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/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));
+    }
+}