This is a simple file chooser class for android

FileChooser.java

import android.app.Activity;
import android.app.Dialog;
import android.os.Environment;
import android.view.View;
import android.view.ViewGroup;
import android.view.WindowManager.LayoutParams;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import java.io.File;
import java.io.FileFilter;
import java.util.Arrays;

public class FileChooser {
    private static final String PARENT_DIR = "..";

    private final Activity activity;
    private ListView list;
    private Dialog dialog;
    private File currentPath;

    // filter on file extension
    private String extension = null;
    public void setExtension(String extension) {
        this.extension = (extension == null) ? null :
                extension.toLowerCase();
    }

    // file selection event handling
    public interface FileSelectedListener {
        void fileSelected(File file);
    }
    public FileChooser setFileListener(FileSelectedListener fileListener) {
        this.fileListener = fileListener;
        return this;
    }
    private FileSelectedListener fileListener;

    public FileChooser(Activity activity) {
        this.activity = activity;
        dialog = new Dialog(activity);
        list = new ListView(activity);
        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override public void onItemClick(AdapterView<?> parent, View view, int which, long id) {
                String fileChosen = (String) list.getItemAtPosition(which);
                File chosenFile = getChosenFile(fileChosen);
                if (chosenFile.isDirectory()) {
                    refresh(chosenFile);
                } else {
                    if (fileListener != null) {
                        fileListener.fileSelected(chosenFile);
                    }
                    dialog.dismiss();
                }
            }
        });
        dialog.setContentView(list);
        dialog.getWindow().setLayout(LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT);
        refresh(Environment.getExternalStorageDirectory());
    }

    public void showDialog() {
        dialog.show();
    }


    /**
     * Sort, filter and display the files for the given path.
     */
    private void refresh(File path) {
        this.currentPath = path;
        if (path.exists()) {
            File[] dirs = path.listFiles(new FileFilter() {
                @Override public boolean accept(File file) {
                    return (file.isDirectory() && file.canRead());
                }
            });
            File[] files = path.listFiles(new FileFilter() {
                @Override public boolean accept(File file) {
                    if (!file.isDirectory()) {
                        if (!file.canRead()) {
                            return false;
                        } else if (extension == null) {
                            return true;
                        } else {
                            return file.getName().toLowerCase().endsWith(extension);
                        }
                    } else {
                        return false;
                    }
                }
            });

            // convert to an array
            int i = 0;
            String[] fileList;
            if (path.getParentFile() == null) {
                fileList = new String[dirs.length + files.length];
            } else {
                fileList = new String[dirs.length + files.length + 1];
                fileList[i++] = PARENT_DIR;
            }
            Arrays.sort(dirs);
            Arrays.sort(files);
            for (File dir : dirs) { fileList[i++] = dir.getName(); }
            for (File file : files ) { fileList[i++] = file.getName(); }

            // refresh the user interface
            dialog.setTitle(currentPath.getPath());
            list.setAdapter(new ArrayAdapter(activity,
                    android.R.layout.simple_list_item_1, fileList) {
                @Override public View getView(int pos, View view, ViewGroup parent) {
                    view = super.getView(pos, view, parent);
                    ((TextView) view).setSingleLine(true);
                    return view;
                }
            });
        }
    }


    /**
     * Convert a relative filename into an actual File object.
     */
    private File getChosenFile(String fileChosen) {
        if (fileChosen.equals(PARENT_DIR)) {
            return currentPath.getParentFile();
        } else {
            return new File(currentPath, fileChosen);
        }
    }
}

MainActivity.java

//...ignoring other code.../

/*Call this method whenever you want to show the file chooser*/

/**
     * Tweak method from website*/
    private void processFile(){
        FileChooser fileChooser = new FileChooser(MainActivity.this);
       
        fileChooser.setFileListener(new FileChooser.FileSelectedListener() {
            @Override
            public void fileSelected(final File file) {
                // ....do something with the file
                String filename = file.getAbsolutePath();
                Log.i("File Name", filename);
                // then actually do something in another module

            }
        });
// Set up and filter my extension I am looking for
        //fileChooser.setExtension("pdf");
        fileChooser.showDialog();
    }

/*According to documentation from Roger Keays*/
/*new FileChooser(activity).setFileListener(new FileSelectedListener() {
    @Override public void fileSelected(final File file) {
        // do something with the file
    }).showDialog();*/

I need help with another problem. When I am at the highest level, ie the root directory of external storage and go to ".." (PARENT_DIR), I get an exception because the app is trying to go up a directory, and it probably can not.
I would like to close the dialogue (dialog.dismiss ()) when this happens, does anyone have any idea where to intercept the exception?

        at de.example.myapp.FileChooser.refresh(FileChooser.java:103)
        at de.example.myapp.FileChooser$1.onItemClick(FileChooser.java:51)
        at android.widget.AdapterView.performItemClick(AdapterView.java:318)
        at android.widget.AbsListView.performItemClick(AbsListView.java:1159)```

EDIT - SOLVED IT: 
```    public FileChooser(Activity activity) {
        this.activity = activity;
        dialog = new Dialog(activity);
        list = new ListView(activity);
        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override public void onItemClick(AdapterView<?> parent, View view, int which, long id) {
                String fileChosen = (String) list.getItemAtPosition(which);
                if (fileChosen.equalsIgnoreCase(PARENT_DIR) && currentPath.getAbsolutePath().equalsIgnoreCase(startPath.getAbsolutePath())) {
                    dialog.dismiss();
                } else {
                    File chosenFile = getChosenFile(fileChosen);
                    if (chosenFile.isDirectory()) {
                        refresh(chosenFile);
                    } else {
                        if (fileListener != null) {
                            fileListener.fileSelected(chosenFile);
                        }
                        dialog.dismiss();
                    }
                }
            }
        });
        dialog.setContentView(list);
        dialog.getWindow().setLayout(LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT);
        startPath = Environment.getExternalStorageDirectory();
        refresh(startPath);
    }```

The variable "startPath" is defined in FileChooser.java and is only set once you create the dialog as shown above.

        this.activity = activity;
        dialog = new Dialog(activity);
        list = new ListView(activity);
        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override public void onItemClick(AdapterView<?> parent, View view, int which, long id) {
                String fileChosen = (String) list.getItemAtPosition(which);
                if (fileChosen.equalsIgnoreCase(PARENT_DIR) && currentPath.getAbsolutePath().equalsIgnoreCase(startPath.getAbsolutePath())) {
                    dialog.dismiss();
return;
                } else {
                    File chosenFile = getChosenFile(fileChosen);
                    if (chosenFile.isDirectory()) {
                        refresh(chosenFile);
                    } else {
                        if (fileListener != null) {
                            fileListener.fileSelected(chosenFile);
                        }
                        dialog.dismiss();
                    }
                }
            }
        });
        dialog.setContentView(list);
        dialog.getWindow().setLayout(LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT);
        startPath = Environment.getExternalStorageDirectory();
        refresh(startPath);
    }```

Environment.getExternalStorageDirectory() is deprecated. Does anybody know how to access the root directory now?