Java tutorial
import javax.swing.JFrame; import javax.swing.JTextArea; 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 UpcaseFilter extends DocumentFilter { public void insertString(DocumentFilter.FilterBypass fb, int offset, String text, AttributeSet attr) throws BadLocationException { fb.insertString(offset, text.toUpperCase(), attr); } //no need to override remove(): inherited version allows all removals public void replace(DocumentFilter.FilterBypass fb, int offset, int length, String text, AttributeSet attr) throws BadLocationException { fb.replace(offset, length, text.toUpperCase(), attr); } public static void main(String[] args) { DocumentFilter dfilter = new UpcaseFilter(); JTextArea jta = new JTextArea(); JTextField jtf = new JTextField(); ((AbstractDocument) jta.getDocument()).setDocumentFilter(dfilter); ((AbstractDocument) jtf.getDocument()).setDocumentFilter(dfilter); JFrame frame = new JFrame("UpcaseFilter"); frame.getContentPane().add(jta, java.awt.BorderLayout.CENTER); frame.getContentPane().add(jtf, java.awt.BorderLayout.SOUTH); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setSize(200, 120); frame.setVisible(true); } }