Java Tutorial � Building GUIs by Vd4179ak


									  Java Tutorial – Application Building

            Extending our Application

                                                         Daniel Bryant

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      Today we will be extending the project from the previous Java
       Tutorial Lab
      Therefore, you must have completed all the work in Lab 1
       before starting this sheet

      You should have NetBeans set up correctly (i.e. with the
       appropriate directories Mounted (specified in the first Lab)

      You should have version 4 of the MainFrame class (file name
       on the website: MainFrame4.class) compiled and working
      Version 4 of the class allows you to specify and load an image
       into your application

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      In this lab will be using our simple GUI to load an image and
       pass this to the “Dither” application you have previously
       looked at in the first week of Dr Tang’s labs

      This avoids having to specify the image name as an Argument
       to your application each time you execute it and also allows
       you to change the image when the application is running

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Adding the Dither Code
      You should have the directory containing the class files (directory name
       “Classes”) and the directory containing the file already mounted

      If not, mount Classes and Apps>Chap05 into NetBeans

      You should also have the directory containing your “sampleGUI” package
       mounted - Mount >> GuiTutorial

      Copy the file into your package (which should be named
           You can do this by right-clicking on the file in NetBeans and selecting
            “copy” and then right clicking on your package and selecting “paste>>copy”

      Because of the way the Dither code is written you will also need to specify the
       working directory. Right-click>>Properties>>Executor>>Working Directory

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Adding the Dither Code
      We have included the Dither class into our package so that we
       can easily reference the class in our code
      Of course you could place the Dither class into another
       package and “import” this into your code. Ask if you are

      We will no longer be executing the Dither class directly as you
       did in the first lab.
      Instead we will execute our MainFrame class and use this to
       call the Dither class
           When the user clicks a button on our GUI we want the Dither
            class to be executed
      Therefore we will need to copy the code in the “public static
       void main (String[] args)” method of the Dither class to a
       button in our GUI application (Why?)
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Modifying Our Application

      Open the MainFrame code in NetBeans

      Choose one of the top buttons to be
       responsible for loading the Dither
       application and alter the button’s text
       accordingly (e.g. so that it reads “Dither the
       image” in the GUI)

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Modifying the Event Handler
      Now we will modify the code of that buttons Event handler
      If you look at the code below the button you will see the
       following code
           loadApp1Button.addActionListener(new DoloadApp1());
      This code adds an “ActionListener” to the button named
      An ActionListener simply monitors the button for any events
       that might occur, such as the user clicking the button.
      When the user clicks the button an Event object is created -
       the ActionListener is informed and passes details of this event
       onto the object specified
      In our code we have specified that a new object named
       DoloadApp1 should be created, sent the details of the event
       and then executed
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Modifying the Event Handler
      Therefore, we need to modify the Event Handler code in the
       appropriate inner class (DoloadApp1() in our example)
      You should copy and then modify the code from the Dither
       class “public static void main (String[] argv)” method into the
       actionPerformed() method of the DoloadApp1() class

      You will notice the name of the image to be loaded is specified
       in the following code in the Dither class “main” method:
           JFrame frame = new Dither(argv[0]);
           argv[0] is the name of the argument you provided to the class at
            run-time using NetBeans (remember when you had to specify
            the filename as an argument?)

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Modifying the Event Handler
      Therefore, you will have to modify this variable to
       refer to the file you have loaded using our file
       selector window
           The file variable is named myPicture
           Because this variable is a File Object and we need to pass
            the name of the file into the Dither class, we will have to
            use the .getName() method on the file variable
           String fileName = myPicture.getName();

      The complete code for the actionPerformed method
       is given overleaf, but please do try and understand
       why you have moved it to this class and why the
       modifications have been made

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actionPerformed code
public void actionPerformed(ActionEvent e) {
       try {
             JFrame frame = new Dither(myPicture.getName());
       catch (IOException iox)
          System.out.println("Error loading the image file");

            catch (ImageDecoderException idx)
              System.out.println("Decode Error loading the image file");

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Running your code
      Compile all the classes in your package and execute your “Main”
       class in order to display the GUI

      Click on the load picture button and select a JPEG Image from the
       Images directory (which you specified as the working directory for

      Click the “open a picture” button to load the image

      Click on the button you have modified labelled “Dither the Image” and
       the Dither window should be displayed with the image you have
       selected previously

      You can minimize the Dither window, specify another image using
       the above method and then open another Dither window which will
       display the new image

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And Finally…
      As soon as you click the close window button on
       any window (i.e. the main application or any Dither
       windows) the whole application will close down

      This is a little annoying but easily fixed. Can you
       determine why this happens and modify the code

      Hint: the Dither code in your package needs to be
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