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diff --git a/Java/Advanced-Java/src/DesignPatterns/TaxCalculator2020.java b/Advanced-Java/src/DesignPatterns/TaxCalculator2020.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/TaxCalculator2020.java
rename to Advanced-Java/src/DesignPatterns/TaxCalculator2020.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/User.java b/Advanced-Java/src/DesignPatterns/User.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/User.java
rename to Advanced-Java/src/DesignPatterns/User.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/memento/Editor.java b/Advanced-Java/src/DesignPatterns/memento/Editor.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/memento/Editor.java
rename to Advanced-Java/src/DesignPatterns/memento/Editor.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/memento/EditorState.java b/Advanced-Java/src/DesignPatterns/memento/EditorState.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/memento/EditorState.java
rename to Advanced-Java/src/DesignPatterns/memento/EditorState.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/memento/History.java b/Advanced-Java/src/DesignPatterns/memento/History.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/memento/History.java
rename to Advanced-Java/src/DesignPatterns/memento/History.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/memento/Main.java b/Advanced-Java/src/DesignPatterns/memento/Main.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/memento/Main.java
rename to Advanced-Java/src/DesignPatterns/memento/Main.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/state/BrushTool.java b/Advanced-Java/src/DesignPatterns/state/BrushTool.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/state/BrushTool.java
rename to Advanced-Java/src/DesignPatterns/state/BrushTool.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/state/Canvas.java b/Advanced-Java/src/DesignPatterns/state/Canvas.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/state/Canvas.java
rename to Advanced-Java/src/DesignPatterns/state/Canvas.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/state/Main.java b/Advanced-Java/src/DesignPatterns/state/Main.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/state/Main.java
rename to Advanced-Java/src/DesignPatterns/state/Main.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/state/Selection.java b/Advanced-Java/src/DesignPatterns/state/Selection.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/state/Selection.java
rename to Advanced-Java/src/DesignPatterns/state/Selection.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/state/Tool.java b/Advanced-Java/src/DesignPatterns/state/Tool.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/state/Tool.java
rename to Advanced-Java/src/DesignPatterns/state/Tool.java
diff --git a/Java/Advanced-Java/src/DesignPatterns/state/ToolType.java b/Advanced-Java/src/DesignPatterns/state/ToolType.java
similarity index 100%
rename from Java/Advanced-Java/src/DesignPatterns/state/ToolType.java
rename to Advanced-Java/src/DesignPatterns/state/ToolType.java
diff --git a/Java/Advanced-Java/src/FunctionalProgramming/Gender.java b/Advanced-Java/src/FunctionalProgramming/Gender.java
similarity index 100%
rename from Java/Advanced-Java/src/FunctionalProgramming/Gender.java
rename to Advanced-Java/src/FunctionalProgramming/Gender.java
diff --git a/Java/Advanced-Java/src/FunctionalProgramming/Main.java b/Advanced-Java/src/FunctionalProgramming/Main.java
similarity index 100%
rename from Java/Advanced-Java/src/FunctionalProgramming/Main.java
rename to Advanced-Java/src/FunctionalProgramming/Main.java
diff --git a/Java/Advanced-Java/src/FunctionalProgramming/Person.java b/Advanced-Java/src/FunctionalProgramming/Person.java
similarity index 100%
rename from Java/Advanced-Java/src/FunctionalProgramming/Person.java
rename to Advanced-Java/src/FunctionalProgramming/Person.java
diff --git a/Java/Advanced-Java/src/FunctionalProgramming/_Function.java b/Advanced-Java/src/FunctionalProgramming/_Function.java
similarity index 100%
rename from Java/Advanced-Java/src/FunctionalProgramming/_Function.java
rename to Advanced-Java/src/FunctionalProgramming/_Function.java
diff --git a/Java/Advanced-Java/src/FunctionalProgramming/_Predicate.java b/Advanced-Java/src/FunctionalProgramming/_Predicate.java
similarity index 100%
rename from Java/Advanced-Java/src/FunctionalProgramming/_Predicate.java
rename to Advanced-Java/src/FunctionalProgramming/_Predicate.java
diff --git a/Java/Advanced-Java/src/FunctionalProgramming/_Supplier.java b/Advanced-Java/src/FunctionalProgramming/_Supplier.java
similarity index 100%
rename from Java/Advanced-Java/src/FunctionalProgramming/_Supplier.java
rename to Advanced-Java/src/FunctionalProgramming/_Supplier.java
diff --git a/Java/Advanced-Java/src/Optionals/_Optionals.java b/Advanced-Java/src/Optionals/_Optionals.java
similarity index 100%
rename from Java/Advanced-Java/src/Optionals/_Optionals.java
rename to Advanced-Java/src/Optionals/_Optionals.java
diff --git a/Java/Advanced-Java/src/csv_reader/CSVReader.java b/Advanced-Java/src/csv_reader/CSVReader.java
similarity index 100%
rename from Java/Advanced-Java/src/csv_reader/CSVReader.java
rename to Advanced-Java/src/csv_reader/CSVReader.java
diff --git a/Java/Advanced-Java/src/csv_reader/data.csv b/Advanced-Java/src/csv_reader/data.csv
similarity index 100%
rename from Java/Advanced-Java/src/csv_reader/data.csv
rename to Advanced-Java/src/csv_reader/data.csv
diff --git a/Java/Advanced-Java/src/dsa_koffman_ch2/JUnitTest.java b/Advanced-Java/src/dsa_koffman_ch2/JUnitTest.java
similarity index 100%
rename from Java/Advanced-Java/src/dsa_koffman_ch2/JUnitTest.java
rename to Advanced-Java/src/dsa_koffman_ch2/JUnitTest.java
diff --git a/Java/Advanced-Java/src/dsa_koffman_ch2/OrderedList.java b/Advanced-Java/src/dsa_koffman_ch2/OrderedList.java
similarity index 100%
rename from Java/Advanced-Java/src/dsa_koffman_ch2/OrderedList.java
rename to Advanced-Java/src/dsa_koffman_ch2/OrderedList.java
diff --git a/Java/Advanced-Java/src/dsa_koffman_ch2/TestOrderedList.java b/Advanced-Java/src/dsa_koffman_ch2/TestOrderedList.java
similarity index 100%
rename from Java/Advanced-Java/src/dsa_koffman_ch2/TestOrderedList.java
rename to Advanced-Java/src/dsa_koffman_ch2/TestOrderedList.java
diff --git a/Java/Advanced-Java/src/interfaces/Employee.java b/Advanced-Java/src/interfaces/Employee.java
similarity index 100%
rename from Java/Advanced-Java/src/interfaces/Employee.java
rename to Advanced-Java/src/interfaces/Employee.java
diff --git a/Java/Advanced-Java/src/interfaces/Person.java b/Advanced-Java/src/interfaces/Person.java
similarity index 100%
rename from Java/Advanced-Java/src/interfaces/Person.java
rename to Advanced-Java/src/interfaces/Person.java
diff --git a/Java/Advanced-Java/src/interfaces/TaxCollector.java b/Advanced-Java/src/interfaces/TaxCollector.java
similarity index 100%
rename from Java/Advanced-Java/src/interfaces/TaxCollector.java
rename to Advanced-Java/src/interfaces/TaxCollector.java
diff --git a/Java/Advanced-Java/src/interfaces/TaxPayer.java b/Advanced-Java/src/interfaces/TaxPayer.java
similarity index 100%
rename from Java/Advanced-Java/src/interfaces/TaxPayer.java
rename to Advanced-Java/src/interfaces/TaxPayer.java
diff --git a/Java/Advanced-Java/src/java8_features/FunctionalInterfaces.java b/Advanced-Java/src/java8_features/FunctionalInterfaces.java
similarity index 100%
rename from Java/Advanced-Java/src/java8_features/FunctionalInterfaces.java
rename to Advanced-Java/src/java8_features/FunctionalInterfaces.java
diff --git a/Java/Advanced-Java/src/java8_features/Lambda.java b/Advanced-Java/src/java8_features/Lambda.java
similarity index 100%
rename from Java/Advanced-Java/src/java8_features/Lambda.java
rename to Advanced-Java/src/java8_features/Lambda.java
diff --git a/Java/Advanced-Java/src/java8_features/LocalDateTimeExample.java b/Advanced-Java/src/java8_features/LocalDateTimeExample.java
similarity index 100%
rename from Java/Advanced-Java/src/java8_features/LocalDateTimeExample.java
rename to Advanced-Java/src/java8_features/LocalDateTimeExample.java
diff --git a/Java/Advanced-Java/src/java8_features/MethodReference.java b/Advanced-Java/src/java8_features/MethodReference.java
similarity index 100%
rename from Java/Advanced-Java/src/java8_features/MethodReference.java
rename to Advanced-Java/src/java8_features/MethodReference.java
diff --git a/Java/Advanced-Java/src/java8_features/Stream.java b/Advanced-Java/src/java8_features/Stream.java
similarity index 100%
rename from Java/Advanced-Java/src/java8_features/Stream.java
rename to Advanced-Java/src/java8_features/Stream.java
diff --git a/Java/Advanced-Java/src/java_gc/FinalizeExample.java b/Advanced-Java/src/java_gc/FinalizeExample.java
similarity index 100%
rename from Java/Advanced-Java/src/java_gc/FinalizeExample.java
rename to Advanced-Java/src/java_gc/FinalizeExample.java
diff --git a/Java/Advanced-Java/src/java_gc/WeakHashMapExample.java b/Advanced-Java/src/java_gc/WeakHashMapExample.java
similarity index 100%
rename from Java/Advanced-Java/src/java_gc/WeakHashMapExample.java
rename to Advanced-Java/src/java_gc/WeakHashMapExample.java
diff --git a/Java/Advanced-Java/src/java_gc/WeakReferenceExample.java b/Advanced-Java/src/java_gc/WeakReferenceExample.java
similarity index 100%
rename from Java/Advanced-Java/src/java_gc/WeakReferenceExample.java
rename to Advanced-Java/src/java_gc/WeakReferenceExample.java
diff --git a/Design-patterns/src/singleton/Captain.java b/Design-patterns/src/singleton/Captain.java
new file mode 100644
index 0000000..dd9c36a
--- /dev/null
+++ b/Design-patterns/src/singleton/Captain.java
@@ -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;
+    }
+}
diff --git a/Design-patterns/src/singleton/SingletonExample.java b/Design-patterns/src/singleton/SingletonExample.java
new file mode 100644
index 0000000..6d8e430
--- /dev/null
+++ b/Design-patterns/src/singleton/SingletonExample.java
@@ -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");
+        }
+
+    }
+}
diff --git a/interview-preparation.iml b/interview-preparation.iml
index 81ac92b..9dbde82 100644
--- a/interview-preparation.iml
+++ b/interview-preparation.iml
@@ -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" />