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

Answer 1

If protected, your instance of Join cannot access the mStructure in other instances (relRight, relLeft) outside the package.

EDIT:

The table here explains this situation fairly well. I marked the culprit in your question with []s

Access Levels
Modifier    Class Package Subclass  World
public      Y     Y       Y         Y
protected   Y    [Y]      Y         N
no modifier Y     Y       N         N
private     Y     N       N         N