Singleton Design Pattern

Singleton pattern is one of the simplest design patterns in Java. This type of design pattern comes under creational pattern as this pattern provides one of the best way to create an object.
This pattern involves a single class which is responsible to creates own object while making sure that only single object get created. This class provides a way to access its only object which can be accessed directly without need to instantiate the object of the class.

Implementation
We're going to create a SingleObject class. SingleObject class have its constructor as private and have a static instance of itself. SingleObject class provides a static method to get its static instance to outside world.SingletonPatternDemo, our demo class will use SingleObject class to get a SingleObject object.

Class Diagram


Steps 
Use the following steps to implement the above mentioned design pattern.
Step 1
Create a Singleton Class.
SingleObject.java
public class SingleObject {
           //create an object of SingleObject
           private static SingleObject instance = new SingleObject();
           //make the constructor private so that this class cannot be
           //instantiated
           private SingleObject(){}
           //Get the only object available
           public static SingleObject getInstance(){
                      return instance;
           }
           public void showMessage(){
                      System.out.println("Hello World!");
           }
}
Step 2
Get the only object from the singleton class.
SingletonPatternDemo.java
public class SingletonPatternDemo {
           public static void main(String[] args) {
                      //illegal construct
                      //Compile Time Error: The constructor SingleObject() is not visible
                      //SingleObject object = new SingleObject();
                      //Get the only object available
                      SingleObject object = SingleObject.getInstance();
                      //show the message object.showMessage();
           }
}

Step 3
Verify the output.
Hello World!

Abstract Factory Pattern

Abstract Factory patterns works around a super-factory which creates other factories. This factory is also called as Factory of factories. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.
In Abstract Factory pattern an interface is responsible for creating a factory of related objects, without explicitly specifying their classes. Each generated factory can give the objects as per the Factory pattern.
Implementation

We're going to create a Shape and Color interfaces and concrete classes implementing these interfaces. We creates an abstract factory class AbstractFactory as next step. Factory classesShapeFactory and ColorFactory are defined where each factory extends AbstractFactory. A factory creator/generator class FactoryProducer is created.

AbstractFactoryPatternDemo, our demo class uses FactoryProducer to get a AbstractFactory object. It will pass information (CIRCLE / RECTANGLE / SQUARE for Shape) to AbstractFactory to get the type of object it needs. It also passes information (RED / GREEN / BLUE for Color) to AbstractFactory to get the type of object it needs.

Class Diagram
 

Steps 
Use the following steps to implement the above mentioned design pattern.

Step 1 
Create an interface.
Shape.java 

public interface Shape { 
      void draw(); 
}

Step 2 
Create concrete classes implementing the same interface.
Rectangle.java 

public class Rectangle implements Shape { 
   @Override 
    public void draw() { 
         System.out.println("Inside Rectangle::draw() method."); 
     } 
}

Square.java 
public class Square implements Shape { 
      @Override 
       public void draw() { 
                 System.out.println("Inside Square::draw() method."); 
       } 
} 

Circle.java 
public class Circle implements Shape { 
      @Override 
       public void draw() { 
                 System.out.println("Inside Circle::draw() method.");
       }
 }

Step 3 
Create an interface for Colors. 
Color.java 
public interface Color { 
    void fill(); 
}

Step4 
Create concrete classes implementing the same interface.
Red.java 
 public class Red implements Color { 
          @Override public void fill() { 
                    System.out.println("Inside Red::fill() method."); 
          } 
} 
Green.java 
public class Green implements Color { 
          @Override public void fill() { 
                    System.out.println("Inside Green::fill() method."); 
          } 
} 
Blue.java 
public class Blue implements Color { 
          @Override 
          public void fill() { 
                    System.out.println("Inside Blue::fill() method."); 
          } 
}   

Step 5 
Create an Abstract class to get factories for Color and Shape Objects. 
AbstractFactory.java 
public abstract class AbstractFactory { 
          abstract Color getColor(String color); 
          abstract Shape getShape(String shape) ; 
}

Step 6 
Create Factory classes extending AbstractFactory to generate object of concrete class based on given information.
ShapeFactory.java 
public class ShapeFactory extends AbstractFactory { 
          @Override 
          public Shape getShape(String shapeType){ 
                    if(shapeType == null){ 
                              return null; 
                   } 
                    if(shapeType.equalsIgnoreCase("CIRCLE")){ 
                              return new Circle(); 
                    } else if(shapeType.equalsIgnoreCase("RECTANGLE")){ 
                              return new Rectangle(); 
                    } else if(shapeType.equalsIgnoreCase("SQUARE")){ 
                              return new Square(); 
                    } 
                    return null; 
          } 
          @Override 
          Color getColor(String color) { 
                    return null; 
          } 
}

ColorFactory.java 
public class ColorFactory extends AbstractFactory { 
          @Override 
          public Shape getShape(String shapeType){ 
                    return null; 
          } 
          @Override 
          Color getColor(String color) { 
                    if(color == null){ 
                              return null; 
                    } 
                    if(color.equalsIgnoreCase("RED")){ 
                              return new Red(); 
                    } else if(color.equalsIgnoreCase("GREEN")){ 
                              return new Green(); 
                    } else if(color.equalsIgnoreCase("BLUE")){ 
                              return new Blue(); 
                    } return null; 
          } 
}

Step 7 
Create a Factory generator/producer class to get factories by passing an information such as Shape or Color
FactoryProducer.java 
public class FactoryProducer { 
          public static AbstractFactory getFactory(String choice {
                    if(choice.equalsIgnoreCase("SHAPE")){ 
                              return new ShapeFactory(); 
                    } else if(choice.equalsIgnoreCase("COLOR")){ 
                              return new ColorFactory(); 
                    } return null; 
          } 
}

Step 8 
Use the FactoryProducer to get AbstractFactory in order to get factories of concrete classes by passing information such as type. 
AbstractFactoryPatternDemo.java 
public class AbstractFactoryPatternDemo { 
          public static void main(String[] args) { 
                     //get shape factory 
                    AbstractFactory shapeFactory = FactoryProducer.getFactory("SHAPE"); 
                    //get an object of Shape Circle 
                    Shape shape1 = shapeFactory.getShape("CIRCLE"); 
                    //call draw method of Shape Circle 
                    shape1.draw(); 
          
                     //get an object of Shape Rectangle 
                    Shape shape2 = shapeFactory.getShape("RECTANGLE"); 
                    //call draw method of Shape Rectangle 
                    shape2.draw(); 
          
                     //get an object of Shape Square 
                    Shape shape3 = shapeFactory.getShape("SQUARE"); 
                    //call draw method of Shape Square 
                    shape3.draw(); 
           
                    //get color factory 
                    AbstractFactory colorFactory = FactoryProducer.getFactory("COLOR");
                    //get an object of Color Red 
                    Color color1 = colorFactory.getColor("RED"); 
                    //call fill method of  Red 
                    color1.fill(); 
                    //get an object of Color Green 
                    Color color2 = colorFactory.getColor("Green"); 
                    //call fill method of Green 
                    color2.fill(); 
                    //get an object of Color Blue 
                    Color color3 = colorFactory.getColor("BLUE"); 
                    //call fill method of Color Blue 
                    color3.fill(); 
          } 
}

Step 9 
Verify the output. 
Inside Circle::draw() method. 
Inside Rectangle::draw() method. 
Inside Square::draw() method. 
Inside Red::fill() method. 
Inside Green::fill() method. 
Inside Blue::fill() method.