Question

我正在尝试将PrintWrite数据写入文件，但只创建java文件而不编写任何数据。

数据存储在ArrayList中，我认为这个问题是因为sData.size（）方法。当用户点击保存图标时，使用操作方法部分，表格将没有任何数据。用户单击该按钮可将数据添加到Jtable。

末尾的部分代码会显示错误消息java.lang.NullPointerException。

    public class ee extends JFrame {
            String[] columnNames = {"#", "c1", "c2", "data column"};

            public final JTable table;
            JToolBar toolBar;
            private final JTextField enterText,startTime,endTime;


            static DefaultTableModel cModel ;
            JButton Obutton;
            int  counter = 1;
          public static ArrayList<String> sData = new ArrayList<String>();
   public static ArrayList<String> eData = new ArrayList<String>();
   public static ArrayList<String> tData = new ArrayList<String>();

                 public static void main(String[] args) throws IOException 
               {
                   ee is = new ee("iss");

               }


            //the constrictor 
            public ee(String title){
            cModel =new DefaultTableModel(columnNames, 0);       
            table = new JTable(cModel);

行动方法

Action saveAction = new AbstractAction("Save", saveIcon) {
                @Override
                public void actionPerformed(ActionEvent e) {
                    File myFile = new File("the-file-name.txt");
                    try (PrintWriter writer = new PrintWriter(myFile)) {   

                    for (int i =0 ; i< sData.size(); i++)
                    {

                    writer.println(i+1 + "\n");
                    writer.println(sData.get(i)+" --> " + eData.get(i));
                    writer.println(tData.get(i)+"\n");

                    }
                    }catch(FileNotFoundException ioEx){

                    } 

                }
            };  
        }

从JTable获取数据并将其保存到ArrayList

public static ArrayList<String> getTableSTimeData(){

     sData = new ArrayList<String>();

        for (int i=0; i < cModel.getRowCount(); i++){      
               Object val = cModel.getValueAt(i, 1); // value of ith row and 3rd column
               sData.add((String) val);

            }

    return sData;
}

数据插入JTable的方式

public class MenuBarMethod implements ActionListener{

    @Override
    public void actionPerformed(ActionEvent a){
        Object buttonPressed=a.getSource();

       if(buttonPressed.equals(Obutton)){
           String STime = startTime.getText();
           String ETime = endTime.getText();
           String text = enterText.getText();
           clearEntity();
           cModel.addRow(new Object[] {getCounter(),STime, ETime, text});

       }
       }
    }


}

Answer 1

在声明时初始化ArrayList以避免NullPointerException

public class ee extends JFrame {

   ...
   public static List<String> sData = new ArrayList<String>();
   public static List<String> eData = new ArrayList<String>();
   public static List<String> tData = new ArrayList<String>();
   ...
}

为什么Java只创建文件而不向其写入任何数据

1 个答案: