为什么我的ItemListener不起作用？

Question

I'm trying to implements ItemListener in JCheckBox. The following code works:

public class Test {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        JPanel p = new JPanel();
        p.setLayout(new FlowLayout());

        int size = 5;
        JLabel[] labels = new JLabel[size];
        for (int i = 1; i < labels.length; i++) {
            labels[i] = new JLabel();
            labels[i].setText("" + i);
            ;

            labels[i].setPreferredSize(new Dimension(25, 25));
            p.add(labels[i]);
        }

        JCheckBox[] checkBoxs = new JCheckBox[size];
        for (int i = 1; i < checkBoxs.length; i++) {
            checkBoxs[i] = new JCheckBox();
            checkBoxs[i].setSelected(true);
            p.add(checkBoxs[i]);
            checkBoxs[i].addItemListener(new MyItemListener(labels[i]));
        }

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 400);
        frame.setContentPane(p);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}

and the listener class:

public class MyItemListener implements ItemListener {

JLabel label = new JLabel();

public MyItemListener(JLabel label) {
    super();
    this.label = label;
}

@Override
public void itemStateChanged(ItemEvent e) {
    if (e.getStateChange() == ItemEvent.SELECTED) {
        label.setVisible(true);
        label.revalidate();
        label.repaint();
    } else {
        label.setVisible(false);
    }
}

}

But now, I'm trying to do the same things with separate class, but it doesn't work:

Test.java

public class Test {
public static void main(String[] args) {
    JFrame frame = new JFrame();
    JPanel p = new JPanel();
    p.setLayout(new FlowLayout());

    PanelOfLabel panelOfLabel = new PanelOfLabel();
    panelOfLabel.setBorder(new LineBorder(Color.red));
    p.add(panelOfLabel);

    PanelOfBoxes panelOfBoxes = new PanelOfBoxes();
    panelOfBoxes.setBorder(new LineBorder(Color.green));
    p.add(panelOfBoxes);

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(400, 400);
    frame.setContentPane(p);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}

}

PanelOfLabel.java

public class PanelOfLabel extends JPanel {

private JLabel[] labels = new JLabel[5];

public PanelOfLabel() {
    super();
    createLabels();
}

public void createLabels() {
    for (int i = 1; i < labels.length; i++) {
        labels[i] = new JLabel();
        labels[i].setText("" + i);
        labels[i].setPreferredSize(new Dimension(25, 25));
        this.add(labels[i]);
    }

}

PanelOfBoxes.java

public class PanelOfBoxes extends JPanel {

private JCheckBox[] checkBoxs = new JCheckBox[5];
private PanelOfLabel[] PanelOfLabels = new PanelOfLabel[5];

public PanelOfBoxes() {
    super();
    createBoxes();
}

public void createBoxes() {
    for (int i = 1; i < checkBoxs.length; i++) {
        checkBoxs[i] = new JCheckBox();
        checkBoxs[i].setSelected(true);
        this.add(checkBoxs[i]);
        checkBoxs[i].addItemListener(new MyListener(PanelOfLabels[i]));
    }
}

}

and MyListener.java

public class MyListener implements ItemListener {

private PanelOfLabel panelOfLabels;

public MyListener(PanelOfLabel panelOfLabels) {
    super();
    this.panelOfLabels = panelOfLabels;
}

@Override
public void itemStateChanged(ItemEvent e) {
    if (e.getStateChange() == ItemEvent.SELECTED) {
        panelOfLabels.setVisible(true);
        panelOfLabels.revalidate();
        panelOfLabels.repaint();
    } else {
        panelOfLabels.setVisible(false);
    }
}

}

Where is the mistake that i made ? Can somebody help me please ?

Answer 1

This doesn't make sense:

private PanelOfLabel[] PanelOfLabels = new PanelOfLabel[5];

public PanelOfBoxes() {
    super();
    createBoxes();
}

public void createBoxes() {
    for (int i = 1; i < checkBoxs.length; i++) {
        checkBoxs[i] = new JCheckBox();
        checkBoxs[i].setSelected(true);
        this.add(checkBoxs[i]);
        checkBoxs[i].addItemListener(new MyListener(PanelOfLabels[i]));
    }
}

Since each PanelOfLabels item has not been constructed, each item will be null. You're passing a null reference into your MyListener constructor.

And shouldn't you instead be passing in one JLabel into that constructor and not an array of JPanel?

eg,

import java.awt.Color;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.*;

import javax.swing.*;
import javax.swing.border.LineBorder;

public class Test {
   public static void main(String[] args) {
      JFrame frame = new JFrame();
      JPanel p = new JPanel();
      p.setLayout(new FlowLayout());

      PanelOfLabel panelOfLabel = new PanelOfLabel();
      panelOfLabel.setBorder(new LineBorder(Color.red));
      p.add(panelOfLabel);

      PanelOfBoxes panelOfBoxes = new PanelOfBoxes(panelOfLabel); //!!
      panelOfBoxes.setBorder(new LineBorder(Color.green));
      p.add(panelOfBoxes);

      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setSize(400, 400);
      frame.setContentPane(p);
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }
}

class PanelOfLabel extends JPanel {
   private JLabel[] labels = new JLabel[5];

   public PanelOfLabel() {
      super();
      createLabels();
   }

   public void setLabelVisibility(int index, boolean visible) {
      if (index < 0 || index >= labels.length) {
         throw new ArrayIndexOutOfBoundsException(index);
      }

      labels[index].setVisible(visible);
   }

   public void createLabels() {
      for (int i = 1; i < labels.length; i++) {
         labels[i] = new JLabel();
         labels[i].setText("" + i);
         labels[i].setPreferredSize(new Dimension(25, 25));
         this.add(labels[i]);
      }

   }
}

class PanelOfBoxes extends JPanel {

   private JCheckBox[] checkBoxs = new JCheckBox[5];
   // !! private PanelOfLabel[] PanelOfLabels = new PanelOfLabel[5];
   private PanelOfLabel panelOfLabel; //!!

   public PanelOfBoxes(PanelOfLabel panelOfLabel) {
      super();
      this.panelOfLabel = panelOfLabel; //!!
      createBoxes();
   }

   public void createBoxes() {
      for (int i = 1; i < checkBoxs.length; i++) {
         checkBoxs[i] = new JCheckBox();
         checkBoxs[i].setSelected(true);
         this.add(checkBoxs[i]);
         // !!
         checkBoxs[i].addItemListener(new MyListener(i, panelOfLabel)); //!!
      }
   }
}

class MyListener implements ItemListener {

   private PanelOfLabel panelOfLabels;
   private int index;

   // !!
   public MyListener(int index, PanelOfLabel panelOfLabels) {
      super();
      this.index = index; // !!
      this.panelOfLabels = panelOfLabels;
   }

   @Override //!!
   public void itemStateChanged(ItemEvent e) {
      boolean visible = e.getStateChange() == ItemEvent.SELECTED;
      panelOfLabels.setLabelVisibility(index, visible);
      Container parent = panelOfLabels.getParent();
      parent.repaint();
   }
}

---

Edit
My attempt at a Model-View-Control (MVC) solution:

import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import java.beans.IndexedPropertyChangeEvent;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.util.ArrayList;
import java.util.List;

import javax.swing.*;
import javax.swing.event.*;

public class TestMvc {

   private static final int SIZE = 6;

   private static void createAndShowGui() {
      TestModel model = new TestModel(SIZE);
      TestView view = new TestView();
      new TestControl(model, view);

      JFrame frame = new JFrame("TestMvc");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(view.getMainPanel());
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

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

class TestControl {

   private TestModel model;
   private TestView view;

   public TestControl(final TestModel model, final TestView view) {
      this.model = model;
      this.view = view;
      view.setModel(model);
      TestViewCheckBoxBox checkBoxBox = view.getCheckBoxBox();
      for (int i = 0; i < model.getSize(); i++) {
         checkBoxBox.addCheckBoxItemListener(i, new MyItemListener(i));
      }

      model.addPropertyChangeListener(new ModelPropertyChangeListener());
   }

   private class MyItemListener implements ItemListener {
      private int index;

      public MyItemListener(int index) {
         this.index = index;
      }

      @Override
      public void itemStateChanged(ItemEvent iEvt) {
         model.setValue(index, iEvt.getStateChange() == ItemEvent.SELECTED);
      }
   }

   private class ModelPropertyChangeListener implements PropertyChangeListener {
      @Override
      public void propertyChange(PropertyChangeEvent pcEvt) {
         if (TestModel.VALUE.equals(pcEvt.getPropertyName())) {
            IndexedPropertyChangeEvent ipcEvt = (IndexedPropertyChangeEvent) pcEvt;
            int index = ipcEvt.getIndex();
            boolean visible = (Boolean)ipcEvt.getNewValue();
            view.getLabelBox().setLabelVisible(index, visible);
         }
      }
   }

}

class TestView {
   private JPanel mainPanel = new JPanel();
   private TestModel model;
   private TestViewLabelBox labelBox;
   private TestViewCheckBoxBox checkBoxBox;

   public void setModel(TestModel model) {
      this.model = model;
      mainPanel.setLayout(new GridLayout(1, 0, 5, 0));
      labelBox = new TestViewLabelBox(model.getSize());
      checkBoxBox = new TestViewCheckBoxBox(model.getSize());

      mainPanel.add(labelBox.getMainPanel());
      mainPanel.add(checkBoxBox.getMainPanel());
   }

   public TestViewLabelBox getLabelBox() {
      return labelBox;
   }

   public TestViewCheckBoxBox getCheckBoxBox() {
      return checkBoxBox;
   }

   public JComponent getMainPanel() {
      return mainPanel;
   }

}

class TestViewLabelBox {
   private JPanel mainPanel = new JPanel();
   private List<JLabel> labelList = new ArrayList<>();

   public TestViewLabelBox(int count) {
      mainPanel.setBorder(BorderFactory.createLineBorder(Color.black));
      mainPanel.setLayout(new GridLayout(1, 0, 5, 0));
      for (int i = 0; i < count; i++) {
         JLabel label = new JLabel(String.valueOf(i + 1), SwingConstants.CENTER);
         labelList.add(label);
         mainPanel.add(label);
      }
   }

   public void setLabelVisible(int index, boolean visible) {
      if (index < 0 || index >= labelList.size()) {
         throw new ArrayIndexOutOfBoundsException(index);
      }
      String text = visible ? String.valueOf(index + 1) : "";
      labelList.get(index).setText(text);
   }

   public JComponent getMainPanel() {
      return mainPanel;
   }

}

class TestViewCheckBoxBox {
   private JPanel mainPanel = new JPanel();
   private List<JCheckBox> checkBoxList = new ArrayList<>();

   public TestViewCheckBoxBox(int count) {
      mainPanel.setBorder(BorderFactory.createLineBorder(Color.black));
      mainPanel.setLayout(new GridLayout(1, 0, 5, 0));
      for (int i = 0; i < count; i++) {
         JCheckBox checkBox = new JCheckBox("", true);
         checkBoxList.add(checkBox);
         mainPanel.add(checkBox);
      }
   }

   public void addCheckBoxItemListener(int index, ItemListener itemListener) {
      if (index < 0 || index >= checkBoxList.size()) {
         throw new ArrayIndexOutOfBoundsException(index);
      }
      checkBoxList.get(index).addItemListener(itemListener);
   }

   public JComponent getMainPanel() {
      return mainPanel;
   }
}

class TestModel {
   public static final String VALUE = "value";
   private List<Boolean> values = new ArrayList<>();
   private SwingPropertyChangeSupport pcSupport = new SwingPropertyChangeSupport(
         this);

   public TestModel(int size) {
      for (int i = 0; i < size; i++) {
         values.add(Boolean.TRUE);
      }
   }

   public void addPropertyChangeListener(PropertyChangeListener listener) {
      pcSupport.addPropertyChangeListener(listener);
   }

   public int getSize() {
      return values.size();
   }

   public void removePropertyChangeListener(PropertyChangeListener listener) {
      pcSupport.removePropertyChangeListener(listener);
   }

   public boolean getValue(int index) {
      if (index < 0 || index >= values.size()) {
         throw new ArrayIndexOutOfBoundsException(index);
      }
      return values.get(index);
   }

   public void setValue(int index, boolean value) {
      if (index < 0 || index >= values.size()) {
         throw new ArrayIndexOutOfBoundsException(index);
      }
      boolean oldValue = getValue(index);
      boolean newValue = value;
      values.set(index, value);
      pcSupport.fireIndexedPropertyChange(VALUE, index, oldValue, newValue);
   }
}

Answer 2

private PanelOfLabel[] PanelOfLabels = new PanelOfLabel[5];

Here you're creating an array of PanelOfLabel, containing null elements.

checkBoxs[i].addItemListener(new MyListener(PanelOfLabels[i]));

and here you add a listener to your checkboxes, and pass one of the null elements to the listener.

First decide what you want to make visible/invisible in your ItemListener. Is it one of your JLabels, or is it the only PanelOfLabel instance created in the main class? My guess is that it's the labels. So the listener should be identical to the one you have in your working example. But you have to make sure that the labels passed to the listener are the ones created and added in PanelOfLabel.

Side note: you're confusing yourself, and us, by not respecting the Java naming conventions. Variables start with a lowercase letter.