The first step is to set the project up in Eclipse.

1. Start up Eclipse
2. Right click the package explorer and click on New->Android Project
3. In the new Project dialog box, fill out the following
   • Project Name: FSExplorer1
   • Package Name: org.lombardos.android.FSExplorer1
   • Activity Name: FileList
   • Application Name: Filesystem Explorer
4. Click Finish to complete the Project dialog box.

New Android Project (Click to enlarge)

Display the Root File System

The next step is to modify the FileList so that the files and directories on the root directory are displayed. This is similar to how the Notes are displayed in the Notepad, except we don't get our info from the embedded SQL database, we get it from the file system on the Android phone.

1. Add a string named "no_files" to res/values/strings.xml. It's value should be "This directory is empty". This string will be displayed when there are no files or subdirectories within a directory.

2. Create a new layout called directory_list.xml. It should look the same as below. The format of this file is covered step four of the first exercise in the tutorial.

   
         <?xml version="1.0" encoding="utf-8"?>
         <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content">
   
           <ListView id="@id/android:list"
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"/>
           <TextView id="@id/android:empty"
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
                 android:text="@string/no_files"/>
         </LinearLayout>
   

3. Create another new layout called file_row.xml. It should look the same as below. The format of this file is covered step five of the first exercise in the tutorial.

         <?xml version="1.0" encoding="utf-8"?>
         <TextView id="@+id/text1"
             xmlns:android="http://schemas.android.com/apk/res/android"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"/>
   

   This layout is used to display each individual file or subdirectory within a directory.

4. Modify the onCreate() function in FileList.java so that it reads as so:

   
             public void onCreate(Bundle icicle) {
                 super.onCreate(icicle);
                 setContentView(R.layout.directory_list);
                 fill(new File("/").listFiles());
             }
   

   We changed the first parameter of setContentView() to R.layout.directory_list so that the list of Mp3s would appear instead of the "main" view. We also added a call to the fill() method. We haven't written fill() yet, but we'll get there in a second.

5. Change the class that FileList extends from "Activity" to "ListActivity". It should look like this:

   
         public class FileList extends ListActivity {
   

6. Create a private field, items, that holds the items in the current directory. This should go just under the "public class FileList" line.

   
             private List<String> items = null;
   

7. The private fill() method takes a File[] array, and displays each file name on the screen in the list.

   
         1    private void fill(File[] files) {
         2        items = new ArrayList<String>();
         3        for( File file : files )
         4            items.add(file.getPath());
         5        ArrayAdapter<String> fileList = new ArrayAdapter<String>(this, R.layout.file_row, items);
         6        setListAdapter(fileList);
         7    }
   

   1. Lines 2-4 take the files[] array and make an ArrayList<String> out of each file's name. "items" is a class field because it will be used later when the user clicks on one of the files or directories.

   2. Line 5 creates an ArrayAdapter<String> out of the items using the file_row layout. This is what marries the list of List<> of files to the list on the screen.
   3. Finally, on line six, the current screen's content is set to the list of file names.

               <>   

8. You may have to hit Shift-Control-O to Organize Imports.

9. Save all your files and run. You should have a list of directories on your emulated Android phone.

10. Click on them and nothing happens -- next step fixes that.

Now that we can see the root directory structure, let's see if we can display some sub directories.

1. We're going to need some way to navigate back to the root of the tree, or we'll eventually get stuck on a leaf with no way back. A quick solution is to give our users a way to hop all the way back to the root from wherever they are. To implement this, the first step is to add a string to the strings.xml file. The name should be to_top. The value should be "[Back to Top]".

2. Next we need to add the "to_top" string to the item list in the fill() method. Add this line just after the "items = new ArrayList<String>(); line in the fill method:

   
           items.add(getString(R.string.to_top));
   

   Note that using the getString(R.string.<name>) is preferable to including the string as a final field within your class (or even worse, just putting the string in-line). Moving all of your strings to the resource directory goes a long way toward internationalizing your code if you ever need to do so.

3. When the user clicks on a list item, we have the opportunity to intercept that click and do something interesting with it. To that end we override the onListItemClick() method of ListActivity. Here's a method that will do nicely:

   
         1    @Override
         2    protected void onListItemClick(ListView l, View v, int position, long id) {
         3        int selectionRowID = (int)getSelectionRowID();
         4        if (selectionRowID==0){
         5            fillWithRoot();
         6        } else {
         7            File file = new File(items.get(selectionRowID));
         8            if (file.isDirectory())
         9                fill(file.listFiles());
         10           else
         11               AlertDialog.show(this, "Not a Directory", "That's a file, not a directory", "Cancel", false);
         12       }
         13   }
   

   Notes:
   • Line 3 reveals which line number the user clicked on.
   • On line five, we display the root of the file system again if the user chose line 0.
   • Line nine descends into the sub directory that the user chose.
   • Line 11 shows how to display a simple error dialog box to the user.

4. Now we need to create the fillWithRoot() method. It does the same thing as the last line of our onCreate() method.

   
             private void fillWithRoot() {
                 fill(new File("/").listFiles());
             }
   

5. Don't forget to replace the fill() method in the onCreate() method with fillWithRoot().

The complete source code file for this project, ready for importing into Eclipse, is available for download, here (32K Zip archive)