JTextField salary=new JTextField(10);
salary.setDocument(new NumOnlyDocument());

class NumOnlyDocument extends PlainDocument{
    public void insertString(int offset, String s, AttributeSet attrSet)
        throws BadLocationException {
      try {
        Integer.parseInt(s);
      }
      catch (NumberFormatException ex) {
        return;
      }
      super.insertString(offset,s,attrSet);
    }
}