explore external storage and list all .txt files in all directories

FileExplorer.java

import android.os.Bundle;
import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

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

public class FileExplorer extends AppCompatActivity {

    private TextView tvPath;
    private ListView listView;

    private List<FileData> allFiles;
    private List<FileData> dirFiles;
    private Stack<File> fileStack;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_file_explorer);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        initViews();

        fileStack = new Stack<>();
        fileStack.push(new File(Environment.getExternalStorageDirectory(), ""));
        dirFiles = new ArrayList<>();
        allFiles = getDirFiles();

        FileAdapter adapter = new FileAdapter(this, allFiles);
        listView.setAdapter(adapter);

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
                String fName = allFiles.get(i).getfName();
                File file = new File(fileStack.peek(), fName);

                if (file.isFile()) {
                    Toast.makeText(getApplicationContext(), "this is a txt file", Toast.LENGTH_LONG).show();
                }else {
                    fileStack.push(new File(fileStack.peek(), fName));
                    populateListView();
                }
            }
        });
    }

    private List<FileData> getDirFiles() {
        dirFiles.clear();
        File dir = new File(fileStack.peek(), "");

        for (File file : dir.listFiles()) {
            if ((file.isFile() && file.getName().endsWith(".txt")) || file.isDirectory()) {
                dirFiles.add(new FileData(file.getName(), file.isDirectory()));
            }
        }
        tvPath.setText(dir.toString());
        return dirFiles;
    }

    private void populateListView() {
        allFiles = getDirFiles();
        listView.setAdapter(new FileAdapter(FileExplorer.this, allFiles));
    }

    public void clickEvent(View view) {

        switch (view.getId()) {
            case R.id.iv_prev_dir:
                if (fileStack.size() > 1) {
                    fileStack.pop();
                    populateListView();
                }
                break;
            /*  handle additional click events  */
        }
    }

    private void initViews() {
        listView = findViewById(R.id.lv_files);
        tvPath = findViewById(R.id.tv_path);
    }
}