Creational Patterns : Singleton

Properties:

-          Ensures that only one instance is created.

-          Global access is provided to single instance.

-          Conventionally has a public static method named getInstance/getXYZ ,where XYZ stands for class name, to return the class instance.

-          Suitable for creating factory instances and system wide unique resources.

Implementation:

-          Constructor should be made private/protected to prevent arbitrary instance creation.

-          Single instance is created and assigned to a public static member of the class. This class member is conventionally named as instance.

-          Class instance is exposed to outside world by a public static method named getInstance/getXYZ ,where XYZ stands for class name.

Java Standard Library Implementations:

-          java.lang.Runtime

-          java.awt.Desktop

-          java.awt.print.PrinterJob

Example Usage:

PrinterJob pj = PrinterJob.getPrinterJob();

if (pj.printDialog()) {

    try {

        pj.print();

    }

    catch (PrinterException e) {

        e.printStackTrace();

    }

 }