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.