Simple-File-Manager/app/src/main/java/com/simplemobiletools/filemanager/activities/MainActivity.java

package com.simplemobiletools.filemanager.activities;

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.os.Environment;
import android.support.v4.app.ActivityCompat;
import android.support.v4.app.FragmentManager;
import android.support.v7.app.AppCompatActivity;

import com.simplemobiletools.filemanager.Constants;
import com.simplemobiletools.filemanager.R;
import com.simplemobiletools.filemanager.Utils;
import com.simplemobiletools.filemanager.fragments.ItemsFragment;

import butterknife.ButterKnife;

public class MainActivity extends AppCompatActivity implements ItemsFragment.ItemInteractionListener {
    private static final int STORAGE_PERMISSION = 1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);
    }

    @Override
    protected void onResume() {
        super.onResume();
        tryInitFileManager();
    }

    private void tryInitFileManager() {
        if (Utils.hasStoragePermission(getApplicationContext())) {
            initRootFileManager();
        } else {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, STORAGE_PERMISSION);
        }
    }

    private void initRootFileManager() {
        final String path = Environment.getExternalStorageDirectory().toString();
        openPath(path);
    }

    private void openPath(String path) {
        final Bundle bundle = new Bundle();
        bundle.putString(Constants.PATH, path);

        final ItemsFragment fragment = new ItemsFragment();
        fragment.setArguments(bundle);
        fragment.setListener(this);
        getSupportFragmentManager().beginTransaction().replace(R.id.fragment_holder, fragment).addToBackStack("").commit();
    }

    @Override
    public void onBackPressed() {
        final FragmentManager manager = getSupportFragmentManager();
        if (manager.getBackStackEntryCount() == 1)
            finish();
        else {
            super.onBackPressed();
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);

        if (requestCode == STORAGE_PERMISSION) {
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                initRootFileManager();
            } else {
                Utils.showToast(getApplicationContext(), R.string.no_permissions);
                finish();
            }
        }
    }

    @Override
    public void itemClicked(String path) {
        openPath(path);
    }
}