Java: How to create a public class that can be subclassed within package, while preventing outsiders from inheriting?

Question

Suppose I am writing a package. I have the following inheritance hierarchy, all within my package:

public class Container {
    public void size() {
        System.out.println(10);
    }
}

public final class Bucket extends Container {}

public final class Bag extends Container {}

public class Item {
    Container container; // is aware of what container it's in. Will be initialized on construction.
    public Container getContainer() {
        return container;
    } 
}

I want to prevent others from subclassing Container. But I can't make it final because Bucket and Bag are subclasses. And I have to make it public or else outsiders cannot call getContainer(). Is there a way to do this in Java? Otherwise, what do people do in this type of situation?

Answer 1

To prevent a class from being subclassed by classes in other packages, make all the constructors package-private.

If the class doesn't have any constructors, it has the implicit default constructor, which is public, so you need to explicitly create a package-private no-arg constructor.

package foo;
public class Container {
  Container () {/*package-private*/}
  public void size() { System.out.println(10); }
}

---

package foo;
public final class Bucket extends Container {}

---

package bar;
public final class Oops extends Container {}

Compile Error: Implicit super constructor Container() is not visible for default constructor. Must define an explicit constructor

---

public final class Oops extends Container {
  public Oops() {}
}

Compile Error: Implicit super constructor Container() is not visible. Must explicitly invoke another constructor

Of course, there are no other constructors, so it cannot be done.