State en Java

State es un patrón de diseño de comportamiento que permite a un objeto cambiar de comportamiento cuando cambia su estado interno.

El patrón extrae comportamientos relacionados con el estado, los coloca dentro de clases de estado separadas y fuerza al objeto original a delegar el trabajo de una instancia de esas clases, en lugar de actuar por su cuenta.

Aprende más sobre el patrón State

Complejidad:

Popularidad:

Ejemplos de uso: El patrón State se utiliza habitualmente en Java para convertir las enormes máquinas de estados basadas en declaraciones switch, en objetos.

Aquí tienes algunos ejemplos del patrón State en las principales bibliotecas Java:

javax.faces.lifecycle.LifeCycle#execute() (controlado por FacesServlet: el comportamiento depende de la fase actual (estado) del ciclo de vida de las JSF)

Identificación: El patrón State se puede reconocer por métodos que cambian su comportamiento dependiendo del estado del objeto, controlado externamente.

Interfaz de un reproductor multimedia

En este ejemplo, el patrón State permite a los mismos controles del reproductor multimedia comportarse de forma diferente, dependiendo del estado actual de reproducción. La clase principal del reproductor contiene una referencia a un objeto de estado que realiza la mayor parte del trabajo para el reproductor. Algunas acciones pueden acabar sustituyendo el objeto de estado por otro, lo cual cambia la forma en la que el reproductor reacciona a las interacciones del usuario.

states

states/State.java: Interfaz común de estado

package refactoring_guru.state.example.states;

import refactoring_guru.state.example.ui.Player;

/**
 * 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();
}

states/LockedState.java

package refactoring_guru.state.example.states;

import refactoring_guru.state.example.ui.Player;

/**
 * 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...";
    }
}

states/ReadyState.java

package refactoring_guru.state.example.states;

import refactoring_guru.state.example.ui.Player;

/**
 * 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...";
    }
}

states/PlayingState.java

package refactoring_guru.state.example.states;

import refactoring_guru.state.example.ui.Player;

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();
    }
}

ui

ui/Player.java: Código principal del reproductor

package refactoring_guru.state.example.ui;

import refactoring_guru.state.example.states.ReadyState;
import refactoring_guru.state.example.states.State;

import java.util.ArrayList;
import java.util.List;

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;
    }
}

ui/UI.java: GUI del reproductor

package refactoring_guru.state.example.ui;

import javax.swing.*;
import java.awt.*;

public class UI {
    private Player player;
    private static JTextField textField = new JTextField();

    public UI(Player player) {
        this.player = player;
    }

    public void init() {
        JFrame frame = new JFrame("Test player");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel context = new JPanel();
        context.setLayout(new BoxLayout(context, BoxLayout.Y_AXIS));
        frame.getContentPane().add(context);
        JPanel buttons = new JPanel(new FlowLayout(FlowLayout.CENTER));
        context.add(textField);
        context.add(buttons);

        // Context delegates handling user's input to a state object. Naturally,
        // the outcome will depend on what state is currently active, since all
        // states can handle the input differently.
        JButton play = new JButton("Play");
        play.addActionListener(e -> textField.setText(player.getState().onPlay()));
        JButton stop = new JButton("Stop");
        stop.addActionListener(e -> textField.setText(player.getState().onLock()));
        JButton next = new JButton("Next");
        next.addActionListener(e -> textField.setText(player.getState().onNext()));
        JButton prev = new JButton("Prev");
        prev.addActionListener(e -> textField.setText(player.getState().onPrevious()));
        frame.setVisible(true);
        frame.setSize(300, 100);
        buttons.add(play);
        buttons.add(stop);
        buttons.add(next);
        buttons.add(prev);
    }
}

Demo.java: Código de inicialización

package refactoring_guru.state.example;

import refactoring_guru.state.example.ui.Player;
import refactoring_guru.state.example.ui.UI;

/**
 * Demo class. Everything comes together here.
 */
public class Demo {
    public static void main(String[] args) {
        Player player = new Player();
        UI ui = new UI(player);
        ui.init();
    }
}