Google Drive API - Android - How to obtain a drive file id?

You could use the DriveAPI Query method, to retrieve any information about an specific file. you will need to define a query object as the following:

Query query = new Query.Builder()
    .addFilter(Filters.eq(SearchableField.TITLE, "HelloWorld.java"))
    .build();

And set a callback function to iterate on the results:

Drive.DriveApi.query(googleApiClient, query)
        .setResultCallback(new OnChildrenRetrievedCallback() {

    @Override
    public void onChildrenRetrieved(MetadataBufferResult result) {
        // Iterate over the matching Metadata instances in mdResultSet
    }
});

You can find more information on the topic here: https://developers.google.com/drive/android/queries

The solution i found for this problem was creating the file from the app. Using the class ("CreateFileActivity.java") from google drive api demo app.

With this class i save the returning Driveid from the new file in a global DriveId variable.

final private ResultCallback<DriveFolder.DriveFileResult> fileCallback = new
        ResultCallback<DriveFolder.DriveFileResult>() {
            @Override
            public void onResult(DriveFolder.DriveFileResult result) {
                if (!result.getStatus().isSuccess()) {
                    Log.e("","Error while trying to create the file");
                    return;
                }
                Id=result.getDriveFile().getDriveId();
                Log.e("","Created a file with content: " + Id);

            }
        };

Then with this id in another method i call the file and read it (If i want i can edit this file information from Google Drive Web App):

 public void leer(){
       DriveFile file = Drive.DriveApi.getFile(getGoogleApiClient(),Id);
       file.open(mGoogleApiClient, DriveFile.MODE_READ_ONLY, null)
               .setResultCallback(contentsOpenedCallback);
   }
ResultCallback<DriveApi.DriveContentsResult> contentsOpenedCallback =
        new ResultCallback<DriveApi.DriveContentsResult>() {
            @Override
            public void onResult(DriveApi.DriveContentsResult result) {
                if (!result.getStatus().isSuccess()) {
                    Log.e("Error:","No se puede abrir el archivo o no se encuentra");
                    return;
                }
                // DriveContents object contains pointers
                // to the actual byte stream
                DriveContents contents = result.getDriveContents();
                BufferedReader reader = new BufferedReader(new InputStreamReader(contents.getInputStream()));
                StringBuilder builder = new StringBuilder();
                String line;
                try {
                    while ((line = reader.readLine()) != null) {
                        builder.append(line);
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
                String contentsAsString = builder.toString();
                Log.e("RESULT:",contentsAsString);
            }
        };