上以下代码。

---

    package mynums;
//Interface for all my number types, Number1, Number2 (not shown), etc
public interface NumberIF
{

    public int getNum();

    public void setNum(int numx);

    //I will have other types of numbers not just Number1.
    //I will have Number2, etc.
    public void add(NumberIF f1, NumberIF f2);
    public void print();
}


    package mynums;
//There will be other types of numbers Number2, Number3
//all doing these operations but different.
//This is just to test the concept.
//But I have a problem here.
public class Number1 implements NumberIF
{
    private int num;

    public Number1()
    {
        num = 1;
    }

    public Number1(int numx)
    {
        num = numx;
    }

    public int getNum()
    {
        return (num);
    }

    public void setNum(int numx)
    {
        num = numx;
    }

    public void add(NumberIF f1, NumberIF f2)
    {
        int numt;

        /**
         * Why to use member variables I must specify the type of class.
         * Why to use member method I do not have to specify type of class.
         */

        //numt = f1.num + f2.num;  ERROR

        // either one works
        //Why accessing member data is different from accessing member method.
        numt = ((Number1)f1).num + ((Number1)f2).num;
        numt = f1.getNum() + f2.getNum();

        num = numt;

    }

    public void print()
    {
        System.out.println(num);
    }

    static public void main(String[] args)
    {
        Number1 f1, f2, f3;

        f1 = new Number1(1);
        f2 = new Number1(2);
        f3 = new Number1(0);

        f3.add(f1, f2); // 1 + 2= 3
        f3.print();
    }
}

Answer 1

Java 是一种静态类型的语言。

静态类型NumberIF 的变量不一定指向Number1 类型的对象，因此这些对象可能没有num 字段。

您只能使用变量的静态类型（及其超类型）的成员。