Add Singleton example to Design Patterns

Author: dawand

Java/Advanced-Java/Advanced-Java.iml renamed to Advanced-Java/Advanced-Java.iml

@@ -0,0 +1,21 @@
+package singleton;
+
+// final is to avoid having non-static nested classes subclassing Captain
+final class Captain {
+
+    private static Captain captain;
+
+    private Captain() {}
+
+    // synchronized is for multithreaded purpose
+    public static synchronized Captain getInstance() {
+        if (captain == null) { // lazy initialization
+            captain = new Captain();
+            System.out.println("A new captain is created :)");
+            return captain;
+        } else {
+            System.out.println("You already have a captain!");
+        }
+            return captain;
+    }
+}

Java/Advanced-Java/out/production/Advanced-Java/AsyncProgramming/CompletableFutureExample.class renamed to Advanced-Java/out/production/Advanced-Java/AsyncProgramming/CompletableFutureExample.class

@@ -0,0 +1,13 @@
+package singleton;
+
+public class SingletonExample {
+    public static void main(String[] args) {
+        Captain captain1 = Captain.getInstance();
+        Captain captain2 = Captain.getInstance();
+
+        if (captain1 == captain2) {
+            System.out.println("Captain 1 and 2 are same instances");
+        }
+
+    }
+}

Java/Advanced-Java/out/production/Advanced-Java/Callbacks/Callbacks.class renamed to Advanced-Java/out/production/Advanced-Java/Callbacks/Callbacks.class

@@ -4,6 +4,7 @@
     <exclude-output />
     <content url="file://$MODULE_DIR$">
       <sourceFolder url="file://$MODULE_DIR$/problems" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/Design-patterns/src" isTestSource="false" />
     </content>
     <orderEntry type="inheritedJdk" />
     <orderEntry type="sourceFolder" forTests="false" />