How to override a class within an Android library project?

Question

The following is my situation:

I have a library project and a project based on it. Now in the library I have two classes A and B, whereby A uses B. In the project

Answer 1

I dont know if this is the best way to do it but i do it this way;

I create a class A from the project and this class extends form the library project

public class A extends libraryProject_A{

//here i put all methods from the new class B to override methods from library class B

}

Answer 2

As I see from your comment you have class A that uses class B

But class B should be different according to which project you're using.

I think you need to create a base class say BaseB that will be an instance variable in class A, and you might have a setter and getter for this Or you can make it a parameter passed to the constructor of class A. and when instantiating A you should choose which one to use.

Let's have a look at code

Class A {
 private BaseB b;
 public A(BaseB aB) {
   b = aB;
 }

 public void set(BaseB aB) {
  b = aB;
 }
 public BaseB get() {
   return b;
 }
}

interface BaseB {
}


// in the library have this
class B implements BaseB {
}

// in your project have the other implementation
class B implements BaseB {
}

// you can then instantiate A like this
A a = new A(new B());
// you can choose which one to use here in the previous statement.

Answer 3

That does not work with the current layout. You have to use the strategy pattern. In your library define LibA with a constructor that takes a object of type LibB in the constructor:

class LibA{
    LibB b;
    public LibA(LibB b)
        this.b = b;
    }
}

Then you can override LibB in your project and create LibA with the class that extends LibB:

class ProjectB extends LibB{

}

LibA a = new LibA(new ProjectB());

---

Answer to Turbos question:

You want to start Project-Activities from your Library. So then move the code that creates the Intent into your Projects, because only in your project you know the type or name of the Activity to be started.

The solution you mentioned in your comment (here) creates the Intent in the library project, by guessing the name of the Activity that should be started. There is nothing really wrong with that but it's not an elegant solution. You can only start Activities that follow that special naming scheme. And because of that you cannot start arbitrary Activities that are visible in your projects like Activities from other libraries, where you cannot change the name of the class.

To move the Intent creation into your libraries you can i.e. use the strategy, template or factory-method pattern. See Design Patterns on Wikipedia for even more (creational) patterns that match your library design.

A simple solution would be:

Create an abstract LibraryActivity and extend your ProjectActivities from it. The LibraryActivity defines an abstract method that returns the Intent. The implementation of that abstract method is done in your ProjectActivities.

abstract class LibActivity{

    private void doSomething(){
        //Library does something

        //and finally calls createIntent() to start an project specific Activity
        startActivity(this.createIntent());
    }

    protected abstract Intent createIntent();
}


class ProjectActivity extends LibActivity{

    protected Intent createIntent(){
        return new Intent(ProjectActivity.this, AnyActivityYouWant.class);
    }        
}