java如何始终包含毫秒，即使日期格式不包含毫秒？

Question

这个例子有两列数据

第1列是不带毫秒的日期，第2列是带毫秒的数据

显示屏允许用户更改日期格式。关键是第1列不应该包含毫秒，而第2列应该始终包含毫秒

如果日期格式为dd-MM-yyy-HH:MM:ss，则在处理第2列中的数据时，代码可以简单地附加“.SSS”，但格式可能会有很大差异。例如，它甚至可以是“MM-dd-yy”，而不包括任何时间参考，但第二列应始终包括时间和毫秒

这是代码。有额外的杂项代码，只是为了确保毫秒在第一次可见

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTable;
import javax.swing.JTextField;
import javax.swing.table.AbstractTableModel;
import javax.swing.table.TableCellRenderer;

public class DateFormatRendererExample 
{

    public static void main(String[] args) 
    {
        DateExampleSetup dateExampleSetup = new DateExampleSetup (); 

    }

}


class DateExampleSetup extends JFrame 
{
    final static Class[] columnClass = new Class[] { Date.class, Date.class };
    String currentDateFormat = "MM-dd-yyyy HH:mm:ss";
    String oldDateFormat = "MM-dd-yyyy HH:mm:ss";
    MyTableModel model;
    
    public DateExampleSetup() 
    {

        model = new MyTableModel();
        model.addRow("05-22-2020 12:12:12", "06-02-2020 20:11:55.123");
        model.addRow("10-18-2020 14:16:17", "01-02-2020 09:09:49.567");
        
        JTable table = new JTable(model);
        table.setDefaultRenderer(Date.class, new MyDateCellRenderer(this));
        JTextField dateFormatField = new JTextField(20);
        dateFormatField.setText(currentDateFormat);
        JButton activateDateFormat = new JButton("Activate new Date Format");
        activateDateFormat.addActionListener(new ActionListener(){

            @Override
            public void actionPerformed(ActionEvent arg0) 
            {
                
                setCurrentDateFormat(dateFormatField.getText());
                MyTableModel newModel = new MyTableModel ();
                newModel.addRow("05-22-2020 12:12:12", "06-02-2020 20:11:55.1234");
                newModel.addRow("10-18-2020 14:16:17", "02-02-2020 09:09:49.5678");
                table.setModel(newModel);
                newModel.fireTableDataChanged();
                setOldDateFormat(getCurrentDateFormat());
            }
            
        });
        setLayout(new BorderLayout());
        add(dateFormatField, BorderLayout.NORTH);
        add(table, BorderLayout.CENTER);
        add(activateDateFormat, BorderLayout.SOUTH);
        setVisible(true);
        setSize(500,300);
        
    }
    public String getCurrentDateFormat()
    {
        return currentDateFormat;
    }
    public void setCurrentDateFormat(String newCurrent)
    {
        currentDateFormat = newCurrent;
    }
    public String getOldDateFormat()
    {
        return oldDateFormat;
    }
    public void setOldDateFormat(String newCurrent)
    {
        currentDateFormat = newCurrent;
    }

}

class MyDateCellRenderer extends JLabel implements TableCellRenderer 
{
    DateExampleSetup des;
    boolean firstTimeRow1 = true;
    boolean firstTimeRow2 = true;
    
    public MyDateCellRenderer(DateExampleSetup des)
    {
        super();
        this.des = des;
    }
      public Component getTableCellRendererComponent(JTable table, Object value, boolean isSelected,
          boolean hasFocus, int row, int col) {
          
        try 
        {
            ///Just to get the milliseconds to show the first time.
            if (firstTimeRow1 && row == 0 && col == 1)
            {
                SimpleDateFormat tempDateFormat = new SimpleDateFormat("MM-dd-yyyy HH:mm:ss.SSS");
                Date tempDate = null;
                tempDate = tempDateFormat.parse((String)value);
                value = tempDateFormat.format(tempDate);
                setText(value.toString());
                firstTimeRow1 = false;
            }
            else if (firstTimeRow2 && row == 1 && col == 1)
            {
                SimpleDateFormat tempDateFormat = new SimpleDateFormat("MM-dd-yyyy HH:mm:ss.SSS");
                Date tempDate = null;
                tempDate = tempDateFormat.parse((String)value);
                value = tempDateFormat.format(tempDate);
                setText(value.toString());
                firstTimeRow2 = false;
            }
            else
            {
                SimpleDateFormat oldDateFormat = new SimpleDateFormat(des.getOldDateFormat());
                SimpleDateFormat newDateFormat = new SimpleDateFormat(des.getCurrentDateFormat());
                Date date = null;
                date = oldDateFormat.parse((String)value);
                value = newDateFormat.format(date);
                setText(value.toString());
            }
        } 
        catch (ParseException e) 
        {
            e.printStackTrace();
        }

        return this;
      }
    }

class MyTableModel extends AbstractTableModel {

    private List<String[]> rows;
    private String columnNames[] = { "Regular Date ", "Date with milliseconds"};

    public MyTableModel() {
        rows = new ArrayList<>(25);
    }
    
    @Override
    public int getRowCount() {
        return rows.size();
    }

    @Override
    public int getColumnCount() {
        return 2;
    }

    @Override
    public Class<?> getColumnClass(int columnIndex) {
        return Date.class;
    }

    @Override
    public Object getValueAt(int rowIndex, int columnIndex) {
        String[] row = rows.get(rowIndex);
        return row[columnIndex];
    }
    
    @Override
    public String getColumnName(int col) {
        return columnNames[col];
    }
    
    public void addRow(String s1, String s2) 
    {
        String[] row = new String[getColumnCount()];
        row[0] = s1;
        row[1] = s2;
        rows.add(row);
        fireTableRowsInserted(getRowCount(), getRowCount());
    }            
} 

执行时，将显示以下帧。注意，第二列有毫秒

当日期更改为“dd-MM-yyyy”并按下“激活新日期格式”按钮时，第二列中的日期会丢失小时、分钟和秒，尤其是毫秒

是否可以对一列使用单一日期格式（几乎可以是任何格式），而对另一列使用相同的格式，同时确保HH:mm:ss。sss总是存在吗