Searching specific file types from an emulator's storage [android studio]

//MainActivity.java
//Hosts the app's fragments and handles communication between them
//I'm skipping the phone parts of the app since my revised one is for tablets only

package com.iekproject.siegfried.libraryapp;

import android.content.Intent;
import android.graphics.Color;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.support.design.widget.FloatingActionButton;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.LinearLayout;
import android.widget.TextView;

import java.io.File;
import java.util.ArrayList;



public class MainActivity extends AppCompatActivity
    implements LibraryFragment.LibraryFragmentListener, DetailFragment.DetailFragmentListener,
        AddEditFragment.AddEditFragmentListener {

    private static final String AUTHORITY = "com.iekproject.siegfried.libraryapp";

    //key for storing a book's Uri in a Bundle passed to a fragment
    public static final String BOOK_URI = "book_uri";

    private LibraryFragment libraryFragment; //displays library aka book list

    private File root;
    private ArrayList<File> fileList = new ArrayList<File>();
    private LinearLayout view;

    FloatingActionButton btn;
    int PICKFILE_RESULT_CODE=1;

    //displays LibraryFragment when MainActivity first loads
   @Override
    protected void onCreate(Bundle savedInstanceState) {

       super.onCreate(savedInstanceState);
       //Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
       //setSupportActionBar(toolbar);
       setContentView(R.layout.activity_main);

        //if layout contains fragmentContainer, the phone layout is in use. Create and display
        //a LibraryFragment
        if (savedInstanceState == null && findViewById(R.id.fragmentContainer) != null) {
            //create LibraryFragment
            libraryFragment = new LibraryFragment();

            //add the fragment to the FrameLayout
            FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
            transaction.add(R.id.fragmentContainer, libraryFragment);
            transaction.commit(); //displays LibraryFragment
        }
        else {
            libraryFragment =
                    (LibraryFragment) getSupportFragmentManager().
                            findFragmentById(R.id.DetailFragment);
        }


       //view = (LinearLayout) findViewById(R.id.DetailFragment);

       //getting SDcard root path
       root = new File(Environment.getExternalStorageDirectory()
               .getAbsolutePath());
       getfile(root);

       for (int i = 0; i < fileList.size(); i++) {
           TextView textView = new TextView(this);
           textView.setText(fileList.get(i).getName());
           textView.setPadding(5, 5, 5, 5);

           System.out.println(fileList.get(i).getName());

           if (fileList.get(i).isDirectory()) {
               textView.setTextColor(Color.parseColor("#FF0000"));
           }
           view.addView(textView);
       }

       btn = (FloatingActionButton) findViewById(R.id.addButton);
       btn.setOnClickListener(new View.OnClickListener() {
           @Override
           public void onClick(View v) {
               Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
               intent.setType("file/*");
               startActivityForResult(intent,PICKFILE_RESULT_CODE);
           }
       });

   }

    public ArrayList<File> getfile(File dir) {
        File listFile[] = dir.listFiles();
        if (listFile != null && listFile.length > 0) {
            for (int i = 0; i < listFile.length; i++) {

                if (listFile[i].isDirectory()) {
                    fileList.add(listFile[i]);
                    getfile(listFile[i]);

                }
                else {
                    if (listFile[i].getName().endsWith(".mobi")
                            || listFile[i].getName().endsWith(".epub")) {
                        fileList.add(listFile[i]);
                    }
                }

            }
        }
    return fileList;
    }


        @Override
            protected void onActivityResult(int requestCode, int resultCode, Intent data) {
            super.onActivityResult(requestCode, resultCode, data);
            if(resultCode==PICKFILE_RESULT_CODE){
                Log.d("TAG", "File Uri " +data.getData());
            }
        }


   public boolean isExternalStorageReadable() {
        String state = Environment.getExternalStorageState();
        if (Environment.MEDIA_MOUNTED.equals(state) ||
                Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
            return true;
        }
        return false;
   }

    //displays DetailFragment for selected book
    @Override
    public void onBookSelected(Uri bookUri) {
            getSupportFragmentManager().popBackStack();
            displayBook(bookUri, R.id.rightPaneContainer);
    }

    //displays AddEditFragment to add a new book. Possibly what I'll also have to change to make it
    //scan/update the book list
    @Override
    public void onAddBook() {
            displayAddEditFragment(R.id.rightPaneContainer, null);
    }

    //displays a book
    private void displayBook(Uri bookUri, int viewID) {
        DetailFragment detailFragment = new DetailFragment();

        //specify book's Uri as an argument to the DetailFragment
        Bundle arguments = new Bundle();
        arguments.putParcelable(BOOK_URI, bookUri);
        detailFragment.setArguments(arguments);

        //use a FragmentTransaction to display the DetailFragment
        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        transaction.replace(viewID, detailFragment);
        transaction.addToBackStack(null);
        transaction.commit(); //causes DetailFragment to display
    }

    //displays fragment for adding new or editing existing book
    private void displayAddEditFragment(int viewID, Uri bookUri) {
        AddEditFragment addEditFragment = new AddEditFragment();

        //if editing existing book, provide bookUri as an argument
        if (bookUri != null) {
            Bundle arguments = new Bundle();
            arguments.putParcelable(BOOK_URI, bookUri);
            addEditFragment.setArguments(arguments);
        }

        //use a FragmentTransaction to display the AddEditFragment
        FragmentTransaction transaction =
                getSupportFragmentManager().beginTransaction();
        transaction.replace(viewID, addEditFragment);
        transaction.addToBackStack(null);
        transaction.commit(); //causes AddEditFragment to display
    }

    //return to book list when displayed book deleted
    @Override
    public void onBookDeleted() {
        //removes top of back stack
        getSupportFragmentManager().popBackStack();
        libraryFragment.updateLibrary(); //refresh book list
    }

    //displays the AddEditFragment to edit an existing book. Maybe it can be used as Move or sth
   /*@Override
    public void onEditBook(Uri bookUri) {
        displayAddEditFragment(R.id.rightPaneContainer, bookUri);
    }*/

    //update GUI after the new book or updated book saved
    @Override
    public void onAddEditCompleted(Uri bookUri) {
        //removes top of back stack
        getSupportFragmentManager().popBackStack();
        libraryFragment.updateLibrary(); //refresh book list

        if (findViewById(R.id.fragmentContainer) == null){ //tablet
            //removes top of back stack
            getSupportFragmentManager().popBackStack();

            //on tablet, displays the book that was just added or edited
            displayBook(bookUri, R.id.rightPaneContainer);
        }
    }
}