Textfield aceitar apenas nÚmeros

/**
* @author Fernando Paiva
/*
public class NumerosInteiros extends PlainDocument {
	private int maxlength;
	public NumerosInteiros(){};

        public NumerosInteiros(int maxlength){
        	super();
                this.maxlength = maxlength;
	}

	public void insertString(int offs, String str, AttributeSet a){
	try{
        	Integer.parseInt(str);
	} catch (NumberFormatException ex){
        	str = "";
	}
	try{
        	boolean fixedLengh = (!((getLength() + str.length()) > maxlength));
        	if (maxlength == 0 || fixedLengh)
        	super.insertString(offs, str, a);
	} catch (BadLocationException e){
        	e.printStackTrace();
	}
    }
}

//usar
JTextField txtField = new JTextField();
txtField.setDocument(new NumerosInteiros(int len));