Is it possible for an interface to be available in only one package and child packages?

Question

Is it possible for an interface to be available in only one package and child packages?

I defined an interface with a standard modifier:

package com.mycompany.myapp.dao; import java.io.Serializable; interface BaseDao<T, Id extends Serializable> { public void create(T t); public T readById(Id id); public void update(T t); public void delete(T t); } 

Now I have a child package where I want to define a class that implements BaseDao . So I wrote this code:

 package com.mycompany.myapp.dao.jpa; import java.io.Serializable; public class BaseDaoJpa<T, Id extends Serializable> implements BaseDao<T, Id> { ... } 

But I get this error:

BaseDao cannot be allowed for type

So is this a Java restriction for an interface, or am I doing it wrong?

thanks

Answer 1

In Java, there is no such thing as a "child package". Do not be fooled by the dots. com.mycompany.myapp.dao and com.mycompany.myapp.dao.jpa are two separate packages that have nothing to do with each other.

So, to answer your question: no, it is impossible to make the interface visible only to child packages. You can make your interface public, but then it will be visible to all other packages.

Answer 2

Take a look at Java Acess Modifiers : https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

In the table you can see that by default or Without a modifier is limited to only the same class or other classes in one package. Since I understand that you want it to be visible to other packages, but not to the whole world, and for this you need to use a protected modifier, but this is not possible, since this is unacceptable, so go back to your question, you cannot: - (

Answer 3

If you are looking for some way to hide and provide only what you want for the rest of your Java application, you might need a component, so take a look at OSGi. This question is a good start to reading before jumping (or not) into it.

Answer 4

It looks like you have extensions where you need tools.

 public class BaseDaoJpa<T, Id extends Serializable> implements BaseDao<T, Id> { ... }