Add state design pattern implementations & tests

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());
+    }
+}

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

@@ -0,0 +1,20 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.machine;
+
+record StoppedState() implements MediaPlayerState {
+
+    @Override
+    public void pressPlay(MediaPlayer player) {
+        System.out.println("▶ Starting music...");
+        player.setState(new PlayingState());
+    }
+
+    @Override
+    public void pressPause(MediaPlayer player) {
+        System.out.println("⏸ Can't pause! Music is stopped.");
+    }
+
+    @Override
+    public void pressStop(MediaPlayer player) {
+        System.out.println("⏹ Already stopped!");
+    }
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/behavioral/state/flow/TrafficLightStateTest.java

@@ -0,0 +1,31 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.flow;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import pl.mperor.lab.common.TestUtils;
+
+import java.util.stream.Stream;
+
+public class TrafficLightStateTest {
+
+    @Test
+    public void testTrafficLightStateChanges() {
+        var out = TestUtils.setTempSystemOut();
+        var trafficLight = new TrafficLight();
+
+        Stream.generate(() -> {
+            trafficLight.display();
+            trafficLight.change();
+            return trafficLight;
+        }).limit(4).count();
+
+        Assertions.assertLinesMatch("""
+                🔴 STOP! The light is RED.
+                🟢 GO! The light is GREEN.
+                🟡 Caution! The light is YELLOW.
+                🔴 STOP! The light is RED.
+                """.lines(), out.lines().stream());
+
+        TestUtils.resetSystemOut();
+    }
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/behavioral/state/machine/MediaPlayerStateTest.java

@@ -0,0 +1,30 @@
+package pl.mperor.lab.java.design.pattern.behavioral.state.machine;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import pl.mperor.lab.common.TestUtils;
+
+public class MediaPlayerStateTest {
+
+    @Test
+    public void shouldBehaveLikeStateMachine() {
+        var out = TestUtils.setTempSystemOut();
+
+        var player = new MediaPlayer();
+        player.pressPlay();  // Stopped -> Playing
+        player.pressPause(); // Playing -> Paused
+        player.pressPlay();  // Paused -> Playing
+        player.pressStop();  // Playing -> Stopped
+        player.pressPause(); // Invalid
+
+        Assertions.assertLinesMatch("""
+                ▶ Starting music...
+                ⏸ Pausing music...
+                ▶ Resuming music...
+                ⏹ Stopping music...
+                ⏸ Can't pause! Music is stopped.
+                """.lines(), out.lines().stream());
+
+        TestUtils.resetSystemOut();
+    }
+}