浅析Java设计模式编程中的单例模式和简单工厂模式

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

public class SingletonDemo { 
    private static volatile SingletonDemo instance = null; 
  
    private SingletonDemo() {    } 
  
    public static SingletonDemo getInstance() { 
        if (instance == null) { 
            synchronized (SingletonDemo .class){ 
                if (instance == null) { 
                    instance = new SingletonDemo (); 
                } 
           } 
        } 
        return instance; 
    } 
} 

   public class Singleton implements Serializable { 
... 
 
// This method is called immediately after an object of this class is deserialized. 
// This method returns the singleton instance. 
protected Object readResolve() { 
  return getInstance(); 
} 
} 

public class ProductFactory{ 
  public Product createProduct(String ProductID){ 
    if (id==ID1) 
      return new OneProduct(); 
    if (id==ID2)  
      return new AnotherProduct(); 
    ... // so on for the other Ids 
     
    return null; //if the id doesn't have any of the expected values 
  } 
  ... 
}