Allow textfield to input only number [Java]

Question

I know this has been asked and answered many times but I still can't get the answer that I really need. Hopefully this time, somebody can help me and I thank you in advance. :)

This is what I want in my program, I want user to limit to input only numbers. Whenever they input letters and others there will be a prompt message. I can do that, there is a prompt message for letters and other char but the inputted value still remain, I want it to be cleared.

Please see my code.

private void txtQty1KeyTyped(java.awt.event.KeyEvent evt) {                                 
    txtQty1.addKeyListener(new KeyAdapter() {});
    char char_input = evt.getKeyChar();
    if (((char_input < '0') || (char_input > '9')) && (char_input != '\b'))
    {
        JOptionPane.showMessageDialog(this, "Number only!","Invalid Input",JOptionPane.ERROR_MESSAGE);
        txtQty1.setText(" ");
    }

}    

Though I clear my textfield, the character that I input still appears. Any help would be much appreciated. Thank you! :)

Answer 1

You need to create a subclass of DocumentFilter class and use a regular expression to match each inserted string/character if they are digits or not and perform actions accordingly.

Below is a fully working sample code of this working. Thanks to @camickr for pointing out using DocumentFilter is more up-to-date than the old way of extending JTextField to achieve the same result.

import java.awt.BorderLayout;
import java.util.regex.Pattern;

import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.text.AbstractDocument;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.DocumentFilter;

public class TestDocumentFilter {

    public static void main(String... args) {
        new TestDocumentFilter();
    }

    public TestDocumentFilter() {
        JTextField textField = new JTextField(10);
        ((AbstractDocument) textField.getDocument()).setDocumentFilter(new CustomDocumentFilter());

        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(new BorderLayout(5, 5));
        frame.getContentPane().add(textField, BorderLayout.NORTH);
        frame.setSize(400, 200);
        frame.setVisible(true);
    }

    private class CustomDocumentFilter extends DocumentFilter {

        private Pattern regexCheck = Pattern.compile("[0-9]+");

        @Override
        public void insertString(FilterBypass fb, int offs, String str, AttributeSet a) throws BadLocationException {
            if (str == null) {
                return;
            }

            if (regexCheck.matcher(str).matches()) {
                super.insertString(fb, offs, str, a);
            }
        }

        @Override
        public void replace(FilterBypass fb, int offset, int length, String str, AttributeSet attrs)
                throws BadLocationException {
            if (str == null) {
                return;
            }

            if (regexCheck.matcher(str).matches()) {
                fb.replace(offset, length, str, attrs);
            }
        }
    }
}