assetsフォルダーに格納したテキストファイルを読み込みます。

InputStream inputStream = null;
BufferedReader reader = null;
try {
    inputStream = getResources().getAssets().open("ファイル名");
    reader = new BufferedReader(
              new InputStreamReader(inputStream));
    String line;
    StringBuilder sb = new StringBuilder();
    while ((line = reader.readLine()) != null) {
        Log.v("reader", line);
        if(line != ""){
            sb.append(line);
        }
    }
    String str = sb.toString();
    Toast.makeText(this, str, Toast.LENGTH_LONG).show();

} catch (IOException e) {
    e.printStackTrace();
} catch (SQLException e) {
    e.printStackTrace();
} catch (Exception e) {
    e.printStackTrace();
} finally {
    try {
        reader.close();
        inputStream.close();
    } catch (IOException e) {
        e.printStackTrace();
    }

}