Finish state pattern

Author: LBeghini

README.md

@@ -2,9 +2,11 @@
 
 ## State
 
-State pattern is used when you need to control the state of an object.
+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.
 
-For example, if we want to create a keep track of the state of a music, that follows the diagram:
+### Without State Design Pattern
+
+If we want to create a keep track of the state of a music, that follows the diagram:
 
 <img src="./resources/StateDiagram.png" width="800px" />
 
@@ -20,7 +22,41 @@ public class Music {
 }
 ```
 
-and create a MusicState interface:
+where `MusicState` is an enum:
+
+```java
+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:
+
+```java
+    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;
+            }
+        }
+
+```
+
+### With State Design Pattern
+
+Let's transform our `MusicState` enum in an interface:
 
 ```java
 public interface MusicState {
@@ -37,3 +73,117 @@ public interface MusicState {
 }
 
 ```
+
+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:
+
+```java
+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 "⏹️";
+    }
+
+}
+```
+
+```java
+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:
+
+```java
+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!

src/main/java/com/language/programming/model/Music.java

@@ -13,6 +13,26 @@ public Music(String title, String artist) {
         this.state = Play.getInstance();
     }
 
+    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);
+    }
+
+    public void setState(MusicState state) {
+        this.state = state;
+    }
+
     public String getTitle() {
         return title;
     }

src/main/java/com/language/programming/model/states/Pause.java

@@ -18,26 +18,25 @@ public String getState() {
 
     @Override
     public String play(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        music.setState(Play.getInstance());
+        return "▶️";
     }
 
     @Override
     public String pause(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
     }
 
     @Override
     public String skip(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        music.setState(Skip.getInstance());
+        return "⏭️";
     }
 
     @Override
     public String stop(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
+
     }
 
 }

src/main/java/com/language/programming/model/states/Play.java

@@ -18,26 +18,25 @@ public String getState() {
 
     @Override
     public String play(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
     }
 
     @Override
     public String pause(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        music.setState(Pause.getInstance());
+        return "⏸";
     }
 
     @Override
     public String skip(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        music.setState(Skip.getInstance());
+        return "⏭️";
     }
 
     @Override
     public String stop(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        music.setState(Stop.getInstance());
+        return "⏹️";
     }
 
 }

src/main/java/com/language/programming/model/states/Skip.java

@@ -18,26 +18,22 @@ public String getState() {
 
     @Override
     public String play(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
     }
 
     @Override
     public String pause(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
     }
 
     @Override
     public String skip(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
     }
 
     @Override
     public String stop(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
     }
 
 }

src/main/java/com/language/programming/model/states/Stop.java

@@ -18,26 +18,22 @@ public String getState() {
 
     @Override
     public String play(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
     }
 
     @Override
     public String pause(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
     }
 
     @Override
     public String skip(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
     }
 
     @Override
     public String stop(Music music) {
-        // TODO Auto-generated method stub
-        return null;
+        return "⚠️ Operation not allowed";
     }
 
 }