文章作者：Tyan
博客：noahsnail.com | CSDN | 简书
Item 4: Enforce noninstantiability with a private constructor
Occasionally you’ll want to write a class that is just a grouping of static methods and static fields. Such classes have acquired a bad reputation because some people abuse them to avoid thinking in terms of objects, but they do have valid uses. They can be used to group related methods on primitive values or arrays, in the manner of java.lang.Math or java.util.Arrays. They can also be used to group static methods, including factory methods (Item 1), for objects that implement a particular interface, in the manner of java.util.Collections. Lastly, they can be used to group methods on a final class, instead of extending the class.
有时你会想写一个只包含一组静态方法和静态变量的类。这种类的名声很不好，因为有些人滥用它们来避免思考如何面向对象，但它们确实是有用的。它们可以用来以java.lang.Math或java.util.Arrays的方式来组织与基本类型或数组相关的方法。它们也可以用来以java.util.Collections的方式来组织实现特定接口对象的静态方法，包括工厂方法（Item 1）。最后，它们可以用来组织一个fianl类的方法，从而代替扩展这个类。
Such utility classes were not designed to be instantiated: an instance would be nonsensical. In the absence of explicit constructors, however, the compiler provides a public, parameterless default constructor. To a user, this constructor is indistinguishable from any other. It is not uncommon to see unintentionally instantiable classes in published APIs.
这种工具类被设计成不能实例化：它的实例是没有意义的。然而，在缺少显式构造函数的情况下，编译器会提供一个公有的无参构造默认函数。对用户而言，这个构造函数与其它的构造函数没有任何差别。在发布的APIs中看到无意义的可实例化类是很罕见的。
Attempting to enforce noninstantiability by making a class abstract does not work. The class can be subclassed and the subclass instantiated.
Furthermore, it misleads the user into thinking the class was designed for inheritance (Item 17). There is, however, a simple idiom to ensure noninstantiability. A default constructor is generated only if a class contains no explicit constructors, so a class can be made noninstantiable by including a private constructor:
企图通过声明一个类为抽象类来强制类不能被实例化是行不通的。这个类可以被子类化，子类可以被实例化。而且，它会使用户误认为这个类是为继承而设计的（Item 17）。然而有一些简单的习惯用法可以确保类不能被实例化。如果一个类没有显式的构造函数，会产生默认的构造函数，因此，一个含有私有构造函数的类不能被实例化：
1 | // Noninstantiable utility class |
Because the explicit constructor is private, it is inaccessible outside of the class. The AssertionError isn’t strictly required, but it provides insurance in case the constructor is accidentally invoked from within the class. It guarantees that the class will never be instantiated under any circumstances. This idiom is mildly counterintuitive, as the constructor is provided expressly so that it cannot be invoked. It is therefore wise to include a comment, as shown above.
因为显式构造函数是私有的，因此类的外部不能访问构造函数。AssertionError不是必须的，但它可以避免类内部无意的调用构造函数。这种习惯用法有点违背直觉，似乎构造函数的提供就是为了它不能被调用一样。因此明智的做法是在类中加上注释，像上面的例子一样。
As a side effect, this idiom also prevents the class from being subclassed. All constructors must invoke a superclass constructor, explicitly or implicitly, and a subclass would have no accessible superclass constructor to invoke.
这种习惯用法的一个副作用就是阻止了类的子类化。子类的所有的构造函数必须调用父类的构造函数，无论是显式的或隐式的，但这种情况下子类不能调用父类构造函数。
