State pattern is used when you need to control the state of an object. It will avoid to have nested many ifs and elses to control the state flow.
If we want to create a keep track of the state of a music, that follows the diagram:
we would implement a class like below:
public class Music {
private String title;
private String artist;
private MusicState state;
// constructor, getters and setters were ommited for simplification
}where MusicState is an enum:
public enum MusicState {
PLAY, SKIP, PAUSE, STOP;
}Then we have to implement ifs and elses in a method called setState, like so, to control the states:
public void setState(MusicState state){
if(this.state.equals(MusicState.SKIP) || this.state.equals(MusicState.STOP)){
return;
}
if(this.state.equals(MusicState.PLAY)){
this.state = state;
return;
}
if(this.state.equals(MusicState.PAUSE)){
if(state.equals(MusicState.STOP)){
return;
}else {
this.state = state;
return;
}
}Let's transform our MusicState enum in an interface:
public interface MusicState {
String getState();
String play(Music music);
String pause(Music music);
String skip(Music music);
String stop(Music music);
}and now create for each state a class that implements our interface, and each method will have a different return depending of the class. Let's take Play and Pause as examples:
public class Play implements MusicState {
private static Play instance = new Play();
public static Play getInstance() {
return instance;
}
@Override
public String getState() {
return "▶️";
}
@Override
public String play(Music music) {
return "⚠️ Operation not allowed";
}
@Override
public String pause(Music music) {
music.setState(Pause.getInstance());
return "⏸";
}
@Override
public String skip(Music music) {
music.setState(Skip.getInstance());
return "⏭️";
}
@Override
public String stop(Music music) {
music.setState(Stop.getInstance());
return "⏹️";
}
}public class Pause implements MusicState {
private static Pause instance = new Pause();
public static Pause getInstance() {
return instance;
}
@Override
public String getState() {
return "⏸";
}
@Override
public String play(Music music) {
music.setState(Play.getInstance());
return "▶️";
}
@Override
public String pause(Music music) {
return "⚠️ Operation not allowed";
}
@Override
public String skip(Music music) {
music.setState(Skip.getInstance());
return "⏭️";
}
@Override
public String stop(Music music) {
return "⚠️ Operation not allowed";
}
}
}Note that these classes are singletons because they don't need more than one instance to work on our code.
Finally, we have to update our Music class to implement the method to each state:
public class Music {
// attributes, constructor, getters and setters were ommited for simplification
public String play() {
return state.play(this);
}
public String pause() {
return state.pause(this);
}
public String skip() {
return state.skip(this);
}
public String stop() {
return state.stop(this);
}
}and we're done!