A simple design pattern example.
Concept: A creational pattern. You basically need one "global-type" object to exist. Implementation is pretty cut-and-dry.
Pros: We can be sure that there is only one access point to the class.
Cons: Anything "global in nature" leads to tightly coupled classes. We want loosely coupled classes. Unit testing individual classes becomes more difficult.
Example: In the following example, we exhibit how the Singleton Pattern can be used to have a single logger for an application.
To accomplish the Singleton Pattern, we:
- Create a Singleton class.
- Make the class constructor
private. - Define a public static method called
getInstance()so only one single instance of the class ever gets created and returned.
When using Java, in order to make the code thread-safe and to avoid needing synchronization, we:
- Create a static inner class helper to lazily create the singleton instance.
- We return the Singleton Helper's INSTANCE when
getInstance()is called.
In this example, the class has an ArrayList of type String that can have entries added to it to using the public void log(String message) method.
It also returns the entire log using the public List<String> getLog() method.