java - 无法从其他包实现抽象类

Question

出于某种原因，我似乎无法在定义它的包之外实现抽象类。 package1 中的抽象类不能在 package2 中的类中实现。为什么这不是合法的 Java？

package com.stackoverflow.abstraction.package1;

abstract public class BaseClass {
    abstract Long foo();
}

package com.stackoverflow.abstraction.package1;

public class Implement1 extends BaseClass {
    @Override
    Long foo() {
        return null;
    }
}


package com.stackoverflow.abstraction.package2;

import com.stackoverflow.abstraction.package1.BaseClass;

/** Compiling this class will output
* - Implement2 is not abstract and does not override abstract method foo() in BaseClass
* - error: method does not override or implement a method from a supertype
*/

public class Implement2 extends BaseClass {
    @Override
    Long foo() {
        return null;
    }
}

运行: OS X 10.6.8 - Java(TM) SE 运行时环境(构建 1.6.0_31-b04-415-10M3646) - OpenJDK 运行时环境(build 1.7.0-u4-b13-20120301) 尝试了两个 Java 版本。当然不是同时:)

Answer 1

该方法是包级可见性，因此您不能在其他包中覆盖它。

abstract Long foo();

If a method has no modifier (the default, also known as package-private), it is visible only within its own package

如果您确实需要覆盖它，请将其设置为 protected。

protected abstract Long foo();

信息

• Controlling Access to Members of a Class
• Java Modifier Summary