这是我在面试中软国际时候的一道笔试题。
编程题: 写一个Singleton出来。
Singleton模式
Singleton可以说是《Design Pattern》中最简单也最实用的一个设计模式。那么,什么是Singleton?
顾名思义,Singleton就是确保一个类只有唯一的一个实例。Singleton主要用于对象的创建,这意味着,如果某个类采用了 Singleton 模式,则在这个类被创建后,它将有且仅有一个实例可供访问。很多时候我们都会需要Singleton模式,最常见的比如我们希望整个应用程序中只有一个连 接数据库的Connection实例;又比如要求一个应用程序中只存在某个用户数据结构的唯一实例。我们都可以通过应用Singleton模式达到目的。
一眼看去,Singleton似乎有些像全局对象。但是实际上,并不能用全局对象代替Singleton模式,这是因为:其一,大量使用全局对象会使得程 序质量降低,而且有些编程语言例如C#,根本就不支持全局变量。其二,全局对象的方法并不能阻止人们将一个类实例化多次:除了类的全局实例外,开发人员仍 然可以通过类的构造函数创建类的多个局部实例。而Singleton模式则通过从根本上控制类的创建,将”保证只有一个实例”这个任务交给了类本身,开发 人员不可能再有其它途径得到类的多个实例。这一点是全局对象方法与Singleton模式的根本区别。
Singleton模式的实现
Singleton模式的实现基于两个要点:
1)不直接用类的构造函数,而另外提供一个Public的静态方法来构造类的实例。通常这个方法取名为Instance。Public保证了它的全局可见性,静态方法保证了不会创建出多余的实例。
2)将类的构造函数设为Private,即将构造函数”隐藏”起来,任何企图使用构造函数创建实例的方法都将报错。这样就阻止了开发人员绕过上面的Instance方法直接创建类的实例。
通过以上两点就可以完全控制类的创建:无论有多少地方需要用到这个类,它们访问的都是类的唯一生成的那个实例。
以下C#代码展现了两种实现Singleton模式的方式,开发人员可以根据喜好任选其一。
实现方式一:Singleton.cs
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另外一种实现方式是:Singleton.cs:
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编译执行:
Csc Singleton.cs
得到运行结果:
see, only one instance!
Jave 实现方式
第一种形式: 定义一个类,它的构造函数为private的,它有一个static的private的该类变量,在类初始化时实例话,通过一个public的getInstance方法获取对它的引用,继而调用其中的方法。
java 代码
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第二种形式:
java 代码
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其他形式:
定义一个类,它的构造函数为private的,所有方法为static的。
一般认为第一种形式要更加安全些.