Add singleton examples with tests

Author: mperor

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/singleton/LazyInitializedSingleton.java

@@ -0,0 +1,22 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+public class LazyInitializedSingleton {
+
+    private static LazyInitializedSingleton instance;
+    private final long time = System.currentTimeMillis();
+
+    private LazyInitializedSingleton() {
+    }
+
+    public static synchronized LazyInitializedSingleton getInstance() {
+        if (instance == null) {
+            instance = new LazyInitializedSingleton();
+        }
+        return instance;
+    }
+
+    public long getTime() {
+        return time;
+    }
+
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/singleton/Singleton.java

@@ -0,0 +1,19 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+public class Singleton {
+
+    private static final Singleton instance = new Singleton();
+    private final long time = System.currentTimeMillis();
+
+    private Singleton() {
+    }
+
+    public static Singleton getInstance() {
+        return instance;
+    }
+
+    public long getTime() {
+        return time;
+    }
+
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/creational/singleton/LazyInitializedSingletonTest.java

@@ -0,0 +1,27 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.CompletableFuture;
+
+public class LazyInitializedSingletonTest {
+
+    @Test
+    public void shouldOnlyAllowToLazyCreateOneInstanceOfSingleton() {
+        var first = CompletableFuture.supplyAsync(() -> {
+            System.out.println("First ...");
+            return LazyInitializedSingleton.getInstance();
+        });
+        var second = CompletableFuture.supplyAsync(() -> {
+            System.out.println("Second  ...");
+            return LazyInitializedSingleton.getInstance();
+        });
+
+        LazyInitializedSingleton firstResult = first.join();
+        LazyInitializedSingleton secondResult = second.join();
+
+        Assertions.assertSame(firstResult, secondResult);
+        Assertions.assertEquals(firstResult.getTime(), secondResult.getTime());
+    }
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/creational/singleton/SingletonTest.java

@@ -0,0 +1,22 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.TimeUnit;
+
+public class SingletonTest {
+
+    @Test
+    public void shouldOnlyAllowToCreateOneInstanceOfSingleton() throws InterruptedException {
+        var instance = Singleton.getInstance();
+        long time = instance.getTime();
+
+        TimeUnit.MILLISECONDS.sleep(50);
+        long timeAfterBreak = Singleton.getInstance().getTime();
+
+        Assertions.assertSame(instance, Singleton.getInstance());
+        Assertions.assertEquals(time, timeAfterBreak);
+    }
+
+}