当我的子类位于不同的包中时，为什么我的子类不能访问其超类的受保护变量？ [英] Why can't my subclass access a protected variable of its superclass, when it's in a different package?

问题描述

我在包 database.relation 中有一个抽象类 relation 及其子类加入，在包 database.operations 中。 relation 有一个名为 mStructure 的受保护成员。

I have an abstract class, relation in package database.relation and a subclass of it, Join, in package database.operations. relation has a protected member named mStructure.

在加入：

public Join(final Relation relLeft, final Relation relRight) {
        super();
        mRelLeft = relLeft;
        mRelRight = relRight;
        mStructure = new LinkedList<Header>();
        this.copyStructure(mRelLeft.mStructure);

        for (final Header header :mRelRight.mStructure) {
        if (!mStructure.contains(header)) {
            mStructure.add(header);
        }
    }
}

在线

this.copyStructure(mRelLeft.mStructure);

和

for (final Header header : mRelRight.mStructure) {

我收到以下错误：

字段Relation.mStructure不可见

The field Relation.mStructure is not visible

如果我将两个类放在同一个包中，这样就可以了。任何人都可以解释这个问题吗？

If I put both classes in the same package, this works perfectly. Can anyone explain this issue?

推荐答案

它有效，但只有孩子们试图访问它拥有变量，而不是其他实例的变量（即使它属于同一个继承树）。

It works, but only you the children tries to access it own variable, not variable of other instance ( even if it belongs to the same inheritance tree ).

请参阅此示例代码以更好地理解它：

See this sample code to understand it better:

//in Parent.java
package parentpackage;
public class Parent {
    protected String parentVariable = "whatever";// define protected variable
}

// in Children.java
package childenpackage;
import parentpackage.Parent;

class Children extends Parent {
    Children(Parent withParent ){
        System.out.println( this.parentVariable );// works well.
        //System.out.print(withParent.parentVariable);// doesn't work
    } 
}

如果我们尝试使用 withParent.parentVariable 进行编译，我们得到：

If we try to compile using the withParent.parentVariable we've got:

Children.java:8: parentVariable has protected access in parentpackage.Parent
    System.out.print(withParent.parentVariable);

可以访问，但只能访问自己的变量。

It is accessible, but only to its own variable.

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