Why JTextField.setText will fire DocumentListener's removeUpdate() before changedUpdate()?

Question

This is my code:

import javax.swing.*;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.Document;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Frame extends JFrame {

    private JTextField txt1 = new JTextField(10);
    private JTextField txt2 = new JTextField(10);
    private JButton btn = new JButton("Set Text");

    public Frame() {
        super("Latihan");
        setLayout(new FlowLayout());
        btn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                txt1.setText("TEST"); txt2.setText("TEST2");
            }
        });

        txt1.getDocument().addDocumentListener(new TheDocumentListener("txt1"));
        txt2.getDocument().addDocumentListener(new TheDocumentListener("txt2"));

        add(txt1);
        add(txt2);
        add(btn);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        pack();
        setVisible(true);
    }

    public static void main (String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new Frame();
            }
        });
    }
}

class TheDocumentListener implements DocumentListener {

    private String source;

    public TheDocumentListener(String source) {
        this.source = source;
    }
    @Override
    public void insertUpdate(DocumentEvent e) {
        System.out.println("insertUpdate from " + source);
    }

    @Override
    public void removeUpdate(DocumentEvent e) {
        System.out.println("removeUpdate from " + source);
    }

    @Override
    public void changedUpdate(DocumentEvent e) {
        System.out.println("changedUpdate from " + source);
    }
}

When I click on the JButton for the first time, only insertUpdate() will be called:

insertUpdate from txt1
insertUpdate from txt2

But if I click the button again, removeUpdate() will be called before insertUpdate():

removeUpdate from txt1
insertUpdate from txt1
removeUpdate from txt2
insertUpdate from txt2

Is this expected behaviour or something wrong in my code?

Can I make insertUpdate the only method that was being called when performing JTextField.setText? I want to make sure removeUpdate is being called only when user delete text in the text field. How to do that?

tenorsax · Accepted Answer · 2013-03-05T01:25:27.003

This is the expected behavior of string replacement. What setText() actually does is remove the whole string and set a new one. Here is the implementation of JTextField.setText():

public void setText(String t) {
    try {
        Document doc = getDocument();
        if (doc instanceof AbstractDocument) {
            ((AbstractDocument)doc).replace(0, doc.getLength(), t,null);
        }
        else {
            doc.remove(0, doc.getLength());
            doc.insertString(0, t, null);
        }
    } catch (BadLocationException e) {
    UIManager.getLookAndFeel().provideErrorFeedback(JTextComponent.this);
    }
}

As you can see, AbstractDocument.replace() is executed for AbstractDocument docs. Otherwise, remove() and insert() are executed.

From AbstractDocument.replace() documentation:

Deletes the region of text from offset to offset + length, and replaces it with text. It is up to the implementation as to how this is implemented, some implementations may treat this as two distinct operations: a remove followed by an insert, others may treat the replace as one atomic operation.

So it depends on the document implementation. PlainDocument for example inherits basic implementation of AbstractDocument. A PlainDocument is the default document for text fields.

You can always create you own document implementation if needed, or maybe installing a document filter. See Using Text Components tutorial for details. Not sure though, what is the reason behind this behavior change you're trying to achieve.

The reason is for simulating JTextField binding because setText() is not bound. When it comes to mutual binding, it should be: change to other bound property will propagated to JTextField.text; and call to JTextField.text should be propagated to the bound property. But what really happen now is change to JTextField.text will sometimes fire 1 or 2 document listener events and it's hard to decide when to stop the mutual binding. — jocki, Mar 05 '13 at 04:21

Why JTextField.setText will fire DocumentListener's removeUpdate() before changedUpdate()?

1 Answers1