使用构造函数初始化变量

我有两个类，第一个是我的主类，第二个是我的编辑框架类。

public class RecordTableGUI extends JFrame implements ActionListener { String newName; public RecordTableGUI(String newReceivedName) { newName = newReceivedName; System.out.println("new name in new constructor : " + newName); //prints new name correctly } public void actionPerformed(ActionEvent e) { if (e.getSource() == editButton) { Object oldName = table.getValueAt(table.getSelectedRow(), 1); System.out.println("old name: " + oldName); // prints old name correctly this.setVisible(false); new UpdateGUI(String.valueOf(oldName)); System.out.println("new name in problem area: " + newName); // why null? } } } 

我的第二个类（UpdateGUI）在它的构造函数中给出了oldName，在编辑之后，当我点击okButton ，它将newName发送到我的第一个类。

我的第二课：

 public class UpdateGUI extends JFrame implements ActionListener { String oldName, newName; public UpdateGUI(String oldname) { oldName = oldname; .... } public void actionPerformed(ActionEvent e) { if (e.getSource() == okButton) { newName = tf.getText(); //tf is JTextfield new RecordTableGUI(newName); this.setVisible(false); } } 

我的问题是为什么newName为null？

更新：

 public class RecordTableGUI extends JFrame implements ActionListener { public RecordTableGUI(String newReceivedName) { setNewName(newReceivedName); } public void actionPerformed(ActionEvent e) { if (e.getSource() == editButton) { Object oldName = table.getValueAt(table.getSelectedRow(), 1); System.out.println("old name: " + oldName); RecordTableGUI recordObject = new RecordTableGUI(); UpdateGUIDialog updDialog = new UpdateGUIDialog(String.valueOf(oldName), recordObject); } } 

UpdateGUIDialog类：

 public class UpdateGUIDialog extends JDialog implements ActionListener { RecordTableGUI recordtablegui; public UpdateGUIDialog(String old, RecordTableGUI recordGUI) { oldName = old; recordtablegui = recordGUI; } @Override public void actionPerformed(ActionEvent e) { if (e.getSource() == okButton) { newName = tf.getText(); recordtablegui.setNewName(newName); this.dispose(); } } } 

输出：

 old name:james //prints correctly new name: null //prints null new name in set method: rrr //prints correctly 

我需要打印rrr而不是null。

Answer 1

Java对象有点像真实对象。 并且new的名字暗示：它创造了一个新的对象。 我们举一个简单的例子：

 Box box1 = new Box(); Box box2 = new Box(); box1.fillWithCandies(candies); 

box1是一个装满糖果的盒子。 box2是一个不同的盒子，它不包含任何东西，因为只有box1填充了糖果。

在您的代码中，updateGUI的actionPerformed()方法使用新名称创建一个新的RecordTableGUI对象。 这不会改变第一个。

如果您希望updateGUI修改现有的RecordTableGUI对象，则需要引用此对象：

 public class updateGUI extends JFrame implements ActionListener { private RecordTableGUI recordTableGUIToUpdateWhenOKIsClicked; public updateGUI(RecordTableGUI recordTableGUIToUpdateWhenOKIsClicked, ...) { this.recordTableGUIToUpdateWhenOKIsClicked = recordTableGUIToUpdateWhenOKIsClicked; ... } public void actionPerformed(ActionEvent e) { if (e.getSource() == okButton) { newName = tf.getText(); this.recordTableGUIToUpdateWhenOKIsClicked.setNewName(newName); } } } 

在使用Swing之前，您应该练习更简单的示例。 您还应该尊重Java命名约定。 updateGui类应该是JDialog，而不是JFrame。

Answer 2

例如（ 没有理由，不打扰使用costructors，getter ….，Swing JComponents被指定为可重用 ）

。

。

来自代码

 import java.awt.BorderLayout; import java.awt.Component; import java.awt.Dimension; import java.awt.Point; import java.awt.event.*; import javax.swing.*; import javax.swing.event.ListSelectionEvent; import javax.swing.event.ListSelectionListener; import javax.swing.table.*; public class TableCheckBox { private static final long serialVersionUID = 1L; private JTable table; private JFrame frame = new JFrame("Popup Table Editor"); // I'm reinvent the wheel see code for Popup Table Editor by @camickr private JDialog dialog = new JDialog(frame, "Edit Table data", true); private JPanel panel = new JPanel(); private JLabel TypeLabel, CompanyLabel, SharesLabel, PriceLabel, BooleanLabel; private JTextField TypeTextField, CompanyTextField; private JFormattedTextField SharesTextField, PriceTextField; private JCheckBox BooleanCheckBox; private JButton saveButton = new JButton("Save changed to JTable"); private Point location; private Object[] columnNames = {"Type", "Company", "Shares", "Price", "Boolean"}; private Object[][] data = { {"Buy", "IBM", new Integer(1000), new Double(80.50), false}, {"Sell", "MicroSoft", new Integer(2000), new Double(6.25), true}, {"Sell", "Apple", new Integer(3000), new Double(7.35), true}, {"Buy", "Nortel", new Integer(4000), new Double(20.00), false} }; private DefaultTableModel model = new DefaultTableModel(data, columnNames) { private static final long serialVersionUID = 1L; @Override public Class getColumnClass(int column) { return getValueAt(0, column).getClass(); } }; public TableCheckBox() { table = new JTable(model); table.setPreferredScrollableViewportSize(table.getPreferredSize()); table.getSelectionModel().setSelectionMode( ListSelectionModel.SINGLE_SELECTION); table.getSelectionModel().addListSelectionListener(new ListSelectionListener() { @Override public void valueChanged(ListSelectionEvent e) { if (!e.getValueIsAdjusting()) { System.out.println(table.getSelectedColumn()); System.out.println(table.getSelectedRow()); } } }); JScrollPane scrollPane = new JScrollPane(table); createPopupMenu(); createDialog(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.add(scrollPane); frame.pack(); frame.setLocation(150, 150); frame.setVisible(true); } private void createPopupMenu() { JPopupMenu popup = new JPopupMenu(); JMenuItem myMenuItem1 = new JMenuItem("Edit Table Data"); myMenuItem1.addActionListener(showingDialog()); popup.add(myMenuItem1); MouseListener popupListener = new PopupListener(popup); table.addMouseListener(popupListener); } private void createDialog() { /* laid to private JPanel panel = new JPanel(); change layout to GBC, SprigLayout valid for follows JComponents private JLabel TypeLabel, CompanyLabel, SharesLabel, PriceLabel, BooleanLabel; private JTextField TypeTextField, CompanyTextField; private JFormattedTextField SharesTextField, PriceTextField; private JCheckBox BooleanCheckBox; private JButton saveButton = new JButton("Save changed to JTable"); */ saveButton.addActionListener(new ActionListener() { @Override public void actionPerformed(ActionEvent e) { //table.setValueAt(JTextField.getText, rowFromListSelectionLIstener, //ColumnFromListSelectionListener + plusMinusCitibus) //table.setValueAt(JFormattedTextField. getValue //or(((Number) textField2.getValue()).doubleValue());, //rowFromListSelectionLIstener, ColumnFromListSelectionListener + plusMinusCitibus) hideDialog();//last code line } }); dialog.add(saveButton, BorderLayout.SOUTH); dialog.setDefaultCloseOperation(JDialog.HIDE_ON_CLOSE); dialog.addWindowListener(new WindowAdapter() { @Override public void windowClosing(WindowEvent e) { hideDialog(); } }); dialog.setPreferredSize(new Dimension(400, 300));// remove this code line dialog.pack(); } private Action showingDialog() { return new AbstractAction("Show Dialog") { private static final long serialVersionUID = 1L; @Override public void actionPerformed(ActionEvent e) { System.out.println("dialog.setVisible(true)"); // // copy value from JTable/XxxTableModel to JComponents placed in JPanel // dialog.setVisible(false); //location = frame.getLocationOnScreen(); int x = location.x - 10; int y = location.y + 50; dialog.setLocation(x, y); Runnable doRun = new Runnable() { @Override public void run() { //dialog.setLocationRelativeTo(frame); dialog.setVisible(true); } }; SwingUtilities.invokeLater(doRun); } }; } private void hideDialog() { System.out.println("dialog.setVisible(false)"); /* reset value for private JTextField TypeTextField, CompanyTextField; private JFormattedTextField SharesTextField, PriceTextField; then after to call dialog.setVisible(false); */ dialog.setVisible(false);//last code line } private class PopupListener extends MouseAdapter { private JPopupMenu popup; PopupListener(JPopupMenu popupMenu) { popup = popupMenu; } @Override public void mousePressed(MouseEvent e) { maybeShowPopup(e); } @Override public void mouseReleased(MouseEvent e) { if (table.getSelectedRow() != -1) { maybeShowPopup(e); } } private void maybeShowPopup(MouseEvent e) { if (e.isPopupTrigger()) { int row = table.rowAtPoint(e.getPoint());// get row that pointer is over if (table.isRowSelected(row)) {// if pointer is over a selected row, show popup Component comp = e.getComponent(); location = comp.getLocationOnScreen(); popup.show(e.getComponent(), e.getX(), e.getY()); } } } } public static void main(String[] args) { SwingUtilities.invokeLater(new Runnable() { @Override public void run() { TableCheckBox frame = new TableCheckBox(); } }); } }