Singleton Design Pattern

singleton-bill-pugh.java

public class Singleton {
    private Singleton() {}

    private static class SingletonHolder {
        private static final Singleton uniqueInstance = new Singleton();
    }

    public static Singleton getInstance() {
        return SingletonHolder.uniqueInstance;
    }
}

singleton-lazy-initialization.java

public class Singleton {
    private volatile static Singleton uniqueInstance;

    private Singleton() {}

    public static Singleton getInstance() {
        if (uniqueInstance == null) {
            synchronized (Singleton.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
}

singleton-pattern.java

public class Singleton {
    private static Singleton uniqueInstance = new Singleton();
    private Map<String, String> kvmap = new ConcurrentHashMap(new HashMap<String, String>());

    private Singleton() {}

    public static Singleton getInstance { return uniqueInstance; }

    public String getValue(String key) { return kvmap.get(key); }
    public Set<String> getKeys() { return kvmap.keySet(); }
}

singleton-server-interface.java

public interface ServerInterface {
    public void config(String[] params);
}

public class ServerSingleton implements ServerInterface {
    private ServerSingleton() {}

    private static class SingletonHolder {
        private static final ServerSingleton uniqueInstance = new ServerSingleton();
    }

    public static ServerSingleton getInstance() {
        return SingletonHolder.uniqueInstance;
    }

    public void config(String[] params) {
        // ...
    }
}

public class Client {
    private final ServerInterface server;
    
    public Client(ServerInterface server) {
        this.server = server;
    }
    
    public void configServer(String[] params) {
        this.server.config(params);
    }
}