Showing posts with the label Singleton Pattern

Use Java enum to create Singletons


Using a State-Machine to Control a Wizard

On previous articles, I wrote about real-world usages for the State and Singleton design patterns. I also wrote about using these two patterns together to create a state-machine. Now, I am going use that state-machine to control a Wizard. I am going to use Java Swing to create the User Interface classes. To create my interface using Java Swing, I am going to create three panels for contents, one panel to provide the navigation controls, and a frame to hold it all together. It will be a simple example, but one you could easily expand should you have the need to create your own wizard. The Frame The frame is the application window. I will not go to much into details about the Java JFrame class, as I assume you have some basic knowledge of Java and Java Swing. If you don't much about Java Swing, I suggest you visit this Oracle tutorial on how to create frames . public class Main extends JFrame { private CardsPanel cards = new CardsPanel(); private Navi

Combining State and Singleton Patterns to Create a State-Machine

In my previous two posts, I discussed real-world applications for the Singleton and State design patterns. In this article, I am going to illustrate how to combine both of these patterns to create a simple wizard. Simple State Design Pattern Implementation In a typical implementation of the design pattern, State is either an interface or abstract class, with each state of the state machine being Singleton classes. My example is going to be slightly different. I will implement the state machine using a state interface and Java enums to implement the Singleton. Using an enum is the recommended way to implement a Singleton in Java. First, let's come up with a simple state interface public interface State { void goNext(Context input); void goPrevious(Context input); } Now that we have an interface defined, we can derive as many states as we need. For this example, three states should be sufficient to demonstrate the wizard's functionality. Jav

Applying the Singleton Design Pattern to Real-World Scenarios

There are many who believe that the Singleton pattern is an anti-pattern. By the title you probably assumed I am not one of those people, and you will be right. I believe the term "anti-pattern" is being used for cases where a pattern is not being used for what it was originally designed for or just implemented incorrectly. I am going to try to keep this short. But I need to explain what the Singleton pattern is before I provide a real-life example of its applicability. The Singleton pattern is one of the original Gang of Four Design Patterns . The Singleton pattern falls under the Creational Pattern category. This means that the purpose of the Singleton pattern is to create objects. The creational purpose of the pattern is to ensure that one and only one object is instantiated in the life of the application. By definition, an anti-pattern is a pattern that is usually ineffective and highly counterproductive. So, why do classify Singleton as an anti-pattern? I fou