How can I identify Arabic letters in an image using Java?
You have to do following methods:

First step: Number plate extraction

You can use OpenCV for this purpose. It has Java api. Also it is ported to Android.

Second step : OCR 

OCR is used to recognise the letters on notice board.

You can train your own OCR for arabic letters and digits using OpenCV.

Or you can use tesseract-OCR. I think it supports arabic also.



Following paper will be useful for you: Paper 1 

Also, this site includes complete code for number plate extraction, but in C++. But it will be a good start.