Examples of singleton classes in the Java APIs

Question

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

Answer 1

Only two examples comes to mind:

• java.lang.Runtime#getRuntime()
• java.awt.Desktop#getDesktop()

See also:

• Real world examples of GoF Design Patterns in Java API

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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.