Add state design pattern implementations with tests (#54)

Author: mperor

DesignPatterns/README.md

@@ -12,6 +12,7 @@ with practical examples and best practices for using design patterns to create r
 - [Command](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/command) 📝
 - [Execute Around Method (EAM)](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/eam) ⭕
 - [Observer](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/observer) 👀
+- [State](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state) 📜
 - [Strategy](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/strategy) 🎯
 - [Template Method](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/template/method) 📋
 

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state/flow/GreenLight.java

@@ -0,0 +1,14 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.flow;
+
+record GreenLight() implements TrafficLightState {
+
+    @Override
+    public void change(TrafficLight trafficLight) {
+        trafficLight.setState(new YellowLight());
+    }
+
+    @Override
+    public void display() {
+        System.out.println("🟢 GO! The light is GREEN.");
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state/flow/RedLight.java

@@ -0,0 +1,14 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.flow;
+
+record RedLight() implements TrafficLightState {
+
+    @Override
+    public void change(TrafficLight trafficLight) {
+        trafficLight.setState(new GreenLight());
+    }
+
+    @Override
+    public void display() {
+        System.out.println("🔴 STOP! The light is RED.");
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state/flow/TrafficLight.java

@@ -0,0 +1,22 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.flow;
+
+class TrafficLight {
+
+    private TrafficLightState state;
+
+    TrafficLight() {
+        this.state = new RedLight();
+    }
+
+    void setState(TrafficLightState state) {
+        this.state = state;
+    }
+
+    void change() {
+        state.change(this);
+    }
+
+    void display() {
+        state.display();
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state/flow/TrafficLightState.java

@@ -0,0 +1,8 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.flow;
+
+sealed interface TrafficLightState permits RedLight, YellowLight, GreenLight {
+
+    void change(TrafficLight trafficLight);
+
+    void display();
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state/flow/YellowLight.java

@@ -0,0 +1,14 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.flow;
+
+record YellowLight() implements TrafficLightState {
+
+    @Override
+    public void change(TrafficLight trafficLight) {
+        trafficLight.setState(new RedLight());
+    }
+
+    @Override
+    public void display() {
+        System.out.println("🟡 Caution! The light is YELLOW.");
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state/machine/MediaPlayer.java

@@ -0,0 +1,26 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.machine;
+
+public class MediaPlayer {
+
+    private MediaPlayerState state;
+
+    public MediaPlayer() {
+        this.state = new StoppedState();
+    }
+
+    public void setState(MediaPlayerState state) {
+        this.state = state;
+    }
+
+    public void pressPlay() {
+        state.pressPlay(this);
+    }
+
+    public void pressPause() {
+        state.pressPause(this);
+    }
+
+    public void pressStop() {
+        state.pressStop(this);
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state/machine/MediaPlayerState.java

@@ -0,0 +1,10 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.machine;
+
+sealed interface MediaPlayerState permits PausedState, PlayingState, StoppedState {
+
+    void pressPlay(MediaPlayer player);
+
+    void pressPause(MediaPlayer player);
+
+    void pressStop(MediaPlayer player);
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state/machine/PausedState.java

@@ -0,0 +1,21 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.machine;
+
+record PausedState() implements MediaPlayerState {
+
+    @Override
+    public void pressPlay(MediaPlayer player) {
+        System.out.println("▶ Resuming music...");
+        player.setState(new PlayingState());
+    }
+
+    @Override
+    public void pressPause(MediaPlayer player) {
+        System.out.println("⏸ Already paused!");
+    }
+
+    @Override
+    public void pressStop(MediaPlayer player) {
+        System.out.println("⏹ Stopping music...");
+        player.setState(new StoppedState());
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state/machine/PlayingState.java

@@ -0,0 +1,21 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.machine;
+
+record PlayingState() implements MediaPlayerState {
+
+    @Override
+    public void pressPlay(MediaPlayer player) {
+        System.out.println("▶ Already playing!");
+    }
+
+    @Override
+    public void pressPause(MediaPlayer player) {
+        System.out.println("⏸ Pausing music...");
+        player.setState(new PausedState());
+    }
+
+    @Override
+    public void pressStop(MediaPlayer player) {
+        System.out.println("⏹ Stopping music...");
+        player.setState(new StoppedState());
+    }
+}