Java API 中单例类的示例

Java API 中单例设计模式的最佳示例有哪些？是个Runtime类单例？

我只想到两个例子：

• java.lang.Runtime#getRuntime() http://java.sun.com/javase/6/docs/api/java/lang/Runtime.html#getRuntime%28%29
• java.awt.Desktop#getDesktop() http://java.sun.com/javase/6/docs/api/java/awt/Desktop.html#getDesktop%28%29

See also:

• Java API 中 GoF 设计模式的真实示例 https://stackoverflow.com/questions/1673841/examples-of-gof-design-patterns/2707195#2707195

Update：要回答 PeterMmm 的（当前已删除？）评论（问我如何知道它是单例），请检查 javadoc 和源代码：

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() {}

它每次都返回相同的实例，并且它有一个private构造函数。