当我在jList中选择项目并使用getSelectedIndex()时，它返回-1

Question

我正在尝试制作一个 ToDoList ，我创建了一个jButton从Database中删除任务，当我检查它总是给出index时（-1）

        if (jListTasks.getSelectedIndex() == -1) {
            JOptionPane.showMessageDialog(null, "No task selected!!");
        } else {
            JOptionPane.showConfirmDialog(null, "Are you sure?!", "Remove task", JOptionPane.YES_NO_OPTION);}

更新：

//      set DefaultListModel
        tasksListModel = new DefaultListModel();
        jListTasks.setModel(tasksListModel);

然后我创建此方法

// to clear and fill list every time method invoked
    private void fillTaskList() {
        tasksListModel.clear();
        try {
            resultSet = pstmt.executeQuery();
            while (resultSet.next()) {
//                System.out.println("xxx");
                tasksListModel.addElement(""" + resultSet.getString("task_name") +
                        """ + " starts at " + resultSet.getString("start_time"));
            }
        } catch (SQLException ex) {
            Logger.getLogger(ToDoFrame.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

这是RemoveButton的ActionListener

jbtnRemoveTask.addActionListener((ActionEvent e) -> {
// to fill the list with latest values in database
            sqlQuery = "SELECT * FROM tasks";
            try {
                pstmt = connection.prepareStatement(sqlQuery);
                fillTaskList();
            } catch (SQLException ex) {
                Logger.getLogger(ToDoFrame.class.getName()).log(Level.SEVERE, null, ex);
            }
// to restore the Frame to default size as I change size during the using of application
            if (ToDoFrame.this.getSize().width > 515 || ToDoFrame.this.getSize().height > 440) {
// implementation for these two methods comes at the end
                decreaseHeight();
                decreaseWidth();
            }
//          System.out.println(jListTasks.getLastVisibleIndex());
//          System.out.println(jListTasks.getSelectedIndex());
// this code is always executed even there is item selected
            if (jListTasks.getSelectedIndex() == -1) {
                JOptionPane.showMessageDialog(null, "No task selected!!");
            } else {
                JOptionPane.showConfirmDialog(null, "Are you sure?!", "Remove task", JOptionPane.YES_NO_OPTION);
                try {
//                  System.out.println(jListTasks.getSelectedValue());
                    String s = jListTasks.getSelectedValue().toString();
                    s = s.substring(s.length() - 5);
//                    System.out.println(s);
// the code for updating database
                    sqlUpdate = "DELETE FROM tasks WHERE start_time = ?";
                    pstmt = connection.prepareStatement(sqlUpdate);
                    pstmt.setString(1, s);
                    pstmt.executeUpdate();
                    JOptionPane.showMessageDialog(null, "Task removed successfully!");
                    sqlQuery = "SELECT * FROM tasks";
                    pstmt = connection.prepareStatement(sqlQuery);
                    fillTaskList();
                } catch (SQLException ex) {
                    Logger.getLogger(ToDoFrame.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        });
// here are the code of decreaseWidth and decreaseHeight methods
private void decreaseWidth() {
    for (int i = ToDoFrame.this.getSize().width; i > 510; i -= 10) {
        ToDoFrame.this.setSize(i, ToDoFrame.this.getSize().height);
        ToDoFrame.this.setLocationRelativeTo(null);
        try {
            Thread.sleep(20);
        } catch (InterruptedException ex) {
            Logger.getLogger(ToDoFrame.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}
 private void decreaseHeight() {
    for (int i = ToDoFrame.this.getSize().height; i > 435; i -= 10) {
        ToDoFrame.this.setSize(ToDoFrame.this.getSize().width, i);
        ToDoFrame.this.setLocationRelativeTo(null);
        try {
            Thread.sleep(20);
        } catch (InterruptedException ex) {
            Logger.getLogger(ToDoFrame.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

注意：在制作方法调整Frame大小之前，代码运行良好，当直接在ActionListener内重新调整大小时

感谢您的帮助

Answer 1

看起来删除按钮的操作侦听器在检索所选索引之前会调用 fillTaskList 方法。重新填充待办事项列表将清除列表中的选择，对jListTasks.getSelectedIndex()的调用将返回 -1。

我认为您只需要在删除所选项目后更新待办事项列表。