java 重写时返回类型疑惑 #848

Zouxxyy · 2020-07-08T10:34:29Z

Java基础知识 1.4.3.2.重写中指出重写时返回值类型相同；我觉得应改为返回类型小于等于父类方法返回类型。

我注意到 issue#723 改正了这一说法，能说说原因吗？感谢！

下面是我在JDK 1.8进行的测试

public class A {

    public Number fun() {
        System.out.println("A");
        return null;
    }
}

class B extends A {

    @Override
    public Integer fun() {
        System.out.println("B");
        return null;
    }

    public static void main(String[] args) {
        A a = new B();
        a.fun();
    }
}

// 输出 B

wangpeipei90 · 2020-07-08T19:26:47Z

你的例子应该是正确的。
Returns type can be different only if the parent class method return type is
a supertype of child class method return type.
See StackOverflow post:

Rorke76753 · 2020-07-24T01:59:33Z

为什么我用跟你相同的代码测试结果不一样呢，我也是jdk8但输出的是子类的内容

public class ChildrenClass extends SuperClass{
    @Override
    public Integer test() {
        System.out.println("int");
        return null;
    }

    public static void main(String[] args) {
        SuperClass clazz = new ChildrenClass();
        clazz.test();
    }
}

class SuperClass {
    public Number test(){
        System.out.println("number");
        return null;
    }
}

每次输出的都是int

Zouxxyy · 2020-07-24T03:02:50Z

为什么我用跟你相同的代码测试结果不一样呢，我也是jdk8但输出的是子类的内容

public class ChildrenClass extends SuperClass{
    @Override
    public Integer test() {
        System.out.println("int");
        return null;
    }

    public static void main(String[] args) {
        SuperClass clazz = new ChildrenClass();
        clazz.test();
    }
}

class SuperClass {
    public Number test(){
        System.out.println("number");
        return null;
    }
}

每次输出的都是int

是的，就是输出子类的，表示重写成功，我当时好像忘记改了，我怎么会写aaa。。。

Zouxxyy closed this Jul 24, 2020

Zouxxyy reopened this Jul 24, 2020

Snailclimb / JavaGuide

java 重写时返回类型疑惑 #848

java 重写时返回类型疑惑 #848

Zouxxyy commented Jul 8, 2020 •

edited

wangpeipei90 commented Jul 8, 2020

Rorke76753 commented Jul 24, 2020 •

edited

Zouxxyy commented Jul 24, 2020

Snailclimb / JavaGuide

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java 重写时返回类型疑惑 #848

java 重写时返回类型疑惑 #848

Comments

Zouxxyy commented Jul 8, 2020 • edited

wangpeipei90 commented Jul 8, 2020

Rorke76753 commented Jul 24, 2020 • edited

Zouxxyy commented Jul 24, 2020

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Zouxxyy commented Jul 8, 2020 •

edited

Rorke76753 commented Jul 24, 2020 •

edited