我想到的只有两个例子：

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

另请参阅 ：

• Java API中GoF设计模式的真实示例

更新 ：要回答PeterMmm（当前已删除？）的注释（问我如何知道它是一个单例），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，并且具有private构造函数。