关于java观察者模式使用

package Test;public class Clinet {    public static void main(String[] args) {        IntervalWindow Iw= new IntervalWindow();        Interval i=new Interval();        i.addObserver(Iw);    }}

package Test;import java.util.Observable;public class Interval extends Observable {    public String start = "0";    public String end = "0";    public String length = "0";    public void calculatestart(String start, String end) {        this.start = start;        this.end = end;        this.length = String.valueOf(Integer.parseInt(start)                - Integer.parseInt(end));        setChanged();        notifyObservers();    }    public void calculateend(String start, String end) {        this.start = start;        this.end = end;        this.length = String.valueOf(Integer.parseInt(start)                - Integer.parseInt(end));        setChanged();        notifyObservers();    }    public void calculatelength(String start, String length) {        this.start = start;        this.length = length;        this.end = String.valueOf(Integer.parseInt(start)                - Integer.parseInt(length));        setChanged();        notifyObservers();    }        }

package Test;import java.awt.Color;import java.awt.GridLayout;import java.awt.event.FocusAdapter;import java.awt.event.FocusEvent;import java.util.Observable;import java.util.Observer;import javax.swing.JFrame;import javax.swing.JTextField;@SuppressWarnings("serial")public class IntervalWindow extends JFrame implements Observer {    JTextField start = new JTextField("0", 10);    JTextField end = new JTextField("0", 10);    JTextField length = new JTextField("0", 10);    GridLayout fl = new GridLayout(3, 1);    public IntervalWindow() {        super("测试观察者模式在GUI中的使用");        this.setSize(400, 400);        this.setLocation(500, 100);        this.setBackground(Color.WHITE);        this.setLayout(fl);        this.add(start);        this.add(end);        this.add(length);        start.addFocusListener(new SymFocus());        end.addFocusListener(new SymFocus());        length.addFocusListener(new SymFocus());        this.setVisible(true);    }    class SymFocus extends FocusAdapter {        private String tf_start;        private String tf_end;        private String tf_length;        private Interval interval = new Interval();        @Override        public void focusLost(FocusEvent event) {            Object obj = event.getSource();            if (obj == start) {                tf_start = start.getText();                tf_end = end.getText();                interval.calculatestart(tf_start, tf_end);            } else if (obj == end) {                tf_start = start.getText();                tf_end = end.getText();                interval.calculateend(tf_start, tf_end);            } else if (obj == length) {                tf_start = start.getText();                tf_length = length.getText();                interval.calculatelength(tf_start, tf_length);            }        }    }    @Override    public void update(Observable o, Object arg1) {        System.out.println("her");        start.setText(((Interval) o).start);        end.setText(((Interval) o).end);        length.setText(((Interval) o).length);    }}