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Creational Patterns : Factory Method


Properties:
-          The same with factory pattern except that Factory class is abstract class and createXYZ method is made abstract.
-          Subclasses of the factory class implements createXYZ method thus subclass determines how instance is created.
-          Identified by abstract Factory class and abstract creation method.


Implementation:
-          Factory class has an abstract method named createXYZ which takes necessary arguments.
-          Abstract createXYZ method is implemented by subclasses of Factory class. A Subclass creates an appropriate concrete XYZ instance and retuns.
-          Client uses returned XYZ instance without ever knowing actual implementation.


Java Standard Library Implementations:
-          javax.xml.parsers.DocumentBuilderFactory
-          javax.xml.parsers.SAXParserFactory


Example Usage:
try {
   
    // returns com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl instance.
    DocumentBuilderFactory factory =
        DocumentBuilderFactory.newInstance();

    // Focus this part:Factory Method pattern is in use.
    // Note That: newDocumentBuilder() is an abstrack method in DocumentBuilderFactory class.
    // Thus newDocumentBuilder() method defined in
    // com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl is used.
    DocumentBuilder builder = factory.newDocumentBuilder();
       
       
    // builder instance is used.
    Document doc = builder.parse(new File(args[0]));
   

} catch (ParserConfigurationException e) {
    e.printStackTrace();
} catch (FactoryConfigurationError e) {
    e.printStackTrace();
} catch (Exception e) {
    e.printStackTrace();
}

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