Java。为什么我不能将接口对象转换为 class 对象？

Question

我有这个界面：

public interface Numeric {
    public Numeric addition(Numeric x,Numeric y);
    public Numeric subtraction(Numeric x,Numeric y);
}

还有这个class：

public class Complex implements Numeric {
    private int real;
    private int img;

    public Complex(int real, int img) {
        this.real = real;
        this.img = img;
    }

    public Numeric addition(Numeric x, Numeric y) {
        if (x instanceof Complex && y instanceof Complex) {
            Complex n1 = (Complex)x;
            Complex n2 = (Complex)y;

            return new Complex(n1.getReal() + n1.getReal(), n2.getImg() + 
            n2.getImg());                 
        } 
        throw new UnsupportedOperationException();
    }

    public Numeric subtraction(Numeric x, Numeric y) {
        if (x instanceof Complex && y instanceof Complex) {
            Complex n1 = (Complex)x;
            Complex n2 = (Complex)y;

            return new Complex(n1.getReal() - n1.getReal(), n2.getImg() - 
            n2.getImg());                 
        } 
        throw new UnsupportedOperationException();
    }

    public int getReal() {
        return real;
    }

    public int getImg() {
        return img;
    }
}

为什么会出现此错误：

incompatible types: Numeric cannot be converted to Complex

当我运行这个代码时：

public class TestNumeric {
    public static void main(String[] args) {
        Complex c1 = new Complex(3, 4);
        Complex c2 = new Complex(1, 2);
        Complex rez;

        rez = rez.addition(c1, c2);
    }
}

错误在行 "rez = rez.addition(c1, c2);"
Complex 实现了 Numeric，所以每个 Numeric 都是 Complex，对吧？我已经在添加方法中完成了转换和检查。为什么会出现此错误，我该如何解决？

Answer 1

addition和subtraction的声明应该如下：

public interface Numeric {
    public Numeric addition(Numeric obj);

    public Numeric subtraction(Numeric obj);
}

和addition和subtraction的执行应该如下：

public Numeric addition(Numeric obj){
    if (obj instanceof Complex){
        Complex n = (Complex)obj;

        return new Complex(this.getReal() + n.getReal(), this.getImg() + 
        n.getImg()); 
    } else {
        throw new UnsupportedOperationException();
    }
}

最后，TestNumeric应该是这样的：

public class TestNumeric {
    public static void main(String[] args) {
        Numeric c1 = new Complex(3, 4);
        Numeric c2 = new Complex(1, 2);
        Numeric rez = c1.addition(c2);
    }
}

[更新]

@OrosTom - 根据您的评论，我在下面添加了您需要放在 class、Complex 中的方法，以便您可以打印结果

@Override
public String toString() {
    return real + " + " + img + "i";
}

注意：请查看 https://docs.oracle.com/javase/tutorial/java/IandI/objectclass.html（部分，toString() 方法）以获取更多详细信息。

在此之后，输出以下代码

public class TestNumeric {
    public static void main(String[] args) {
        Numeric c1 = new Complex(3, 4);
        Numeric c2 = new Complex(1, 2);
        Numeric rez = c1.addition(c2);
        System.out.println(c1);
        System.out.println(c2);
        System.out.println(rez);
    }
}

将是：

3 + 4i
1 + 2i
4 + 6i

Java。为什么我不能将接口对象转换为 class 对象？

Java. Why I can't convert a interface object to a class object?

java

oop

interface

return-type

supertype