Using interface reference variable without instantiation

[Fei Wong]

Hi all, I have a question on the servlet. The code is in below

  

public class chp3 extends HttpServlet {
   
   public void doGet(HttpServletRequest request,[b]HttpServletResponse response[/b]) throws IOException,ServletException{

       [b]response.setContentType("text/html");[/b]

   }

}

May I know why the HttpServletResponse interface reference variable can straight using the method without instantiation? Thanks

[Charles 'King]

Servlets are controlled by the container. The container handles creating request and response objects. The container creates a thread for each request and calls the service method; passing request and response objects as arguments. Go to Sun's site and read about the servlet life cycle. If you have questions on passing objects to methods, please google it.

[Mark E Hansen]

To say this differently, the container created a response object which implements the HttpServletResponse interface. All your code has to know is how to call the methods described by the interface. You don't need to know anything about how the container actually implemented them.

[Jinglong Wang]

Just like this..
I have a interface..

public interface InterfaceDemo {
	public void print();
}

And then, I create a class implement InterfaceDemo interface.

public class InterfaceDemoImpl implements InterfaceDemo {
	public void print() {
		System.out.println("This is InterFace Demo Implements!");
	}
}
	public void printDemo(InterfaceDemo ifd){
		ifd.print();
	}

last i test it.

	public static void main(String[] args) {
		InterfaceDemo ifd = new InterfaceDemoImpl();
		ifd.print();
		
		InterfaceDemoImpl ifdi = new InterfaceDemoImpl();
		ifdi.printDemo(ifdi);
	}

Result:

This is InterFace Demo Implements!
This is InterFace Demo Implements!

we can see,the console print two lines.

this is just like the servlet, when the containner startup, it can create the servlet instance, than run the HttpServlet service method, it didn't know what the implements is.Just run the interface methods...