单例模式

public class Singleton {
    private Singleton(){}
    private static Singleton instance = new Singleton();
    public static Singleton getInstance(){
        return instance;
    }
}

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