difficult design problem?

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  • gerrymcc@indigo.ie
    #1

    difficult design problem?

    // I want to place a panel which contains two canvases (a red one and
    a blue one) on screen,
    // along with two buttons. When a button is pressed the appropriate
    message should be
    // printed in the canvas. The code compiles okay, but I get a
    run-time NullPointerExce ption.
    // I don't understand why - could anyone please explain the problem
    and suggest a solution?
    // Thanks very much for any help,
    // Gerard


    import java.awt.*;
    import java.applet.App let;
    import java.awt.event. *;

    class TypeOneApplet extends Applet implements ActionListener{

    public final int CLEAR = 1;
    public final int DULL = 2;
    Button b1, b2;
    Panel pan;
    P p;
    int theAction = 1;

    public void actionPerformed ( ActionEvent e){
    if( e.getSource() == b1)
    theAction = CLEAR;
    else
    theAction = DULL;
    }

    void methodOne(){ System.out.prin tln(" method One"); }
    void methodTwo(){ System.out.prin tln(" method Two"); }
    }

    public class TypeTwoApplet extends TypeOneApplet {

    public void init(){
    b1 = new Button("clear") ;
    b2 = new Button("dull");
    b1.addActionLis tener(this);
    b2.addActionLis tener(this);
    this.add( b1);
    this.add( b2);
    pan = new Panel();
    p = new P( this);
    pan.add( p);
    this.add( pan);
    System.out.prin tln("exit init");
    }

    void methodTwo(){ System.out.prin tln(" method Two B"); }
    }

    class M extends Canvas implements MouseListener{
    TypeOneApplet toa;
    String message;

    public M(){}
    public M( TypeOneApplet t){
    toa = t;
    message = "ready";
    }

    public void mousePressed( MouseEvent evt){
    if( toa.theAction == toa.CLEAR)
    message = "clear";
    else
    message = "dull";
    repaint();
    }
    public void mouseClicked( MouseEvent e) {}
    public void mouseEntered( MouseEvent e) {}
    public void mouseExited( MouseEvent e) {}
    public void mouseReleased( MouseEvent e) {}

    public void paint( Graphics g){
    setBackground( Color.red);
    setForeground( Color.black);
    g.drawString( message, 5, 5);
    }
    }

    class M2 extends M{
    TypeTwoApplet atoa;

    public M2( TypeTwoApplet t){
    atoa = t;
    }

    public void mousePressed( MouseEvent evt){
    if( atoa.theAction == toa.CLEAR)
    message = "clear 2";
    else
    message = "dull 2";
    repaint();
    }

    public void paint( Graphics g){
    setBackground( Color.blue);
    setForeground( Color.black);
    g.drawString( message, 5, 5);
    }
    }

    class P extends Panel{
    M [] m;

    public P( TypeTwoApplet t){
    m[0] = new M(t);
    m[1] = new M2(t);
    add( m[0]);
    add( m[1]);
    }
    }


    Tags: None
    • VisionSet
      #2
      Re: difficult design problem?

      <gerrymcc@indig o.ie> wrote in message
      news:vyNYa.2646 8$pK2.41791@new s.indigo.ie...
      [color=blue]
      > The code compiles okay, but I get a
      > run-time NullPointerExce ption.[/color]

      Please give the 1st lines of the exception that apply to your code and
      indicate which line of the code the line numbers in the exception trace
      refer to.

      --
      Mike W


        Comment

        • gerrymcc@indigo.ie
          #3
          Null pointer exception, was: difficult design problem?

          Thanks for the suggestion about including error message. Here's the
          run-time NullPointerExce ption, which reads:
          at P.(init)(TypeTw oApplet.java:10 7)
          at TypeTwoApplet.i nit(TypeTwoAppl et.java:43)

          Regarding the previous subject line, I've stripped down and extracted
          the idea of a much larger program to show the overall design, as I
          wasn't certain that I was using the classes correctly. For
          instance, as init() seems to be causing a problem, I wondered if it is
          necessary to have a default init() in an applet in the same way that I
          must supply a default constructor for class M. And I wondered if
          class M2 will communicate with the applet subclasses (as I hope it
          will).

          As for posting to the three newsgroups, I'm unsure if it is supposed
          to be better etiquette to post to one, read the replies, then post to
          another, read the replies there, etc? I've found in the past that by
          posting to two or three closely related groups at once I get
          different, useful replies in each group.

          Thanks again for any help,
          Gerard


          import java.awt.*;
          import java.applet.App let;
          import java.awt.event. *;

          class TypeOneApplet extends Applet implements ActionListener{

          public final int CLEAR = 1;
          public final int DULL = 2;
          Button b1, b2;
          Panel pan;
          P p;
          int theAction = 1;

          public void actionPerformed ( ActionEvent e){
          if( e.getSource() == b1)
          theAction = CLEAR;
          else
          theAction = DULL;
          }

          void methodOne(){ System.out.prin tln(" method One"); }
          void methodTwo(){ System.out.prin tln(" method Two"); }
          }

          public class TypeTwoApplet extends TypeOneApplet {

          public void init(){
          b1 = new Button("clear") ;
          b2 = new Button("dull");
          b1.addActionLis tener(this);
          b2.addActionLis tener(this);
          this.add( b1);
          this.add( b2);
          pan = new Panel();
          p = new P( this);
          pan.add( p);
          this.add( pan);
          System.out.prin tln("exit init");
          }

          void methodTwo(){ System.out.prin tln(" method Two B"); }
          }

          class M extends Canvas implements MouseListener{
          TypeOneApplet toa;
          String message;

          public M(){}
          public M( TypeOneApplet t){
          toa = t;
          message = "ready";
          }

          public void mousePressed( MouseEvent evt){
          if( toa.theAction == toa.CLEAR)
          message = "clear";
          else
          message = "dull";
          repaint();
          }
          public void mouseClicked( MouseEvent e) {}
          public void mouseEntered( MouseEvent e) {}
          public void mouseExited( MouseEvent e) {}
          public void mouseReleased( MouseEvent e) {}

          public void paint( Graphics g){
          setBackground( Color.red);
          setForeground( Color.black);
          g.drawString( message, 5, 5);
          }
          }

          class M2 extends M{
          TypeTwoApplet atoa;

          public M2( TypeTwoApplet t){
          atoa = t;
          }

          public void mousePressed( MouseEvent evt){
          if( atoa.theAction == toa.CLEAR)
          message = "clear 2";
          else
          message = "dull 2";
          repaint();
          }

          public void paint( Graphics g){
          setBackground( Color.blue);
          setForeground( Color.black);
          g.drawString( message, 5, 5);
          }
          }

          class P extends Panel{
          M [] m;

          public P( TypeTwoApplet t){
          m[0] = new M(t);
          m[1] = new M2(t);
          add( m[0]);
          add( m[1]);
          }
          }



            Comment

            • Chris
              #4
              Re: Null pointer exception, was: difficult design problem?

              -----BEGIN PGP SIGNED MESSAGE-----
              Hash: SHA1

              gerrymcc@indigo .ie wrote:

              [snip]
              [color=blue]
              > class P extends Panel{
              > M [] m;
              >
              > public P( TypeTwoApplet t){
              > m[0] = new M(t);
              > m[1] = new M2(t);
              > add( m[0]);
              > add( m[1]);
              > }
              > }[/color]

              Hi,
              I see the problem. You declared a reference to an array of M, but you
              never created the array before trying to store objects in it. You
              should change the declaration to also create the actual array:

              M[] m = new M[2];

              I think this should be the only problem.

              - --
              Chris
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                Comment

                • Roedy Green
                  #5
                  Re: difficult design problem?

                  On Fri, 08 Aug 2003 13:26:35 GMT, gerrymcc@indigo .ie wrote or quoted :
                  [color=blue]
                  >. The code compiles okay, but I get a
                  >run-time NullPointerExce ption.[/color]

                  When it throws the null pointer exception you will get a stack trace
                  that tells you exactly where it occurred and how you got there. That
                  it what you have to look at.
                  --
                  Canadian Mind Products, Roedy Green.
                  Coaching, problem solving, economical contract programming.
                  See http://mindprod.com/jgloss/jgloss.html for The Java Glossary.

                    Comment

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