772: Mainline

Having arranged to access the meter and movie instance variables via getters, you can move into the getters the meter and movie construction operations, calling the meter or movie constructors only when your program first needs meter or movie values.

The principal benefit of on-demand construction is that you eliminate the danger that your program will ask a variable for a value before that value has been supplied. Liberal use of the idiom involved dramatically reduces the occurrence of bugs that throw instances of a NullPointerException exception.

import javax.swing.*;
import java.awt.event.*;
public class MovieApplication extends JFrame {
 public static void main (String argv []) {
  new MovieApplication("Movie Application");
 }
 // Declare instance variables:
 private Meter meter;
 private Movie movie;
 // Define constructor
 public MovieApplication(String title) {
  super(title);
  // Create model
  getMovie();                                           
  // Create and connect view to application
  getContentPane().add("Center", getMeter());           
  // Connect window listener, size and show
  addWindowListener(new LocalWindowListener());
  setSize(300, 100);
  show();
 }
 // Define getters and setters
 public Meter getMeter () {                             
  if (meter == null) {                                  
   setMeter(new Meter(0, 30));                          
  }                                                     
  return meter;                                         
 }                                                      
 public Movie getMovie () {                             
  if(movie == null) {                                   
   setMovie(new Movie (10, 10, 10, "On to Java"));      
  }                                                     
  return movie;                                         
 }                                                      
 public void setMeter (Meter m) {meter = m;}             
 public void setMovie (Movie m) {movie = m;}             
 // Define window adapter        
 private class LocalWindowListener extends WindowAdapter { 
  public void windowClosing(WindowEvent e) { 
   System.exit(0); return; 
}}}