java - Why can't my subclass access a protected variable of its superclass, when it's in a different package?

Question

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.

In Join:

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);
        }
    }
}

On lines

this.copyStructure(mRelLeft.mStructure);

and

for (final Header header : mRelRight.mStructure) {

I get the following error:

The field Relation.mStructure is not visible

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

Answer 1

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 ).

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
    } 
}

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);

It is accessible, but only to its own variable.