how to validate a jtextfield from mvc that enter only numbers

An example of this is the validation that a textfield, its length is equal to 4 but, how would you do so that when entering you are only numbers and only letters?

public void keyTyped(KeyEvent e)
{
    if(e.getSource().equals(t.getTxtdesc()))
    {  
        String Cade=t.getTxtdesc().getText();
        if(Cade.length()==45)
        { 
            JOptionPane.showMessageDialog(null, "Ha superado el limite", "AVISO", JOptionPane.WARNING_MESSAGE);
            e.consume();

        }
    }