public class WelcomeActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_welcome);

        String folderB = "bookfiles_"+1;

        File bookfolder = new File(Environment.getExternalStorageDirectory(), folderB);

        if(!bookfolder.exists()){
            bookfolder.mkdir();
            Log.d("my", "dir. created");
        }
        else {
            Log.d("my", "dir. already exists");
        }

    }
}

11-22 22:55:11.929 16566-16566/? E/Zygote: v2
11-22 22:55:11.930 16566-16566/? E/Zygote: accessInfo : 0