[BAEL-1066] Implementing Singleton pattern in Java (#2361)

* Different Types of Bean Injection in Spring code

* Dataclasses in Kotlin

* Revert "Different Types of Bean Injection in Spring code"

This reverts commit 4b747726b93a9f6bf76d6518792fc77e0d5c2fc9.

* Destructuring Declarations in Kotlin

* Corrections on Destructuring Declarations in Kotlin

* Implementing Singleton pattern in Java

* [BAEL-1066] Implementing Singleton pattern in Java

* Singleton Implementation changes.

core-java/src/main/java/com/baeldung/designpatterns/singleton/ClassSingleton.java

@@ -0,0 +1,28 @@
+package com.baeldung.designpatterns.singleton;
+
+public class ClassSingleton {
+
+    private static ClassSingleton INSTANCE;
+    private String info = "Initial class info";
+    
+    private ClassSingleton(){        
+    }
+    
+    public static ClassSingleton getInstance(){
+        if(INSTANCE == null){
+            INSTANCE = new ClassSingleton();
+        }
+        
+        return INSTANCE;
+    }
+    
+    // getters and setters
+    
+    public String getInfo() {
+        return info;
+    }
+    
+    public void setInfo(String info) {
+        this.info = info;
+    }
+}

core-java/src/main/java/com/baeldung/designpatterns/singleton/EnumSingleton.java

@@ -0,0 +1,26 @@
+package com.baeldung.designpatterns.singleton;
+
+public enum EnumSingleton {
+    
+    INSTANCE("Initial enum info"); //Name of the single instance
+    
+    private String info;
+    
+    private EnumSingleton(String info) {
+        this.info = info;
+    }
+    
+    public EnumSingleton getInstance(){
+        return INSTANCE;
+    }
+    
+    //getters and setters
+
+    public String getInfo() {
+        return info;
+    }
+
+    public void setInfo(String info) {
+        this.info = info;
+    }    
+}

core-java/src/main/java/com/baeldung/designpatterns/singleton/Sandbox.java

@@ -0,0 +1,32 @@
+package com.baeldung.designpatterns.singleton;
+
+public class Sandbox {
+
+    public static void main(String[] args) {
+        
+        //Class singleton
+        
+        ClassSingleton classSingleton1 = ClassSingleton.getInstance();
+        //OurSingleton object1 = new OurSingleton(); // The constructor OurSingleton() is not visible
+        
+        System.out.println(classSingleton1.getInfo()); //Initial class info
+        
+        ClassSingleton classSingleton2 = ClassSingleton.getInstance();
+        classSingleton2.setInfo("New class info");
+        
+        System.out.println(classSingleton1.getInfo()); //New class info
+        System.out.println(classSingleton2.getInfo()); //New class info
+        
+        //Enum singleton
+        
+        EnumSingleton enumSingleton1 = EnumSingleton.INSTANCE.getInstance();
+        
+        System.out.println(enumSingleton1.getInfo()); //Initial enum info
+        
+        EnumSingleton enumSingleton2 = EnumSingleton.INSTANCE.getInstance();
+        enumSingleton2.setInfo("New enum info");
+        
+        System.out.println(enumSingleton1.getInfo()); //New enum info
+        System.out.println(enumSingleton2.getInfo()); //New enum info
+    }
+}