State Pattern

📚 State Pattern 설명

• 무언가의 '상태'가 각각에 해당하는 클래스의 객체로 존재한다.
• 객체의 내부 상태에 따라 행동이 달라져야 할 때, 그리고 상태가 자주 변경될 때 사용하기 적합하다.

[Ex1]

public interface State {
    void open(Door door);
    void close(Door door);
}

// 문이 닫힌 상태
public class ClosedState implements State {
    @Override
    public void open(Door door) {
        System.out.println("Door is now Open.");
        door.setState(new OpenState());
    }

    @Override
    public void close(Door door) {
        System.out.println("Door is already Closed.");
    }
}

// 문이 열린 상태
public class OpenState implements State {
    @Override
    public void open(Door door) {
        System.out.println("Door is already Open.");
    }

    @Override
    public void close(Door door) {
        System.out.println("Door is now Closed.");
        door.setState(new ClosedState());
    }
}

public class Door {

    private State state;  // 상태를 멤버변수로 가진다.
    public Door() {
        this.state = new ClosedState();
    }

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

    public void open() {
        state.open(this);
    }
    public void close() {
        state.close(this);
    }
}

[Ex2]

public interface State {
    void play(VideoPlayer player);
    void stop(VideoPlayer player);
}

public class StoppedState implements State {
    @Override
    public void play(VideoPlayer player) {
        System.out.println("Starting the video.");
        player.setState(new PlayingState());
    }

    @Override
    public void stop(VideoPlayer player) {
        System.out.println("Video is already stopped.");
    }
}

// 플레이 상태
public class PlayingState implements State {
    @Override
    public void play(VideoPlayer player) {
        System.out.println("Video is already playing.");
    }

    @Override
    public void stop(VideoPlayer player) {
        System.out.println("Pausing the video.");
        player.setState(new PausedState());
    }
}

// 정지 상태
public class PausedState implements State {
    @Override
    public void play(VideoPlayer player) {
        System.out.println("Resuming the video.");
        player.setState(new PlayingState());
    }

    @Override
    public void stop(VideoPlayer player) {
        System.out.println("Stopping the video.");
        player.setState(new StoppedState());
    }
}

public class VideoPlayer {
    private State state;   // 상태를 필드로 가진다.

    public VideoPlayer() {
        // Set initial state Stopped
        this.state = new StoppedState();
    }

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

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

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