为什么我的ItemListener不起作用？

Question

我试图在JCheckBox中实现ItemListener。 以下代码有效：

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);
    }
}

和监听器类：

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);
    }
}

}

但是现在，我试图用单独的课做同样的事情，但它不起作用：

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]));
    }
}

}

和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);
    }
}

}

我犯的错误在哪里？有人可以帮帮我吗？

Answer 1

这没有意义：

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]));
    }
}

由于尚未构造每个PanelOfLabels项，因此每个项都将为null。您正在将空引用传递给MyListener构造函数。

你不应该将一个 JLabel传入该构造函数而不是JPanel数组吗？

如，

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();
   }
}

---

修改
我尝试了模型 - 视图 - 控制（MVC）解决方案：

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];

在这里，您要创建一个包含null元素的PanelOfLabel数组。

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

在这里，您可以在复选框中添加一个侦听器，并将其中一个null元素传递给侦听器。

首先在ItemListener中确定要使其可见/不可见的内容。它是您的JLabel之一，还是在主类中创建的唯一PanelOfLabel实例？我的猜测是它是标签。所以听众应该与你工作范例中的听众相同。但是你必须确保传递给监听器的标签是在PanelOfLabel中创建和添加的标签。

旁注：您不会因为不尊重Java命名约定而让自己和我们感到困惑。变量以小写字母开头。