Keep out of reach of Children: Removing protected fields from inheritance

Question

In the spirit of well designed OO, a certain class I am extending has marked one of its fields protected. This class has also generously provided a public setter, yet no getter.

I am extending this class with a base class that is in turn extended by several children. How can I restrict access to the protected variable from my children while still being able to manipulate it privately and set it publicly?

See example below:

public abstract class ThirdPartyClass {
  protected Map propertyMap;

  public void setPropertyMap(Map propertyMap){
    this.propertyMap= propertyMap;
  }

  // Other methods that use propertyMap.
}

public abstract class MyBaseClass extends ThirdPartyClass{
// Accessor methods for entries in propertyMap.
  public getFoo(){
    propertyMap.get("Foo");
  }

  public getBar(){
    propertyMap.get("Bar");
  }

 // etc...
}

public class OneOfManyChildren extends MyBaseClass {
// Should only access propertyMap via methods in MyBaseClass.
}

---

I have already found that I can revoke access by making the field private final in MyBaseClass. However that also hinders using the setter provided by the super class.

I am able to circumvent that limitation with the "cleverness" below yet it also results in maintaining two copies of the same map as well as an O(n) operation to copy over every element.

public abstract class MyBaseClass extends ThirdPartyClass{

  private final Map propertyMap = new HashMap(); // Revokes access for children.

  /** Sets parent & grandparent maps. */
  @Override
  public final void setPropertyMap(Map propertyMap){
    super.setPropertyMap(propertyMap);
    this.propertyMap.clear();
    this.propertyMap.putAll(propertyMap);
  }
}

Are there any better ways of accomplishing this?

Note: This is only one example of the real question: How to restrict access to protected fields without maintaining multiple copies?

Note: I also know that if the field were made private in the first place with a protected accessor, this would be a non-issue. Sadly I have no control over that.

Note: IS-A relatonship (inheritance) required.

Note: This could easily apply to any Collection, DTO, or complex object.

Metaphor for those misunderstanding the question:

This is akin to a grandparent having a cookie jar that they leave accessible to all family members and anyone else in their house (protected). A parent, with young children, enters the house and, for reasons of their own, wishes to prevent their children from digging into the cookie jar ad nauseam. Instead, the child should ask the parent for a chocolate chip cookie and see it magically appear; likewise for a sugar cookie or Oreo. They need never know that the cookies are all stored in the same jar or if there even is a jar (black box). This could be easily accomplished if the jar belonged to the parent, if the grandparent could be convinced to put away the cookies, or if the grandparents themselves did not need access. Short of creating and maintaining two identical jars, how can access be restricted for children yet unimpeded for the parent & grandparent?

Answer 1

This might not be possible for you, but if you could derive an interface from ThirdPartyClass and make ThirdPartyClass implement it ?

Then have MyBaseClass act as a decorator by implementing the interface by delegating to a private member ThirdPartyClassImpl.

I.e.

public interface ThirdParty ...


public class ThirdPartyClass implements ThirdParty



public class MyBaseClass implements ThirdParty {

    private ThirdParty decorated = new ThirdPartyClass();



 public class SubclassOne extends MyBaseClass....

etc

Answer 2

Ok, cheating mode on: How about you overwrite de public setter and change the map implementation to a inner class of MyBaseClass. This implementation could throw a exception on all methods of map you dont want your children to access and your MyBaseClass could expose the methods they should use by using an internal method your map implementation... Still has to solve how the ThirdPartyMethod will access those properties, but you could force your code to call a finalizationMethod on your MyBaseClass before use it... I'm just divagating here

EDIT

Like This:

public abstract class MyBaseClass extends ThirdPartyClass{

    private class InnerMapImpl implements Map{
       ... Throw exception for all Map methods you dont want children to use

       private Object internalGet(K key){
           return delegate.get(key);
       }
    }

    public void setPropertyMap(Map propertyMap){
        this.propertyMap= new InnerMapImpl(propertyMap);
    }

    public Object getFoo(){
        return ((InnerMapImpl) propertyMap).internalGet("Foo");
    }


}

Answer 3

Sadly, there's nothing you can do. If this field is protected, it is either a conscious design decision (a bad one IMO), or a mistake. Either way, there's nothing you can do now about it, as you cannot reduce the accessibility of a field.

I have already found that I can revoke access by making the field private final in MyBaseClass.

This isn't exactly true. What you are doing is called variable hiding. Since you are using the same variable name in your subclass, references to the propertyMap variable now point to your private variable in MyBaseClass. However, you can get around this variable hiding very easily, as shown in the code below:

public class A
    {
    protected String value = "A";

    public String getValue ()
        {
        return value;
        }
    }

public class B extends A
    {
    private String value = "B";
    }

public class C extends B
    {
    public C ()
        {
        // super.value = "C"; --> This isn't allowed, as B.value is private; however the next line works
        ((A)this).value = "C";
        }
    }

public class TestClass
    {
    public static void main (String[] args)
        {
        A a = new A ();
        B b = new B ();
        C c = new C ();

        System.out.println (new A ().getValue ()); // Prints "A"
        System.out.println (new B ().getValue ()); // Prints "A"
        System.out.println (new C ().getValue ()); // Prints "C"
        }
    }

So, there's no way you can "revoke" access to the protected class member in the super class ThirdPartyClass. There aren't a lot of options left to you:

• If your child class do not need to know about the class hierarchy above MyBaseClass (i.e. they won't refer to ThirdPartyClass at all), and if you don't need them to be subclasses of ThirdPartyClass then you could make MyBaseClass a class which does not extend from ThirdPartyClass. Instead, MyBaseClass would hold an instance of ThirdPartyClass, and delegate all calls to this object. This way you can control which part of ThirdPartyClass's API you really expose to your subclasses.

  public class MyBaseClass
      {
      private ThirdPartyClass myclass = new ThirdPartyClass ();
  
      public void setPropertyMap (Map<?,?> propertyMap)
          {
          myclass.setPropertyMap (propertyMap);
          }
      }
  

  If you need a direct access to the propertyMap member of ThirdPartyClass from MyBaseClass, then you could define a private inner class and use it to access the member:

  public class MyBaseClass
      {
      private MyClass myclass = new MyClass ();
  
      public void setPropertyMap (Map<?,?> propertyMap)
          {
          myclass.setPropertyMap (propertyMap);
          }
  
      private static class MyClass extends ThirdPartyClass
          {
          private Map<?,?> getPropertyMap ()
              {
              return propertyMap;
              }
          }
      }
  

• If the first solution doesn't apply to your case, then you should document exactly what subclasses of MyBaseClass can do, and what they shouldn't do, and hope they respect the contract described in your documentation.

Answer 4

I am able to circumvent that limitation with the "cleverness" below yet it also results in maintaining two copies of the same map as well as an O(n) operation to copy over every element.

Laf already pointed out, that this solution can easily be circumvented by casting the child classes into the third party class. But if this is ok for you and you just want to hide the protected parent map from your child classes without maintaining two copies of the map, you could try this:

public abstract class MyBaseClass extends ThirdPartyClass{

    private Map privateMap;

    public Object getFoo(){
            return privateMap.get("Foo");
    }

    public Object getBar(){
            return privateMap.get("Bar");
    }

    @Override
    public final void setPropertyMap(Map propertyMap) {
            super.setPropertyMap(this.privateMap =propertyMap);
    }
}

Note also, that it doesn't really matter, if the parents map is protected or not. If one really wants to access this field through a child class, one could always use reflection to access the field:

public class OneOfManyChildren extends MyBaseClass {
    public void clearThePrivateMap() {
        Map propertyMap;
        try {
            Field field =ThirdPartyClass.class.getDeclaredField("privateMap");
            field.setAccessible(true);
            propertyMap = (Map) field.get(this);
        } catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e) {
            e.printStackTrace();
            return;
        }
        propertyMap.clear();
    }
}

So it actually comes down to the question, why you want the field not to be accessible by the child classes:

1) Is it just for convenience, so it is immediately clear how your api should be used? - then it is perhaps fine to simply hide the field from the sub classes.

2) Is it because of security reasons? Then you should definitely search for another solution and use a special SecurityManager that also prohibits accessing private fields through reflection...

That said there is perhaps another design you could try: Instead of extending the third party class, keep a final inner instance of this class and provide public access to the inner class like this:

public abstract class MyBaseClass {
    private Map privateMap;
    private final ThirdPartyClass thirdPartyClass = new ThirdPartyClass(){
        public void setPropertyMap(Map propertyMap) {
            super.setPropertyMap(MyBaseClass.this.privateMap = propertyMap);
        };
    };

    public Object getFoo(){
        return privateMap.get("Foo");
    }

    public Object getBar(){
        return privateMap.get("Bar");
    }

    public void setPropertyMap(Map propertyMap) {
        thirdPartyClass.setPropertyMap(propertyMap);
    }

    public final ThirdPartyClass asThirdPartyClass(){
        return this.thirdPartyClass;
    }

}

Then, whenever you need to access the third party library with an instance of the third party class, you do something like this:

OneOfManyChildren child;
thirdpartyLibrary.methodThatRequiresThirdPartyClass(child.asThirdPartyClass());

Answer 5

How can I restrict access to the protected variable from my children while still being able to manipulate it privately and set it publicly?

So you want the public to have more rights than you do? You can't do that since they could always just call the public method... it's public.

Answer 6

What about creating another protected variable called propertyMap ? That should over shadow if for your child classes. You can also implement it such that calling any method on it will cause an exception.

However, as accessor methods are defined in the base class, they will not see your second shadowed version and still set it appropriately.

Answer 7

Visibility on variables is just like visibility on methods, you are not going to be able to reduce that visibility. Remember that protected variables are visible outside the direct subclass. It can be accessed from the parent by other members of the package See this Answer for Details

The ideal solution would be to mess with the parent level class. You have mentioned that making the object private is a non-starter, but if you have access to the class but just cannot downscope (perhaps due to existing dependencies), you can jiggle your class structure by abstracting out a common interface with the methods, and having both the ThirdPartyClass and your BaseClass use this interface. Or you can have your grandparent class have two maps, inner and outer, which point to the same map but the grandparent always uses the inner. This will allow the parent to override the outer without breaking the grandparent.

However, given that you call it a 3rd party class, I will assume you have no access at all to the base class.

If you are willing to break some functionality on the master interface, you can get around this with runtime exceptions (mentioned above). Basically, you can override the public variable to throw errors when they do something you do not like. This answer is mentioned above, but I would do it at the variable (Map) level instead of your interface level.

If you want to allow READ ONLY access top the map:

protected Map innerPropertyMap = propertyMap;
propertyMap = Collections.unmodifiableMap(innerPropertyMap)

You can obviously replace propertyMap with a custom implementation of map instead. However, this only really works if you want to disable for all callers on the map, disabling for only some callers would be a pain. (I am sure there is a way to do if(caller is parent) then return; else error; but it would be very very very messy). This means the parents use of the class will fail.

Remember, even if you want to hide it from children, if they add themselves to the same package, they can get around ANY restrictions you put with the following:

ThirdPartyClass grandparent = this;
// Even if it was hidden, by the inheritance properties you can now access this
// Assuming Same Package
grandparent.propertyMap.get("Parent-Blocked Chocolate Cookie")

Thus you have two options:

1. Modify the Parent Object. If you can modify this object (even if you can't make the field private), you have a few structural solutions you can pursue.
2. Change property to fail in certain use-cases. This will include access by the grandparent and the child, as the child can always get around the parent restrictions

Again, its easiest to think about it like a method: If someone can call it on a grandparent, they can call it on a grandchild.

Answer 8

Use a wrapper. A anti decorator pattern, that instead of adding new methods removes them by not providing a method to call it.