为什么会有一个抽象类“ DocumentBuilderFactory”？允许实例化新实例 [英] How come an abstract class "DocumentBuilderFactory" allowed to instantiate new instance

问题描述

最近，我一直在使用XML解析器。这只是我的开始，我设法了解了如何在Java中使用DOM解析器类，即 DocumentBuilderFactory 和 DocumentBuilder 解析XML文档。

Recently, I have been working with XML parsers. This is just beginning for me and I managed to understand how to use DOM parser classes in java i.e. DocumentBuilderFactory and DocumentBuilder to parse an XML document.

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();  
DocumentBuilder db = dbf.newDocumentBuilder();            

我要问自己的是抽象类的来历，例如 DocumentBuilderFactory 和 DocumentBuilder 是否可以实例化新实例？然后在另一个示例中，我看到：

What I am asking myself is how come an abstract classes, such as DocumentBuilderFactory and DocumentBuilder, are allowed to instantiate new instances? And then in another example I see:

Calendar calendar = Calendar.getInstance();  
System.out.println(calendar.get(Calendar.DATE)); 

1. 据我所知，您无法实例化（换句话说， ，创建一个对象）用于抽象类和接口类。我正确吗？


2. 执行 getInstance（）和 newInstancce（）方法创建上述抽象类的实例？

1. As far as I know, you can not instantiate (in other words, create an object) for abstract and interface classes. Am I correct?
2. Do getInstance() and newInstancce() methods create instances of the above abstract classes?

我是否缺少使用抽象类及其新对象的某些东西？

Am I missing something about using an abstract class and its new Objects?

推荐答案

该方法是 抽象工厂方法 ，它返回 DocumentBuilder 的子类，这是一个（具体的）实现

That method is an abstract factory method, which returns a subclass of DocumentBuilder, which is a (concrete) implementation.

确切的对象类别并不重要，您只需要知道它是 DocumentBuilder 。该方法可以返回在运行时决定的实例，或者在其认为合适时预定的实例。

The exact class of the object is not important to know, you only need to know that it's a DocumentBuilder. The method may return an instance decided at runtime, or predetermined as it sees fit.

如果您想知道，可以打印出实际的类，如下所示：

If you are curious to know, you can print out the actual class like this:

 System.out.println(dbf.getClass());

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请注意，方法 newInstance（）不要与 Class 的同名方法混淆，即这两个是不同的：

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Note that the method newInstance() is not to be confused with the method of the same name of Class, ie these two are different:

 // a static method of this class
 DocumentBuilderFactory.newInstance(); 

// an instance method of Class
 DocumentBuilderFactory.class.newInstance();

不幸的名字选择肯定会引起混乱。

An unfortunate choice of name sure to have caused confusion.

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