java保护方法可访问性

java protected method accessibility

提问by JavaUser

In the below code the Consumer class can access the protected method of Parent class.How is it possible since there is no relation between Parent and Consumer class.Please explain

在下面的代码中，消费者类可以访问父类的受保护方法。父类和消费者类之间没有关系，这怎么可能。请解释

class Parent {
    public void method1(){
        System.out.println("PUBLIC METHOD");
    }
    private void method2(){
        System.out.println("PRIVATE METHOD");
    }
    protected void method3(){
        System.out.println("PROTECTED METHOD");
    }
}

public class Consumer {
    public static void main(String[] args){
        Parent parentObj = new Parent();
        parentObj.method1();
        //parentObj.method2();
        parentObj.method3();
    }
}

Thanks

谢谢

采纳答案by atamanroman

protectedmeans: same package or by inheritance. Since your classes are both in the default package(not recommended in real life), protectedenables access. By the way: if you tried to test java access control, you forgot default access(default access= no modifier = package private).

protected表示：相同的包或通过继承。由于您的课程都在default package（现实生活中不推荐）中，因此protected启用访问。顺便说一句：如果您尝试测试 java 访问控制，您忘记了default access( default access= no modifier = package private)。

privateaccess on the other hand means: access from nowhere except this particular class (and non-static inner classes, which are still member of the host-class).

private另一方面，访问意味着：除了这个特定的类（和非静态内部类，它们仍然是宿主类的成员）之外，从任何地方都可以访问。

回答by Adam Crume

First of all, they're in the same package. Secondly, Consumer is an inner class of Parent.

首先，它们在同一个包中。其次，Consumer 是 Parent 的内部类。

回答by Rakesh Goyal

Adam you are right! They are in the same package thats why consumer class is able to access protected method of parent class. Consumer is not an inner class of parent.

亚当你是对的！它们在同一个包中，这就是为什么消费者类能够访问父类的受保护方法。消费者不是父类的内部类。

回答by Monis Iqbal

Due to package protection.

由于包装保护。

http://mindprod.com/jgloss/packagescope.html

http://mindprod.com/jgloss/packagescope.html

回答by polygenelubricants

Here are the relevant excerpts from the Java Language Specification:

以下是 Java 语言规范的相关摘录：

JLS 6.6 Access Control

The Java programming language provides mechanisms for access control, to prevent the users of a packageor classfrom depending on unnecessary details of the implementation of that packageor class. If access is permitted, then the accessed entity is said to be accessible.

JLS 6.6.1 Determining Accessibility

• [...]
• A member/constructor of a reference type is accessible only if the type is accessible and the member/constructor is declared to permit access:
  • public: access is permitted.
  • protected: access is permitted only when one of the following is true:
    • Access to the member or constructor occurs from within thepackagecontaining the class in which theprotectedmember or constructor is declared.
    • Access is correct as described in JLS 6.6.2 Details on protectedAccess.
      • A protectedmember/constructor of an object may be accessed from outside the packagein which it is declared only by code that is responsible for the implementation of that object.
  • private: access is permitted if and only if it occurs within the body of the top level class that encloses the declaration of the member or constructor.
  • Otherwise, we say there is default access, which is permitted only when the access occurs from within the packagein which the type is declared.

JLS 6.6 访问控制

Java编程语言提供了机制的访问控制，以防止一个用户package或class从依赖于该实现的不必要的细节package或class。如果允许访问，则称被访问的实体是可访问的。

JLS 6.6.1 确定可访问性

• [...]
• 仅当类型可访问且成员/构造函数声明为允许访问时，引用类型的成员/构造函数才可访问：
  • public: 允许访问。
  • protected：仅当以下情况之一为真时才允许访问：
    • 对成员或构造函数的访问发生在package包含声明该成员或构造函数的类中protected。
    • 访问是正确的，如JLS 6.6.2protected访问详细信息中所述。
      • 甲protected物体的构件/构造可以从外部的访问package在其声明仅通过代码，负责该对象的执行。
  • private: 当且仅当它出现在包含成员或构造函数声明的顶级类的主体内时，才允许访问。
  • 否则，我们说有默认访问，只有当访问发生在package声明类型的内部时才被允许。

The section in bold is the answer to the question in this scenario: Parentand Consumerbelong to the same package, thus, at the very least, protectedmembers of Parentare accessible from Consumer.

粗体部分是此场景中问题的答案：Parent和Consumer属于相同的package，因此，至少， 的protected成员Parent可以从 访问Consumer。

See also

也可以看看

• Java Tutorials/Controlling Access to Members of a Class

• Java 教程/控制对类成员的访问