Question

java新手，我无法理解为什么我的动作监听器不能在jcombobox上工作。我想我已经按照网络上的其他示例来访问getSelectedItem，但没有任何事情发生。仅供参考，我的项目是一个单位转换器（使用MVC ......但是，这不是我的优先考虑）。非常感谢任何帮助。谢谢，西蒙。

    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;

    import javax.swing.*;


    public class UnitConverterView extends JFrame{

    //variables and components
    private static final long serialVersionUID = -4673040337179571462L;
    private JComboBox<String> unitCategory;

    private JTextField fromValue = new JTextField(7);
    private JComboBox<String> convertFrom;
    private JLabel equalsLabel = new JLabel(" = ");

    private JTextField toValue = new JTextField(7);
    private JComboBox<String> convertTo;


    //constructor
    UnitConverterView(){
    //set up the view and components

        JPanel unitPanel = new JPanel();

        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setSize(600,300);

        String[] categories = {"Length","Weight","Speed","Temperature"};
        unitCategory = new JComboBox<>(categories);

        String[] tofromValues = {" "};
        convertFrom = new JComboBox<>(tofromValues);
        convertTo = new JComboBox<>(tofromValues);


        unitPanel.add(unitCategory);

        unitPanel.add(fromValue);
        unitPanel.add(convertFrom);
        unitPanel.add(equalsLabel);
        unitPanel.add(toValue);
        unitPanel.add(convertTo);

        this.add(unitPanel);

    }

    //get value to convert from
    public int getMeasurement() {
        return Integer.parseInt(fromValue.getText());
    }

    //listen for unitCategory to be selected
    void addUnitCategoryListener(ActionListener listenForUnitCategory) {
        unitCategory.addActionListener(listenForUnitCategory);
    }

class UnitCatListener implements ActionListener { 

        public void actionPerformed(ActionEvent e) {

            /*String unitSelected = (String) unitCategory.getSelectedItem();
            if (e.getSource() == unitCategory) {
                String unitName = (String) unitCategory.getSelectedItem();
                System.out.println("UnitName = " + unitName);
                changeText(unitName);
            }*/

            JComboBox cb = (JComboBox)e.getSource();
            String unitName = (String) cb.getSelectedItem();
            System.out.println("UnitName = " + unitName);

        }

        void changeText(String name) {
            toValue.setText(name);
        }

    }



}

Answer 1

您已声明了一个方法addUnitCategoryListener（），用于将侦听器注册到组合框，但您从未调用此方法。这就是听众从未注册的原因。

在构造函数的末尾添加以下行，那么你应该没问题：

addUnitCategoryListener(new UnitCatListener());

Answer 2

要简单地解决您的问题，请调用您创建的方法在组件上注册侦听器。将其添加到构造函数中：

<?php 
include("../db_connect.php");      
$sql="select * from notifications where userid='$uid' or rec_id='$uid'";
$result=mysqli_query($con,$sql);
?>
<div class="col-sm-10">
<table class="table table-striped ">
<thead style="background-color:skyblue;">
<td></td><td></td><td></td><td></td><td><h3><i  class="fa fa-cogs"></i>Notifications</h3></td><td></td><td></td><td></td></thead>
<thead style="background-color:wheet;" class="table-bordered">
<tr>
<td><b>Sr.No.</b></td>
<td><b>From</b></td>
<td><b>Recevier</b></td>
<td><b>Amount</b></td>
<td><b>Details</b></td>
<td><b>Date</b></td>
<td><b>User Info</b></td>
<td><b>Status</b></td>
</tr>
</thead>
<tbody class="table-bordered ">
    <?php
    if($result)
    {
        $i=1;
        while($row = $result->fetch_assoc()) {

       $sql12="select * from users inner join bank_details on users.userid=bank_details.userid  where users.userid='".$row['rec_id']."'";
       $result12=mysqli_query($con,$sql12);
       $row1 = $result12->fetch_assoc();


            echo "<tr><td>".$i."</td><td>".$row['userid']."</td><td>".$row['rec_id']."</td><td>".$row['amount']."</td><td>".$row['details']."</td><td>".$row['date1']."</td>";
            if($uid == $row['rec_id'])
            {
                echo "<td><button class='btn btn-warning' disabled data-toggle='modal' data-target='#myModal".$i."'>View</button></td>";

            }
            else
            {
            echo "<td><button class='btn btn-success'  data-toggle='modal' data-target='#myModals".$i."'>View</button></td>";   
            }
            if($uid == $row['userid'])
            {
                if($row['status']=="pending")
                {
                echo "<td><button class='btn btn-info' disabled id='sndr'>Pending to Accept</button></td></tr>";
                }
                else if($row['status']=="accepted")
                {
                    echo "<td><button class='btn btn-success' disabled id='sndr'>Request Accepted</button></td></tr>";
                }
                else
                {
                    echo "<td><button class='btn btn-danger' disabled id='sndr'>Request Rejected</button></td></tr>";
                }
            }
            else
            {
                if($row['status']=="pending" )
                {
                ?>
                <td>
                 <form role="form">
                    <div class="form-group">
    <select class="form-control" id="sel1"  >
    <option>Select</option>
    <option value="accepted" onclick="aprvs(<?php echo $row['id']; ?>, this.value);"><span class="btn btn-success"> Accept</span></option>
    <option value="rejected" onclick="aprvs(<?php echo $row['id']; ?>, this.value);"><span class="btn btn-danger">Reject</span></option>
  </select>
  </div>
  </form>
  </td>
  </tr>
  <?php
                }
                    else if($row['status']=="accepted")
                    {
                        echo "<td><button class='btn btn-success' disabled id='sndr'>Request Accepted</button></td></tr>";
                    }
                    else{
                        echo "<td><button class='btn btn-danger' disabled id='sndr'>Request Rejected</button></td></tr>";
                    }
            }

            ?>
            <div    class='modal fade' id='<?php echo "myModals".$i.""; ?>' role='dialog' tabindex="-1" aria-hidden="true">
<div class="modal-dialog" >

  <!-- Modal content-->
  <div class="modal-content">
    <div class="modal-header" style="background-color:green;padding:10px;">
      <button type="button" class="close" data-dismiss="modal" >&times;</button>
      <center>
      <h4 class="modal-title" align="center" style="color:white;">User Details</h4>
      </center>
    </div>
    <div class="modal-body" style="padding:10px;">


            <table class='table table-striped'>

            <thead>
            <tr><td><h4 style='color:blue;'></b>User Details:</b></h4></td><td></td></tr>
            </thead>
            <tbody>
            <tr><td><b>User Id:</b></td><td><?php echo $row1['userid']; ?></td></tr>
            <tr><td><b>Name:</b></td><td><?php echo $row1['name']; ?></td></tr>
            <tr><td><b>Father Name:</b></td><td><?php echo $row1['fname']; ?></td></tr>
            </tbody>
            <!--
            <thead><tr><td><h4 style='color:blue;'></b>Bank Details:</b></h4></td><td></td></tr></thead>
            <tbody>
            <tr><td><b>Bank Name:</b></td><td><?php echo $row1['bname']; ?></td></tr>
            <tr><td><b>Branch:</b></td><td><?php echo $row1['branch']; ?></td></tr>
            <tr><td><b>A/c. No.:</b></td><td><?php echo $row1['account_no']; ?></td></tr>
            <tr><td><b>IFSC Code:</b></td><td><?php echo $row1['ifs']; ?></td></tr>
            <tbody>

            <thead><tr><td><h4 style='color:blue;'></b>Contact Details:</b></h4></td><td></td></tr></thead>
            <tbody>
            <tr><td><b>Mobile No.:</b></td><td><?php echo $row1['mob']; ?></td></tr>
            <tr><td><b>Alternate No.:</b></td><td><?php echo $row1['alt']; ?></td></tr>
            <tr><td><b>E-Mail:</b></td><td><?php echo $row1['email']; ?></td></tr>
            </tbody>-->
            </table> 

    </div>
    <div class="modal-footer">
      <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
    </div>
  </div>

</div>


   <?php                
        echo "</div>";  
            $i++;       
        }               
    }
    else
    {
        echo "<tr><td>No Notifications to show</td></tr>";
    }
?>
</tbody>
</table>
<?php   
 }
 ?>

但是，您可能想知道一些事情：

对于addUnitCategoryListener(new UnitCatListener());，ItemListener通常比ActionListener做得更好。如果用户选择已经选择的项目（基本上什么都不做），前一个不会触发事件。通常在这些情况下你不需要做任何事情。
你不需要额外的方法来注册监听器，你可以直接添加到你的构造函数行
```
JComboBox
```
并删除自定义方法。
从不使用方法unitCategory.addActionListener(new UnitCatListener());和changeText。
使用参数化类型：而不是getMeasurement使用JComboBox。
您不需要JComboBox<String>作为字段 - 本地变量将会这样做 - 因为您以后不需要在任何地方引用它（除非您计划在运行时更改标签的属性）

JComboBox getSelectedItem

2 个答案: