How to read/write to a file that can be accessed by a user not being root?

I would like to write to a file that I can access from the file system without being root.

This is my attempt:

FileOutputStream fos = null;
final String FILE_NAME = "test.txt";

fos = openFileOutput(FILE_NAME, MODE_PRIVATE);
fos.write("test".getBytes());

// Display path of file written to
Toast.makeText(this, "Saved to" + getFilesDir() + "/" + FILE_NAME, Toast.LENGTH_LONG).show();

Which writes to

/data/user/0/com.example.PROJECT_NAME/files/test.txt

which is not accessible without being root.

It would be good if there is a possibility to specify a different, absolute path which I know I can access, such as /data/data/....

My device is a Google Pixel C which unfortunately has no external SD-Card slot to write to.

After I figured out that the possibilities of accessing external storage differ quite a lot in different android versions, I chose to go with the Storage Access Framework (SAF). The SAF is an API (since API level 19), that offers the user a UI to browse through files.

Using an Intent, an UI pops up which lets the user create a file or choose an existing one:

private static final int CREATE_REQUEST_CODE = 40;
private Uri mFileLocation = null;

Intent intent = new Intent(Intent.ACTION_CREATE_DOCUMENT);

intent.addCategory(Intent.CATEGORY_OPENABLE);
intent.setType("text/plain"); // specify file type
intent.putExtra(Intent.EXTRA_TITLE, "newfile.txt"); // default name for file

startActivityForResult(intent, CREATE_REQUEST_CODE);

After the user chose a file onActivityResult(...) gets called. Now it is possible to get the URI of the file by calling resultData.getData();

public void onActivityResult(int requestCode, int resultCode, Intent resultData) {

    if (resultCode == Activity.RESULT_OK)
    {
        if (requestCode == CREATE_REQUEST_CODE)
        {
            if (resultData != null) {
                mFileLocation = resultData.getData();
            }
        } 
    }
}

Now use this URI to write to the file:

private void writeFileContent(Uri uri, String contentToWrite)
{
    try
    {
        ParcelFileDescriptor pfd = this.getContentResolver().openFileDescriptor(uri, "w"); // or 'wa' to append

        FileOutputStream fileOutputStream = new FileOutputStream(pfd.getFileDescriptor());
        fileOutputStream.write(contentToWrite.getBytes());

        fileOutputStream.close();
        pfd.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}