数据输出流不起作用

import java.io.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.*;
import java.util.*;

public class two extends JFrame implements ActionListener
{


JLabel l1,l2;
    JTextArea a1,a2;
    JButton b1;
    JMenuBar bar;
    JMenu menu;
    JMenuItem m1,m2;
    JPanel p;
    ArrayList<String> a;
    DataOutputStream d1;

//程序的gui部分

public void go()
{
    p=new JPanel();
    bar=new JMenuBar();
    menu=new JMenu("File");
    m1=new JMenuItem("Save");
    m1.addActionListener(this);
    m2=new JMenuItem("New");
    menu.add(m1);
    menu.add(m2);


Font f=new Font("Arial",Font.BOLD,16);
l1=new JLabel("Question ");
l1.setAlignmentX(CENTER_ALIGNMENT);
l1.setFont(f);
l2=new JLabel("Answer");
l2.setFont(f);
l2.setAlignmentX(RIGHT_ALIGNMENT);
a1=new JTextArea(10,50);
JScrollPane a4 = new JScrollPane(a1);
a4.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
a4.setMaximumSize(new Dimension(600,220));
a4.setAlignmentX(CENTER_ALIGNMENT);
a2=new JTextArea(10,50);
JScrollPane a3 = new JScrollPane(a2);
a3.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
a3.setMaximumSize(new Dimension(600,220));
a3.setAlignmentX(CENTER_ALIGNMENT);
b1=new JButton("Next Card");
b1.setAlignmentX(LEFT_ALIGNMENT);
b1.addActionListener(this);

bar.add(menu);
setJMenuBar(bar);
BoxLayout g=new BoxLayout(p,BoxLayout.Y_AXIS);
p.setLayout(g);
p.add(l1);
p.add(Box.createRigidArea(new Dimension(15,15)));
p.add(a4);
p.add(Box.createRigidArea(new Dimension(15,15)));
p.add(l2);
p.add(Box.createRigidArea(new Dimension(15,15)));
p.add(a3);
p.add(Box.createRigidArea(new Dimension(25,25)));
p.add(b1);
getContentPane().add(p);
setVisible(true);
setSize(500,600);
setResizable(false);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    }

public static void main(String[] args) {
    two obj=new two();
    obj.go();
}


public void actionPerformed(ActionEvent e)
{
 a=new ArrayList<String>();

//将文本区域内容保存到列表数组的部分

 if(e.getSource()==b1)
{
String l=new String(a1.getText()+"/");
String w=new String (a2.getText()+"\n");
a.add(l);
a.add(w);

//将值添加到arraylist中，如a.size（）的值所示

System.out.println(a.size());
a1.setText("");
a2.setText("");

}

//使用filechooser将listarray保存到文件的方法

if(e.getSource()==m1){

JFileChooser fileSave = new JFileChooser();
int retrival=fileSave.showSaveDialog(this);
File d=fileSave.getSelectedFile();

if (retrival == JFileChooser.APPROVE_OPTION) {

        try{
        System.out.println(d);

//这部分没有被调用

        d1=new DataOutputStream(new FileOutputStream(d+".txt");

        for(String o:a)
        {
        d1.writeChars(o);
     System.out.println("\nIS SUCCESFULLY WRITTEN INTO FILE!");
    }
        d1.close();


    }catch(Exception ex){System.out.println(ex);}
}
}

}
}