/* @author H.U. */

import java.awt.Color;
import java.awt.Font;
import javax.swing.InputVerifier;
import javax.swing.JComponent;
import javax.swing.JTextField;
.........
        inputField.setInputVerifier(myIntInputVerifier);
.........
    private InputVerifier myIntInputVerifier = new IntInputVerifier();

    private class IntInputVerifier extends InputVerifier{
        @Override
        public boolean verify(JComponent input) {
            JTextField inputField = (JTextField) input;
            String text = inputField.getText();
            try {
                int i = Integer.parseInt(text);
                inputField.setForeground(Color.black);
                Font font = inputField.getFont();
                inputField.setFont( new Font(font.getName(), Font.PLAIN, font.getSize()));
                return true;
            }
            catch (NumberFormatException ex) {
                inputField.setForeground(Color.red);
                Font font = inputField.getFont();
                inputField.setFont(new Font(font.getName(), Font.BOLD, font.getSize()));
                return false;
            }
        }
    }
.........