Singleton

A Singleton is a design pattern that ensures a class has exactly one instance in an application and provides a global point of access to that instance. It is perhaps the most simple and widely used pattern, but it can be abused and over used.

Typical Example

The easiest way to create a Singleton in a Java application is through a static field.

public class MySingleton {

    // The single instance, stored in a static field that
    // gets `Lazy Initialized` by the getInstance() method.
    private static MySingleton instance;

    // An `Eager singleton` style would instantiate immediately.
    // Do not do this in addition to Lazy Initialization. Choose ONE!
    private static final MySingleton INSTANCE = new MySingleton();

    // Private constructor prevents direct instantiation
    private MySingleton() { }

    // Public method to get the single instance `lazily`
    public static MySingleton getInstance() {
        if (instance == null) {
            instance = new MySingleton();
        }
        return instance;
    }
}