3.1.1. Singleton Pattern

• The singleton pattern is a software design pattern that restricts the instantiation of a class to one object. The singleton pattern ensures that a class has only one instance and the sole instance of the class can be accessed easily.

• This is useful when exactly one object is needed to coordinate actions across the system.

• ==Implementation==

  • Hide the constructor of the class by declaring all constructors of the class to be private.

  • privode global access to that instance by defining a public static operation (getInstance()) that returns the sole instance of the class.

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

  • Lazy initialization

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

    Using double-checked locking guarantees the class is thread-safe.

    ​

Referenced Links

Singleton Pattern from Wikipedia