文件查看器，怎样实现返回查看功能,该怎么解决

文件查看器，怎样实现返回查看功能
新手刚学习安卓，写了个程序
根目录是scard，点击文件夹可以浏览其子目录所有文件，
比如点击一个文件夹，怎样实现返回父目录,求教高手

主程序代码：
import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;

import android.app.ListActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.Toast;

public class ExplorerActivity extends ListActivity{  

private String rootPath = "/sdcard";
private String currentPath = "";
private ArrayList<String> paths=new ArrayList<String>();

  ListView mListView = null; 
  ArrayList<Map<String,Object>> mData= new ArrayList<Map<String,Object>>();  
  SimpleAdapter adapter = null;
   
  protected void onCreate(Bundle savedInstanceState){ 
super.onCreate(savedInstanceState);
mListView = (ListView)findViewById(R.id.fileList);
currentPath = rootPath;
showList();  
  } 
   
  private void showList(){
  mData = getDir(currentPath);
  adapter = new SimpleAdapter(this,mData,R.layout.mylist,  
  new String[]{"image","title"},new int[]{R.id.list_image,R.id.list_title});  
  mListView.setAdapter(adapter);
  mListView.setSelection(0);
  mListView.setOnItemClickListener(new OnItemClickListener() {  
@Override  
public void onItemClick(AdapterView<?> adapterView, View view, int position,  
long id) {  
currentPath = paths.get(position);
File f = new File(currentPath); 
if(f.isDirectory()){
fresh(currentPath);
} else {
Toast.makeText(ExplorerActivity.this, "this is not a directory", Toast.LENGTH_SHORT).show();
}

}  
});
  }
  private void fresh(String path){
  mData.clear();
mData = getDir(path);
adapter.notifyDataSetChanged();
  }

  private ArrayList<Map<String, Object>> getDir(String path){
mListView = getListView();
paths = new ArrayList<String>();
File f = new File(path);
File fileList[] = f.listFiles();

  int length = fileList.length;  

  for(int i =0; i < length; i++) {  
Map<String,Object> item = new HashMap<String,Object>();  
item.put("image", R.drawable.file_icon);  
item.put("title", fileList[i].getName());
File file=fileList[i];
paths.add(file.getPath());
mData.add(item);  
  }  
 
return mData;
  }
}

main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  android:orientation="vertical" >

<ListView 
android:id="@+id/fileList"
android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  >  
</ListView>
</LinearLayout>

mylist.xml

<?xml version="1.0" encoding="utf-8"?>