javadesign-patterns

Examples of singleton classes in the Java APIs

What are the best examples of the Singleton design pattern in the Java APIs? Is the Runtime class a singleton?

Solution

  • Only two examples comes to mind:

    See also:

    Update: to answer PeterMmm's (currently deleted?) comment (which asked how I knew that it was a singleton), check the javadoc and source code:

    public class Runtime {
    private static Runtime currentRuntime = new Runtime();

    /**
     * Returns the runtime object associated with the current Java application.
     * Most of the methods of class <code>Runtime</code> are instance 
     * methods and must be invoked with respect to the current runtime object. 
     * 
     * @return  the <code>Runtime</code> object associated with the current
     *          Java application.
     */
    public static Runtime getRuntime() { 
        return currentRuntime;
    }

    /** Don't let anyone else instantiate this class */
    private Runtime() {}

    It returns the same instance everytime and it has a private constructor.