Decoupling Classes with Interfaces in OOP

While building a Login module for an Android application, I encountered a common coupling problem. The Login class needed to call three methods on the calling Activity, which created a direct dependency between the two.

The Problem

The Login class was tightly coupled to a specific Activity. Reusing it from a different Activity would require modifying the Login class itself, violating the Open/Closed Principle.

The Solution: Define an Interface

By introducing an interface, we establish a contract that any caller must fulfill:

public interface LoginCallback {
    void onLoginSuccess(User user);
    void onLoginFailed(String error);
    void onLoginProgress(int percent);
}

Now the Login class depends only on the interface, not on any concrete Activity:

public class LoginManager {
    private LoginCallback callback;


    public LoginManager(LoginCallback callback) {
        this.callback = callback;
    }


    public void login(String username, String password) {
        // perform login
        callback.onLoginSuccess(user);
    }
}

Any Activity that implements LoginCallback can use LoginManager without modification. The class is now reusable, testable, and decoupled from its consumers. This is a core principle of good object-oriented design.