Accessing Protected Variable Of An Instance Of A Class That Extends An Abstract Base Class?

Question

I have an abstract class called MyAction which contains a protected enum variable. The class is defined as follows:

package mypackage;

public abstract class MyAction {
    public enum ActionId {
        ACTION1, ACTION2;
    }

    protected ActionId actionId;

    // constructor
    public MyAction(ActionId actionId) {
        this.actionId = actionId;
    }

    public ActionId getActionId() {
        return actionId;
    }    
    ...
    ...
}

I created a specific action, MyAction1, that extends MyAction:

package mypackage;

public class MyAction1 extends MyAction {
    public MyAction1() {
        super(ActionId.ACTION1);
    }
    ...
    ...
}

I have a singleton utility class (in the same package) that creates an instance of MyAction1 and stores it in a HashMap:

package mypackage;

public class MyActionFactory {
    private static MyActionFactory theInstance;
    private HashMap<ActionId, MyAction> actions;

    private MyActionFactory() {
        actions = new HashMap<ActionId, MyAction>();
        MyAction1 myAction1 = new MyAction1();
        actions.put(myAction1.actionId, myAction1); // able to access protected variable actionId
    }

    public static VsActionFactory getInstance() {
        if (theInstance == null)
            theInstance = new VsActionFactory();
        return theInstance;
    }    
    ...
    ...
}

Note that in the method actions.put(myAction1.actionId, myAction1) I am able to access the protected member actionId.

Why is it that I can access the protected member actionId (contained in the base class MyAction) of the instance of MyAction1? I thought protected members were only accessible to subclasses.

Does it have anything to do with MyActionFactory being in the same package as the others?

Protected members are accessible in all classes of same package. http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html — Rohit Jain, Sep 01 '13 at 16:17
Oh. I thought it was only to subclasses. That was an easy answer. :) — Jan Tacci, Sep 01 '13 at 16:20

score 1 · Accepted Answer · answered Sep 01 '13 at 16:18

1

The protected keyword makes things visible within the same package. Which is the case, because both of your classes are in package mypackage.

Here is a nice table, taken from Oracle.com:

enter image description here

answered Sep 01 '13 at 16:18

Martijn Courteaux

67,591
47
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Now I know that. I guess I could have tested it by moving the utility class to another package. – Jan Tacci Sep 01 '13 at 16:20
So there is no way of making a variable in the base class that's only accessible to subclasses and not to other classes in the same package? – Jan Tacci Sep 01 '13 at 16:21
No that is not possible. – Martijn Courteaux Sep 01 '13 at 16:22
I noticed something strange looking at your table. If you don't define a modifier then other classes can access it but not sub-classes (even if the sub-class is defined in the same package)? – Jan Tacci Sep 01 '13 at 16:25
The best way to sum it up is to see that each higher level of access fully encompasses lower levels: it's a strict hierarchy of expanding circles. – Marko Topolnik Sep 01 '13 at 16:25
Good way to look at it! – Jan Tacci Sep 01 '13 at 16:25
@MarkoTopolnik: Indeed. However, to me it feels like the level of package and subclass should be swapped around. What do you think? – Martijn Courteaux Sep 01 '13 at 16:26
1

You mean, swap the Package and Subclas *columns*, so that we get a modifier which allows *only* the subclasses in? The way it is works better for me: a package is a set of closely coupled classes, they are all "friends" in C++ terms. `protected`, on the other hand, is a form of public access, to be used in classes designed for public extension. It would make little sense to allow public extension, but preclude package-private access. – Marko Topolnik Sep 01 '13 at 16:31
Oh, you are right! I was mixing up the concept inner class with subclass. When I saw the table and I was thinking of subclass as of a, innerclass, I was disappointed. Thanks for pointing that out. – Martijn Courteaux Sep 01 '13 at 17:30

score 0 · Answer 2 · answered Sep 01 '13 at 16:24

0

Protected access modifier allows access in the same package + sub classes in other packages.You can remember it as default access plus inheritance.

answered Sep 01 '13 at 16:24

Malwaregeek

2,274
3
15
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Accessing Protected Variable Of An Instance Of A Class That Extends An Abstract Base Class?

2 Answers2