抽象类“DocumentBuilderFactory"怎么来的?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.这对我来说才刚刚开始，我设法了解如何在 Java 中使用 DOM 解析器类，即 DocumentBuilderFactory 和 DocumentBuilder 来解析 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();            

我问自己是如何允许抽象类(例如 DocumentBuilderFactory 和 DocumentBuilder)实例化新实例的?然后在另一个例子中我看到:

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，您不能为抽象类和接口类实例化(换句话说，创建一个对象).我说得对吗?
2. getInstance() 和 newInstancce() 方法是否会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?

我是否遗漏了有关使用抽象类及其新对象的内容?

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

推荐答案

那个方法是一个 抽象工厂方法，它返回一个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.

知道对象的确切类并不重要，您只需要知道它是一个DocumentBuilder.该方法可以返回一个在运行时决定的实例，或者它认为合适的预定实例.

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.

如果你想知道，你可以像这样打印出实际的类:

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

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

<小时>

注意newInstance()方法不要与Class的同名方法混淆，即两者是不同的:

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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();

一个不幸的名字选择肯定会引起混淆.

An unfortunate choice of name sure to have caused confusion.

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