JTextField's setText method doesn't work f

I'm puzzled as to why a JTextField doesn't seem to just "clear out" by using the setText("") method on it, when this is coming from a KeyListener. It works fine from an ActionListener, except that, most amazingly, if the KeyListener method tries to invoke the ActionListener method, with a dummy action event (created on the fly as a simple test), it still leaves the typed text in place.

In other words, when you run it from the command line, if you type, for example, a "3" into the field, you will see the setText("test") method does not wipe out the 3, as I would expect and desire, but rather leaves it in place. You will then see "test3" in the display. I have noted this line with a comment. Clicking the JButton will wipe out the text properly. The JButton and JLabel will change text properly. but the JTextField won't. If you then press the button, you will see that the action event clears out the JTextField properly. Now, if you toggle the commented out line, you can see an attempt to invoke the actionPerformed method from the KeyTyped method!!! And still, when you type a "3" into the text field, it will not get wiped out. I would expect the setText("") method to clear it out, which it won't. And this is even when the keyTyped() method is invoking the same actionPerformed() method as the JTextButton.

Motivation here may help a little. I have a need to trap one particular hot-key which will clear out the JTextField at the moment it is typed, just as if you pressed the "clear" button. And this doesn't seem to work.

I haven't done that much with Swing before, but this is quite puzzling.

My SSCCE code follows:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

class P2 implements KeyListener, ActionListener
{
  JTextField fld;
  JButton btn;
  JLabel  lbl;

  P2()
  {
    JFrame frm = new JFrame("Test");
           fld = new JTextField(10);
    JPanel pnl = new JPanel();
           btn = new JButton("Clear it out");
           lbl = new JLabel("This is a test");

    fld.addKeyListener(this);

    btn.addActionListener(this);

    frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frm.setSize(400,400);
    frm.setLayout(new FlowLayout() );
    pnl.add(fld);
    pnl.add(btn);
    pnl.add(lbl);
    frm.getContentPane().add(pnl);
    frm.setVisible(true);
  }
  public void keyPressed(KeyEvent ke) {}
  public void keyReleased(KeyEvent ke) {}
  public void keyTyped(KeyEvent ke)
  {
     System.out.println("got a pressed key");

//this is the setText method that ought to wipe clean the field comments:
    this.fld.setText("test");
    this.btn.setText("try again");
    this.lbl.setText("got a presseed key");


//toggle this comment to see the invocation of the action event:
//    this.actionPerformed(new ActionEvent( new Object(), 2, "test")  );

  }
  public void actionPerformed(ActionEvent ae)
  {
     fld.setText("");
     fld.selectAll();
  }
  public static void main(String[] args)
  {
    SwingUtilities.invokeLater
    (
      new Runnable()
      {
        public void run()
        {
          new P2();
        }
      }
    );
  }
}

回答1:

This behavior is due to the fact that the KeyEvent will be processed by the field after your KeyListener was fired. You can circumvent it by consuming the event via

ke.consume();

inside your method keyTyped.

Depending on your requirements another way would be to encapsulate the clearing calls inside a SwingUtilities.invokeLater which will be processed after your current event and thus clear the field after it was updated.

回答2:

Here's a code snippet using key bindings to wipe out all text on pressing 'a', implemented to use actions already registered in the field's action map (note that you still need to wrap the code into SwingUtilities.invokeLater - as Howard already suggested - that guarantees it to be processed after the fields internal processing)

    JTextField normal = new JTextField("just a normal field", 10);
    final Action selectAll = normal.getActionMap().get("select-all");
    final Action cut = normal.getActionMap().get("cut-to-clipboard");
    Action combine = new AbstractAction() {

        @Override
        public void actionPerformed(final ActionEvent e) {
            SwingUtilities.invokeLater(new Runnable() {
                @Override
                public void run() {
                    selectAll.actionPerformed(e);
                    cut.actionPerformed(e);

                }
            });
        }

    };
    normal.getActionMap().put("magic-delete-all", combine);
    normal.getInputMap().put(KeyStroke.getKeyStroke("A"), "magic-delete-all");