3: Building application frameworks

An application framework allows you to inherit from a class or set of classes and create a new application, reusing most of the code in the existing classes and overriding one or more methods in order to customize the application to your needs. A fundamental concept in the application framework is the Template Method which is typically hidden beneath the covers and drives the application by calling the various methods in the base class (some of which you have overridden in order to create the application).

For example, whenever you create an applet you’re using an application framework: you inherit from JApplet and then override init( ). The applet mechanism (which is a Template Method) does the rest by drawing the screen, handling the event loop, resizing, etc.

Template method

An important characteristic of the Template Method is that it is defined in the base class and cannot be changed. It’s sometimes a private method but it’s virtually always final. It calls other base-class methods (the ones you override) in order to do its job, but it is usually called only as part of an initialization process (and thus the client programmer isn’t necessarily able to call it directly).

//: c03:TemplateMethod.java
// Simple demonstration of Template Method.
import com.bruceeckel.test.*;

abstract class ApplicationFramework {
  public ApplicationFramework() {
    templateMethod(); // Dangerous!
  }
  abstract void customize1();
  abstract void customize2();
  // "private" means automatically "final":
  private void templateMethod() {
    for(int i = 0; i < 5; i++) {
      customize1();
      customize2();
    }
  }
}

// Create a new "application":
class MyApp extends ApplicationFramework {
  void customize1() { 
    System.out.print("Hello ");
  }
  void customize2() { 
    System.out.println("World!");
  }
}

public class TemplateMethod extends UnitTest {
  MyApp app = new MyApp();
  public void test() {
    // The MyApp constructor does all the work.
    // This just makes sure it will complete 
    // without throwing an exception.
  }
  public static void main(String args[]) {
    new TemplateMethod().test();
  }
} ///:~

The base-class constructor is responsible for performing the necessary initialization and then starting the “engine” (the template method) that runs the application (in a GUI application, this “engine” would be the main event loop). The client programmer simply provides definitions for customize1( ) and customize2( ) and the “application” is ready to run.

Exercises

Create a framework that takes a list of file names on the command line. It opens each file except the last for reading, and the last for writing. The framework will process each input file using an undetermined policy and write the output to the last file. Inherit to customize this framework to create two separate applications:
1) Converts all the letters in each file to uppercase.
2) Searches the files for words given in the first file.

chris@sunpack.com Last Update:09/08/2001