问如何为改造2制作一个单件？

EN

public class Api {
    private static Api instance = null;
    public static final String BASE_URL = "your_base_url";

    // Keep your services here, build them in buildRetrofit method later
    private UserService userService;

    public static Api getInstance() {
        if (instance == null) {
            instance = new Api();
        }

        return instance;
    }

    // Build retrofit once when creating a single instance
    private Api() {
        // Implement a method to build your retrofit
        buildRetrofit(BASE_URL);
    }

    private void buildRetrofit() {
        Retrofit retrofit = ...

        // Build your services once
        this.userService = retrofit.create(UserService.class);
        ...
    }

    public UserService getUserService() {
        return this.userService;
    }
    ...
}

UserService userService = Api.getInstance().getUserService();

public class SingletonClass {

private static volatile SingletonClass sSoleInstance = new SingletonClass();

    //private constructor.
    private SingletonClass(){}

    public static SingletonClass getInstance() {
        return sSoleInstance;
    }
}

public class SingletonClass {

    private static SingletonClass sSoleInstance;

    private SingletonClass(){}  //private constructor.

    public static SingletonClass getInstance(){
        if (sSoleInstance == null){ //if there is no instance available... create new one
            sSoleInstance = new SingletonClass();
        }

        return sSoleInstance;
   }
}

public class Singleton {
private volatile static Singleton singleton;
private Singleton(){}
public static Singleton getSingleton(){
    if (singleton == null) {
        synchronized (Singleton.class) {
            if (singleton == null) {
                singleton = new Singleton();
            }
        }
    }
    return singleton;
}