Textfield keylistener problem

I have the below code

textfield.addKeyListener(new java.awt.event.KeyAdapter() {
            public void keyTyped(java.awt.event.KeyEvent evt) {
            filterStr = filterKeyTyped(evt, textfield.getText().trim());    
            System.out.println(filterStr)
});

private String filterKeyTyped(KeyEvent e, String string) {
        if(e.getKeyChar() == 8 ){ // backspace character was hit
            int length = string.length();
            if(length > 1) {
                string = string.substring(0, length - 1).toUpperCase();
            }
            else {
                string = "";
            }
        }
        else {
            string += e.getKeyChar();
        }

        if (!isNumber(string)) {
            string = "";
        }
       
        return (string);
    }

    public boolean isNumber(String text) {
        try {
            int test = Integer.parseInt(text.trim());
            return true;
        } catch (NumberFormatException nfe) {
            return false;
        }
    }

Works fine except for one problem, if the user selects all of the text in the textfield and hits delete, it only removes one character from the string.
How can i detect if they highlight all the text and delete it so i can set the string back to nothing?