public interface Shape { 
 void draw(); 
} 

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

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

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

public class ShapeFactory { 
 
 //use getShape method to get object of type shape 
 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; 
 } 
} 

public class FactoryPatternDemo { 
 
 public static void main(String[] args) { 
 ShapeFactory shapeFactory = new ShapeFactory(); 
 
 //get an object of Circle and call its draw method. 
 Shape shape1 = shapeFactory.getShape("CIRCLE"); 
 
 //call draw method of Circle 
 shape1.draw(); 
 
 //get an object of Rectangle and call its draw method. 
 Shape shape2 = shapeFactory.getShape("RECTANGLE"); 
 
 //call draw method of Rectangle 
 shape2.draw(); 
 
 //get an object of Square and call its draw method. 
 Shape shape3 = shapeFactory.getShape("SQUARE"); 
 
 //call draw method of circle 
 shape3.draw(); 
 } 
} 

Inside Circle::draw() method. 
Inside Rectangle::draw() method. 
Inside Square::draw() method. 

public class ThisEscape { 
... 
 public ThisEscape(EventSource source) { 
     source.registerListener(new EventListener() { 
                   public void onEvent(Event e) { 
                        doSomething(e); 
               } 
           }); 
 ... 
 } 
} 

public class SafeListener { 
     private final EventListener listener; 
     private SafeListener() { 
         listener = new EventListener() { 
                 public void onEvent(Event e) { 
                doSomething(e); 
          } 
         }; 
  } 

 public static SafeListener newInstance(EventSource source) { 
   SafeListener safe = new SafeListener(); 
   source.registerListener(safe.listener); 
 return safe; 
 } 
} 

Shape shape = shapeFactory.getShape("CIRCLE"); 

((Circle)shape).getRadius();

Shape getColorShape(String type , String color){ 
Shape shape = shapeFactory.getShape(type); 
shape.setColor(color);
return color; 
}

public Holder holder; 
 public void initialize() { 
    holder = new Holder(42); 
  } 
... 
  public class Holder { 
            private int n; 
 
     public Holder(int n) { 
        this.n = n; 
      } 

    public void assertSanity() { 
           if (n != n) 
            throw new AssertionError("This statement is false."); 
      } 
} 

//Thread1 
initialize(); 
 
//Thread2 
if (holder != null){ 
 holder.assertSanity(); 
}

public static Boolean valueOf(boolean b) {
  return b ? Boolean.TRUE : Boolean.FALSE;
}

//Shape.java 
public interface Shape { 
 void draw(); 
} 

//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."); 
 } 
} 

public interface Color { 
 void fill(); 
} 

public class Red implements Color { 
 
 @Override 
 public void fill() { 
 System.out.println("Inside Red::fill() method."); 
 } 
}

public class Green implements Color { 
 
 @Override 
 public void fill() { 
 System.out.println("Inside Green::fill() method."); 
 } 
}

public abstract class AbstractFactory { 
 abstract Color getColor(String color); 
 abstract Shape getShape(String shape) ; 
} 

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; 
 } 
} 

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; 
 } 
} 

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; 
 } 
} 

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(); 
 } 
}

Inside Circle::draw() method. 
Inside Rectangle::draw() method. 
Inside Square::draw() method. 
Inside Red::fill() method. 
Inside Green::fill() method. 
Inside Blue::fill() method. 

public void createSquadron (SquadronFactory factory) {
  Mage mage = factory.createMage();
  Archer archer = factory.createArcher();
  Warrior warrior = factory.createWarrior();
  ...
}

//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!"); 
 } 
} 

//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(); 
 } 
} 

Hello World! 

public class Singleton {  
   private Singleton() {}
 
   private static class SingletonHolder {  
      public static final Singleton instance = new Singleton();  
   }  
 
   public static Singleton getInstance()  {  
      return SingletonHolder.instance;  
   }  
}