BAEL-1997 state design pattern in Java (#4827)

* BAEL-1997 state design pattern in Java

* BAEL-1997 different example code

* BAEL-1997 add additional method to the states

* BAEL-1997 clean up in ReceivedState

patterns/design-patterns/src/main/java/com/baeldung/state/DeliveredState.java

@@ -0,0 +1,25 @@
+package com.baeldung.state;
+
+public class DeliveredState implements PackageState {
+
+    @Override
+    public void next(Package pkg) {
+        pkg.setState(new ReceivedState());
+    }
+
+    @Override
+    public void prev(Package pkg) {
+        pkg.setState(new OrderedState());
+    }
+
+    @Override
+    public void printStatus() {
+        System.out.println("Package delivered to post office, not received yet.");
+    }
+
+    @Override
+    public String toString() {
+        return "DeliveredState{}";
+    }
+
+}

patterns/design-patterns/src/main/java/com/baeldung/state/OrderedState.java

@@ -0,0 +1,24 @@
+package com.baeldung.state;
+
+public class OrderedState implements PackageState {
+
+    @Override
+    public void next(Package pkg) {
+        pkg.setState(new DeliveredState());
+    }
+
+    @Override
+    public void prev(Package pkg) {
+        System.out.println("The package is in it's root state.");
+    }
+
+    @Override
+    public void printStatus() {
+        System.out.println("Package ordered, not delivered to the office yet.");
+    }
+
+    @Override
+    public String toString() {
+        return "OrderedState{}";
+    }
+}

patterns/design-patterns/src/main/java/com/baeldung/state/Package.java

@@ -0,0 +1,26 @@
+package com.baeldung.state;
+
+public class Package {
+
+    private PackageState state = new OrderedState();
+
+    public PackageState getState() {
+        return state;
+    }
+
+    public void setState(PackageState state) {
+        this.state = state;
+    }
+
+    public void previousState() {
+        state.prev(this);
+    }
+
+    public void nextState() {
+        state.next(this);
+    }
+
+    public void printStatus() {
+        state.printStatus();
+    }
+}

patterns/design-patterns/src/main/java/com/baeldung/state/PackageState.java

@@ -0,0 +1,10 @@
+package com.baeldung.state;
+
+public interface PackageState {
+
+    void next(Package pkg);
+
+    void prev(Package pkg);
+
+    void printStatus();
+}

patterns/design-patterns/src/main/java/com/baeldung/state/ReceivedState.java

@@ -0,0 +1,24 @@
+package com.baeldung.state;
+
+public class ReceivedState implements PackageState {
+
+    @Override
+    public void next(Package pkg) {
+        System.out.println("This package is already received by a client.");
+    }
+
+    @Override
+    public void prev(Package pkg) {
+        pkg.setState(new DeliveredState());
+    }
+
+    @Override
+    public void printStatus() {
+        System.out.println("Package was received by client.");
+    }
+
+    @Override
+    public String toString() {
+        return "ReceivedState{}";
+    }
+}

patterns/design-patterns/src/main/java/com/baeldung/state/StateDemo.java

@@ -0,0 +1,19 @@
+package com.baeldung.state;
+
+public class StateDemo {
+
+    public static void main(String[] args) {
+
+        Package pkg = new Package();
+        pkg.printStatus();
+
+        pkg.nextState();
+        pkg.printStatus();
+
+        pkg.nextState();
+        pkg.printStatus();
+
+        pkg.nextState();
+        pkg.printStatus();
+    }
+}

patterns/design-patterns/src/test/java/com/baeldung/state/StatePatternUnitTest.java

@@ -0,0 +1,33 @@
+package com.baeldung.state;
+
+import com.baeldung.state.Package;
+
+import static org.junit.Assert.assertThat;
+import static org.hamcrest.CoreMatchers.instanceOf;
+
+import org.junit.Test;
+
+public class StatePatternUnitTest {
+
+    @Test
+    public void givenNewPackage_whenPackageReceived_thenStateReceived() {
+        Package pkg = new Package();
+
+        assertThat(pkg.getState(), instanceOf(OrderedState.class));
+        pkg.nextState();
+
+        assertThat(pkg.getState(), instanceOf(DeliveredState.class));
+        pkg.nextState();
+
+        assertThat(pkg.getState(), instanceOf(ReceivedState.class));
+    }
+
+    @Test
+    public void givenDeliveredPackage_whenPrevState_thenStateOrdered() {
+        Package pkg = new Package();
+        pkg.setState(new DeliveredState());
+        pkg.previousState();
+
+        assertThat(pkg.getState(), instanceOf(OrderedState.class));
+    }
+}