Achieve Ultimate Excellence

State Design Pattern - Objects internal State Dealing with transitions and behavior

5 min read

Introduction

Design patterns are essential tools that developers use to solve recurring problems in software development. One such pattern is the "State design pattern," which enables objects to alter their behavior when their internal state changes. This pattern promotes cleaner code, better maintainability, and improved flexibility in handling complex state transitions. In this blog post, we'll delve into the State design pattern, explore its components, and demonstrate its implementation in Java with a real-world example.

The State Design Pattern Explained

The State design pattern falls under the behavioral design patterns category and is based on the concept of finite-state machines. It allows an object to change its behavior based on its internal state, making the code more organized and easier to maintain when dealing with multiple states and transitions.

Key Components

Context: The context represents the object whose behavior changes based on its internal state. It maintains a reference to the current state and delegates the state-specific tasks to the current state object.
State: The state interface declares methods that the concrete state classes must implement. These methods define the behavior associated with a specific state.
Concrete States: These are the classes that implement the State interface, defining the behavior associated with a specific state of the context.

Implementing the State Design Pattern in Java

Let's illustrate the State design pattern with an example of an audio player that can be in three states: "Playing," "Paused," and "Stopped."

Step 1: Define the State Interface

// State.java
public interface State {
    void play();
    void pause();
    void stop();
}

Step 2: Implement the Concrete States

// PlayingState.java
public class PlayingState implements State {
    private AudioPlayer audioPlayer;

    public PlayingState(AudioPlayer audioPlayer) {
        this.audioPlayer = audioPlayer;
    }

    public void play() {
        System.out.println("The audio is already playing.");
    }

    public void pause() {
        System.out.println("Pausing the audio.");
        audioPlayer.setState(new PausedState(audioPlayer));
    }

    public void stop() {
        System.out.println("Stopping the audio.");
        audioPlayer.setState(new StoppedState(audioPlayer));
    }
}

// PausedState.java
public class PausedState implements State {
    private AudioPlayer audioPlayer;

    public PausedState(AudioPlayer audioPlayer) {
        this.audioPlayer = audioPlayer;
    }

    public void play() {
        System.out.println("Resuming the audio.");
        audioPlayer.setState(new PlayingState(audioPlayer));
    }

    public void pause() {
        System.out.println("The audio is already paused.");
    }

    public void stop() {
        System.out.println("Stopping the audio.");
        audioPlayer.setState(new StoppedState(audioPlayer));
    }
}

// StoppedState.java
public class StoppedState implements State {
    private AudioPlayer audioPlayer;

    public StoppedState(AudioPlayer audioPlayer) {
        this.audioPlayer = audioPlayer;
    }

    public void play() {
        System.out.println("Playing the audio.");
        audioPlayer.setState(new PlayingState(audioPlayer));
    }

    public void pause() {
        System.out.println("Cannot pause. The audio is stopped.");
    }

    public void stop() {
        System.out.println("The audio is already stopped.");
    }
}

Step 3: Create the Context (AudioPlayer)

// AudioPlayer.java
public class AudioPlayer {
    private State state;

    public AudioPlayer() {
        state = new StoppedState(this);
    }

    public void setState(State state) {
        this.state = state;
    }

    public void play() {
        state.play();
    }

    public void pause() {
        state.pause();
    }

    public void stop() {
        state.stop();
    }
}

Step 4: Putting it all together - Example Usage

public class Main {
    public static void main(String[] args) {
        AudioPlayer audioPlayer = new AudioPlayer();

        audioPlayer.play(); // Output: Playing the audio.
        audioPlayer.pause(); // Output: Pausing the audio.
        audioPlayer.play(); // Output: Resuming the audio.
        audioPlayer.stop(); // Output: Stopping the audio.
        audioPlayer.pause(); // Output: Cannot pause. The audio is stopped.
    }
}

Conclusion

The State design pattern allows us to model complex state transitions in a clean and maintainable way. By encapsulating state-specific behavior in separate classes, we can easily extend our application with new states without modifying existing code. In this blog post, we covered the State design pattern's key components and walked through a practical example of implementing it in Java. Now, armed with this knowledge, you can apply the State design pattern to tackle state-related challenges effectively in your future projects. Happy coding!

Article By,