How come an abstract class "DocumentBuilderFactory" allowed to instantiate new instance

Question

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();            

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. 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?

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

Answer 1

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

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.

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

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

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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();

An unfortunate choice of name sure to have caused confusion.

Answer 2

That is a static abstract factory method , which will return a subtype of DocumentBuilderFactory not the actual instance of DocumentBuilderFactory itself.It is not like what I presume, you understand :

DocumentBuilderFactory dbf = new DocumentBuilderFactory();

DocumentBuilderFactory#newInstance() , Obtain a new instance of a DocumentBuilderFactory. This static method creates a new factory instance. This method uses the following ordered lookup procedure to determine the DocumentBuilderFactory implementation class to load.

newInstance() will return an instance of the implementing class(subtype) of DocumentBuilderFactory, which is not abstract and will assign reference to that object to DocumentBuilderFactory reference variable.