Use a VerifyListener
:
txt_Appname.addVerifyListener(new VerifyListener() {
@Override
public void verifyText(VerifyEvent e) {
String text = e.text;
for (int i = 0; i < text.length(); i++) {
if (!Character.isLetterOrDigit(text.charAt(i)) {
e.doit = false;
return;
}
}
}
});