public class Player {
    private State state;
    private boolean playing = false;
    private List<String> playlist = new ArrayList<>();
    private int currentTrack = 0;

    public Player() {
        this.state = new ReadyState(this);
        setPlaying(true);
        for (int i = 1; i <= 12; i++) {
            playlist.add("Track " + i);
        }
    }

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

    public State getState() {
        return state;
    }

    public void setPlaying(boolean playing) {
        this.playing = playing;
    }

    public boolean isPlaying() {
        return playing;
    }

    public String startPlayback() {
        return "Playing " + playlist.get(currentTrack);
    }

    public String nextTrack() {
        currentTrack++;
        if (currentTrack > playlist.size() - 1) {
            currentTrack = 0;
        }
        return "Playing " + playlist.get(currentTrack);
    }

    public String previousTrack() {
        currentTrack--;
        if (currentTrack < 0) {
            currentTrack = playlist.size() - 1;
        }
        return "Playing " + playlist.get(currentTrack);
    }

    public void setCurrentTrackAfterStop() {
        this.currentTrack = 0;
    }
}

/**
 * Common interface for all states.
 */
public abstract class State {
    Player player;

    /**
     * Context passes itself through the state constructor. This may help a
     * state to fetch some useful context data if needed.
     */
    State(Player player) {
        this.player = player;
    }

    public abstract String onLock();
    public abstract String onPlay();
    public abstract String onNext();
    public abstract String onPrevious();
}

/**
 * Concrete states provide the special implementation for all interface methods.
 */
public class LockedState extends State {

    LockedState(Player player) {
        super(player);
        player.setPlaying(false);
    }

    @Override
    public String onLock() {
        if (player.isPlaying()) {
            player.changeState(new ReadyState(player));
            return "Stop playing";
        } else {
            return "Locked...";
        }
    }

    @Override
    public String onPlay() {
        player.changeState(new ReadyState(player));
        return "Ready";
    }

    @Override
    public String onNext() {
        return "Locked...";
    }

    @Override
    public String onPrevious() {
        return "Locked...";
    }
}

/**
 * They can also trigger state transitions in the context.
 */
public class ReadyState extends State {

    public ReadyState(Player player) {
        super(player);
    }

    @Override
    public String onLock() {
        player.changeState(new LockedState(player));
        return "Locked...";
    }

    @Override
    public String onPlay() {
        String action = player.startPlayback();
        player.changeState(new PlayingState(player));
        return action;
    }

    @Override
    public String onNext() {
        return "Locked...";
    }

    @Override
    public String onPrevious() {
        return "Locked...";
    }
}

public class PlayingState extends State {

    PlayingState(Player player) {
        super(player);
    }

    @Override
    public String onLock() {
        player.changeState(new LockedState(player));
        player.setCurrentTrackAfterStop();
        return "Stop playing";
    }

    @Override
    public String onPlay() {
        player.changeState(new ReadyState(player));
        return "Paused...";
    }

    @Override
    public String onNext() {
        return player.nextTrack();
    }

    @Override
    public String onPrevious() {
        return player.previousTrack();
    }
}

/**
 * Demo class. Everything comes together here.
 */
public class Demo {
    public static void main(String[] args) {
        Player player = new Player();
        player.getState().onPlay();
        player.getState().onLock();
        player.getState().onNext();
        player.getState().onPrevious();
    }
}