扩展单例（单例继承）

在此示例中，基类 Singleton 提供了返回 Hello world! 消息的 getMessage() 方法。

它的子类 UppercaseSingleton 和 LowercaseSingleton 覆盖了 getMessage() 方法，以提供消息的适当表示。

//Yeah, we'll need reflection to pull this off.
import java.lang.reflect.*;

/*
Enumeration that represents possible classes of singleton instance.
If unknown, we'll go with base class - Singleton.
*/
enum SingletonKind {
    UNKNOWN,
    LOWERCASE,
    UPPERCASE
}

//Base class
class Singleton{

    /*
    Extended classes has to be private inner classes, to prevent extending them in 
    uncontrolled manner.
     */
    private class UppercaseSingleton extends Singleton {

        private UppercaseSingleton(){
            super();
        }

        @Override
        public String getMessage() {
            return super.getMessage().toUpperCase();
        }
    }

    //Another extended class.
    private class LowercaseSingleton extends Singleton
    {
        private LowercaseSingleton(){
            super();
        }

        @Override
        public String getMessage() {
            return super.getMessage().toLowerCase();
        }
    }

    //Applying Singleton pattern
    private static SingletonKind kind = SingletonKind.UNKNOWN;

    private static Singleton instance;

    /*
    By using this method prior to getInstance() method, you effectively change the
    type of singleton instance to be created.
     */
    public static void setKind(SingletonKind kind) {
        Singleton.kind = kind;
    }

    /*
    If needed, getInstance() creates instance appropriate class, based on value of
    singletonKind field.
     */
    public static Singleton getInstance() 
        throws  NoSuchMethodException, 
                IllegalAccessException, 
                InvocationTargetException, 
                InstantiationException {

        if(instance==null){
            synchronized (Singleton.class){
                if(instance==null){
                    Singleton singleton = new Singleton();
                    switch (kind){
                        case UNKNOWN:

                            instance = singleton;
                            break;

                        case LOWERCASE:

                            /*
                             I can't use simple

                             instance = new LowercaseSingleton();

                             because java compiler won't allow me to use
                             constructor of inner class in static context,
                             so I use reflection API instead.

                             To be able to access inner class by reflection API,
                             I have to create instance of outer class first.
                             Therefore, in this implementation, Singleton cannot be
                             abstract class.
                             */

                            //Get the constructor of inner class.
                            Constructor<LowercaseSingleton> lcConstructor =
                                    LowercaseSingleton.class.getDeclaredConstructor(Singleton.class);

                            //The constructor is private, so I have to make it accessible.
                            lcConstructor.setAccessible(true);

                            // Use the constructor to create instance.
                            instance = lcConstructor.newInstance(singleton);

                            break;

                        case UPPERCASE:

                            //Same goes here, just with different type
                            Constructor<UppercaseSingleton> ucConstructor =
                                    UppercaseSingleton.class.getDeclaredConstructor(Singleton.class);
                            ucConstructor.setAccessible(true);
                            instance = ucConstructor.newInstance(singleton);
                    }
                }
            }
        }
        return instance;
    }

    //Singletons state that is to be used by subclasses
    protected String message;

    //Private constructor prevents external instantiation.
    private Singleton()
    {
        message = "Hello world!";
    }

    //Singleton's API. Implementation can be overwritten by subclasses.
    public String getMessage() {
        return message;
    }
}

//Just a small test program
public class ExtendingSingletonExample {

    public static void main(String args[]){

        //just uncomment one of following lines to change singleton class

        //Singleton.setKind(SingletonKind.UPPERCASE);
        //Singleton.setKind(SingletonKind.LOWERCASE);

        Singleton singleton = null;
        try {
            singleton = Singleton.getInstance();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        }
        System.out.println(singleton.getMessage());
    }
}