在Java类之间传递数据

Question

I'm having trouble retrieving the variable from my Colour class after i send the index from the GUI after selecting a colour from the drop down list. I can send the index fine and retrieve it from the HashMap, i know this because i use System.out.println to check. Basically my questions are, where have i gone wrong? and What do i need to remember to make sure i don't have this trouble again? Edit: forgot to mention, the button sending the index is in a seperate JPanel which is used for the UI components(buttons and combo boxes).

//edit
class UIPanel extends JPanel{
   public MainPanel gpanel;
    public Integer data;
    public Color colval;
    public Colour col;
public UIPanel(MainPanel panel) {

        col = new Colour();

        gpanel = panel;


        Box btnBox = Box.createHorizontalBox();

        btnBox.add(setBtn = new JButton());
        btnBox.add(Box.createHorizontalGlue());
         JButton setBtn = new JButton("Set");

        final DefaultComboBoxModel colour = new DefaultComboBoxModel();
        colour.addElement("Red");
        final JComboBox colours = new JComboBox(colour);
        JScrollPane colourScroll = new JScrollPane(colours);

        btnBox.setSize(300, 100);
        btnBox.add(Box.createHorizontalGlue());

        add(btnBox, BorderLayout.NORTH);
//end of edit

Button to send Index from GUI class to Colour class    

    setBtn.addActionListener(new ActionListener(){

            public void actionPerformed(ActionEvent e) {

                data= colours.getSelectedIndex();

                col.setCol(data);    
            }
        });

Colour Class where hashmap with the list of colours in.

public class Colour{

    public Color colVal;

    HashMap<Integer, Color> map = new HashMap<Integer, Color>();

    public Colour() {
        map.put(0, Color.RED);
        map.put(1, Color.BLUE);
        map.put(2, Color.YELLOW);
        map.put(3, Color.GREEN);
    }

    public Color setCol(Integer data) {
       //Color colours;
        colVal = map.get(data);
        System.out.println("colour" + colVal);
        return colVal;
    }

    public Color getColVal() {
        return colVal;
    }

And the paint area on the GUI class where the colour will be sent to from the colour class

class MainPanel extends JPanel{    
    //private Colour col;
    int px, py;
    //radius
    public Color colvals;

    public Colour col;

    public MainPanel() {  
        col = new Colour();

        this.addMouseMotionListener(new MouseMotionAdapter() {
            // store drag coordinates and repaint
                public void mouseDragged( MouseEvent event )
                {
                        px = event.getX();
                        py = event.getY();
                        repaint();
                }
                }); // end call to addMouseMotionListener
    }

    public void paint( Graphics g ) {   
        g.setColor(col.colVal);//This is where the colour value will be placed
        System.out.println(col.colVal);
        g.fillOval( px, py, 15, 15 );
    }
}

I'm Probably missing something stupid out but I cant seem to figure it out.

PS: How complicated will it be to make a Vignere Cipher Application?

Answer 1

JComboBox can be used to directly use any Objects as items with only one little thing to consider: It will use the toString for the label to display. (see JComboBox javadoc - Providing a Custom Renderer ).

But instead of using a custom renderer I always preferred to write a very simple helper class once - let's call it ComboBoxItem - that is reusable for any kind of data.

public class ComboBoxItem<T>
{
    private T value;
    private String label;

    public ComboBoxItem(T value, String label)
    {
        this.value = value;
        this.label = label;
    }

    public T getValue()
    {
        return this.value;
    }

    public String getLabel()
    {
        return this.label;
    }

    // important! since this is the workaround ;-)
    public String toString()
    {
        return this.label; // or whatever you like
    }
}

And then populate the JComboBox with ComboBoxItem s instead of String values:

In your code instead of

final DefaultComboBoxModel colour = new DefaultComboBoxModel();
colour.addElement("Red");
colour.addElement("Blue");
colour.addElement("Yellow");
colour.addElement("Green");
colours = new JComboBox(colourValues);

... you will use

final DefaultComboBoxModel colour = new DefaultComboBoxModel();
colour.addElement(new ComboBoxItem<Color>(Color.RED, "Red"));
colour.addElement(new ComboBoxItem<Color>(Color.BLUE, "Blue"));
colour.addElement(new ComboBoxItem<Color>(Color.YELLOW, "Yellow"));
colour.addElement(new ComboBoxItem<Color>(Color.GREEN, "Green"));
colours = new JComboBox(colourValues);

This will make the select contain ComboBoxItem s as values which you can simply access by doing the following:

// instead of getSelectedIndex()
ComboBoxItem<Color> item = (ComboBoxItem) colours.getSelectedItem(); 
Color c = item.getValue();

The same procedure can then be reused for any other kind of values - even complex ones.

Note: If you have a data object with an appropriate toString() representation anyway, you can of course simply use it as a value for the select.

Note2: If a string representation is not enough (eg you want to display the color along with the name), have a look at ListCellRenderer which is able to display the item in any desired way (by returning an arbitrary JComponent).

Answer 2

Your setCol(...) method inside of the Colour class should be getCol(...) since it's functioning as a getter:

public class Colour{

    public Color colVal;

    HashMap<Integer, Color> map = new HashMap<Integer, Color>();

    public Colour() {
        map.put(0, Color.RED);
        map.put(1, Color.BLUE);
        map.put(2, Color.YELLOW);
        map.put(3, Color.GREEN);
    }

    // **** change name ****
    public Color getCol(Integer data) {
       //Color colours;
        colVal = map.get(data);
        System.out.println("colour" + colVal);
        return colVal;
    }

    // **** not sure you need this method
    public Color getColVal() {
        return colVal;
    }

and in your ActionListener, you retrieve the color but never do anything with it. It should be:

    public void actionPerformed(ActionEvent e) {
        data = colours.getSelectedIndex();
        Color color = col.getCol(data); // note name change

        // use Color variable, color, somehow here
        mainPanel.setColVals(color);  // something like this perhaps
        mainPanel.repaint();  // to tell the JVM to repaint the JPanel
    }

Also note that in your JPanel class override you should override the paintComponent method, not the paint method, and don't forget to call the super's method.

ie,

public void setColVals(Color colVals) {
   this.colVals = colVals;
}

@Override
protected void paintComponent(Graphics g)  {   

    super.paintComponent(g);

    g.setColor(colVal);
    // System.out.println(colVal);
    g.fillOval( px, py, 15, 15 );
}

---

Edit a better answer:

• Get rid of Colour entirely.
• Use an enum to match your Color to a String and create a JComboBox model out of that enum.
• Using an enum will prevent you're having to use magic numbers with the risk that the wrong number has been used, a number that doesn't match a color.
• Also, by using an enum, it is trivial to change your code and add more colors. Just add a new item to the enum, and the rest of the program will adapt to the change.
• Add a PropertyChangeListener from the MainPanel to the UIPanel and listen for changes to its "bound" Color property.
• Use RenderingHints with a Graphics2D object to smooth out the jaggies from your circle drawing.

---

import java.awt.*;
import java.awt.event.*;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import javax.swing.*;

public class ColorListenerPanel extends JPanel {
   public ColorListenerPanel() {
      UIPanel uiPanel = new UIPanel();
      MainPanel mainPanel = new MainPanel(uiPanel);

      setLayout(new BorderLayout());
      add(mainPanel, BorderLayout.CENTER);
      add(uiPanel, BorderLayout.PAGE_START);
   }

   private static void createAndShowGui() {
      ColorListenerPanel mainPanel = new ColorListenerPanel();

      JFrame frame = new JFrame("ColorListenerPanel");
      frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

class MainPanel extends JPanel {
   private static final int PREF_W = 800;
   private static final int PREF_H = 650;
   private static final int OVAL_WIDTH = 16;
   private int px, py;
   private Color color = MyColors.values()[0].getColor();

   public MainPanel(UIPanel uiPanel) {

      this.addMouseMotionListener(new MouseMotionAdapter() {
         // store drag coordinates and repaint
         public void mouseDragged(MouseEvent event) {
            px = event.getX();
            py = event.getY();
            repaint();
         }
      });

      uiPanel.addPropertyChangeListener(UIPanel.COLOR, new UiListener());
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);
      g.setColor(color);
      g.fillOval(px - OVAL_WIDTH / 2, py - OVAL_WIDTH / 2, OVAL_WIDTH, OVAL_WIDTH);
   }

   @Override
   public Dimension getPreferredSize() {
      if (isPreferredSizeSet()) {
         return super.getPreferredSize();
      }
      return new Dimension(PREF_W, PREF_H);
   }

   private class UiListener implements PropertyChangeListener {
      @Override
      public void propertyChange(PropertyChangeEvent pcEvt) {
         // not really needed since our listener is added using
         // this property name
         if (!UIPanel.COLOR.equals(pcEvt.getPropertyName())) {
            return;
         }

         color = (Color) pcEvt.getNewValue();
         repaint();
      }
   }
}



enum MyColors {
   RED("Red", Color.RED), 
   BLUE("Blue", Color.BLUE), 
   YELLOW("Yellow", Color.YELLOW), 
   GREEN("Green", Color.GREEN);

   private String name;
   private Color color;

   private MyColors(String name, Color color) {
      this.name = name;
      this.color = color;
   }

   public String getName() {
      return name;
   }

   public Color getColor() {
      return color;
   }

   @Override
   public String toString() {
      return name;
   }

}

class UIPanel extends JPanel {
   public static final String COLOR = "color";
   private MainPanel gpanel;
   private Integer data;
   private Color color;
   private DefaultComboBoxModel<MyColors> comboModel = new DefaultComboBoxModel<>();
   private JComboBox<MyColors> colorsCombo = new JComboBox<>(comboModel);
   SetColorAction setColorAction = new SetColorAction("Set", KeyEvent.VK_S);

   public UIPanel() {
      setLayout(new BoxLayout(this, BoxLayout.LINE_AXIS));
      add(colorsCombo);
      add(Box.createHorizontalStrut(5));
      add(new JButton(setColorAction));
      colorsCombo.addActionListener(setColorAction);
      add(Box.createHorizontalGlue());

      for (MyColors myColor : MyColors.values()) {
         comboModel.addElement(myColor);
      }
   }

   public void setColor(Color color) {
      Color oldValue = this.color;
      Color newValue = color;
      this.color = color;
      firePropertyChange(COLOR, oldValue, newValue);
   }

   private class SetColorAction extends AbstractAction {

      public SetColorAction(String name, int mnemonic) {
         super(name);
         putValue(MNEMONIC_KEY, mnemonic);
      }

      @Override
      public void actionPerformed(ActionEvent evt) {
         MyColors selection = (MyColors) colorsCombo.getSelectedItem();
         if (selection != null) {
            setColor(selection.getColor());            
         }
      }
   }
}

Answer 3

In this Function

 public void paint( Graphics g ) {   
    g.setColor(col.colVal);//This is where the colour value will be placed
    System.out.println(col.colVal);
    g.fillOval( px, py, 15, 15 );
}

You are using g.setColor(col.colVal); col.colVal will give you the default value assigned to it probably null ,You should use col.getColVal() as this is the getter method you have created for the colVal attribute.

And also in the setter declaration public Color setCol(Integer data) the return type you are using is Color but when you are using this function in your GUI class without a Color variable which can accept a Value returned by your setter . I don't understand the need of returning values from a Setter method.