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 MainClass { public static void main(String[] args) throws Exception { JTextField field = new JTextField(30); ((AbstractDocument) (field.getDocument())).setDocumentFilter(new DocumentFilter() { public void insertString(FilterBypass fb, int offset, String string, AttributeSet attr) throws BadLocationException { System.out.println("insert"); fb.insertString(offset, string.toUpperCase(), attr); } public void replace(FilterBypass fb, int offset, int length, String string, AttributeSet attr) throws BadLocationException { System.out.println("replace"); fb.replace(offset, length, string.toUpperCase(), attr); } }); JFrame frame = new JFrame("User Information"); frame.getContentPane().add(field); frame.pack(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); } }