I have a large text file that is in the assets folder, and I have a button that reads the file's next line successfully. However, I want to read the previous line in case the user clicks another button.
Reading the whole file to memory is not an option. The file lines are not numbered.
InputStream is = getResources().getAssets().open("abc.txt"); String result= convertStreamToString(is); public static String convertStreamToString(InputStream is) throws IOException { Writer writer = new StringWriter(); char[] buffer = new char[2048]; try { Reader reader = new BufferedReader(new InputStreamReader(is, "UTF-8")); int n; while ((n = reader.read(buffer)) != -1) { writer.write(buffer, 0, n); } } finally { is.close(); } String text = writer.toString(); return text; } 
StringBuilder than the StringWriter used in the sample above, but otherwise it should work fine.
If you only need to keep track of one previous line, you can do something like the following, keeping track of the previous line through each iteration (I assumed you were using a reader; for this example, BufferedReader):
String previous = null, line; // null means no previous line while (line = yourReader.readLine()) { // Do whatever with line // If you need the previous line, use: if (yourCondition) { if (previous != null) { // Do whatever with previous } else { // No previous line } } previous = line; } If you need to keep track of more than one previous line, you may have to expand that into an array, but you will be keeping a huge amount in memory if your file is large--as much as if you'd read the entire file, once you get to the last line.
There is no simple way in Java or Android to read the previous line, only the next (as it is easier in file I/O to more forward than backward).
One alternative I can think of is to keep a line marker (starting at 0), and as you advance through the lines, increase it. Then, to go backwards, you have to read the file line by line again, until you get to that line minus one. If you need to go backwards, go to that new line minus one, and so on. It would be a heavy operation, most likely, but would suit your needs.
Edit: If nothing above will work, there is also a method to read in a file backwards, in which you may be able to use to find the previous line by iterating forward. Just an alternative idea, but definitely not an easy one to implement.
public class LoadFromAltLoc extends Activity { //a handle to the application's resources private Resources resources; //a string to output the contents of the files to LogCat private String output; /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); //get the application's resources resources = getResources(); try { //Load the file from the raw folder - don't forget to OMIT the extension output = LoadFile("from_raw_folder", true); //output to LogCat Log.i("test", output); } catch (IOException e) { //display an error toast message Toast toast = Toast.makeText(this, "File: not found!", Toast.LENGTH_LONG); toast.show(); } try { //Load the file from assets folder - don't forget to INCLUDE the extension output = LoadFile("from_assets_folder.pdf", false); //output to LogCat Log.i("test", output); } catch (IOException e) { //display an error toast message Toast toast = Toast.makeText(this, "File: not found!", Toast.LENGTH_LONG); toast.show(); } } //load file from apps res/raw folder or Assets folder public String LoadFile(String fileName, boolean loadFromRawFolder) throws IOException { //Create a InputStream to read the file into InputStream iS; if (loadFromRawFolder) { //get the resource id from the file name int rID = resources.getIdentifier("fortyonepost.com.lfas:raw/"+fileName, null, null); //get the file as a stream iS = resources.openRawResource(rID); } else { //get the file as a stream iS = resources.getAssets().open(fileName); } //create a buffer that has the same size as the InputStream byte[] buffer = new byte[iS.available()]; //read the text file as a stream, into the buffer iS.read(buffer); //create a output stream to write the buffer into ByteArrayOutputStream oS = new ByteArrayOutputStream(); //write this buffer to the output stream oS.write(buffer); //Close the Input and Output streams oS.close(); iS.close(); //return the output stream as a String return oS.toString(); } } 
