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();*/

Came across this when in need and I cannot thank you enough! 💯

Attempt to get length of null array in the Filechooser

Thanks a lot, exactly what I was looking for.
Only thing I needed to change was
public FileChooser(Activity activity)
into
public FileChooser(Activity activity, String extension, String path)
because I wanted to look only into a specific folder and see only files with specific extension in the first list view.

Thanks you, itś great. I had a problem: when the dialog was displayed, it did not filter the files based on the file extension. For this, only the method "Refresh (File Path)" had to run, so for example by changing to another folder.

I solved that as follows:
I duplicated the method "refresh (File path)" to a seconds method without variable only using "currentpath" for refreshing. I call this method "(fileChooser.refresh()" directly after "fileChooser.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?