I´d like to use some OCR algorithm (Image to Text) into an Android app (Java). I have previously used several algorithms but for a PC (Matlab) app. Which library or algorithm should I use? Computational cost and response time are important issues.
As far as i know there are no native opensource Java OCR SDKs. There are Java APIs which wrap calls for native interfaces, for example, for one of the most popular opensource OCR engines - Tesseract (http://groups.google.com/group/tesseract-ocr/) - there are some Java wrappers like tesjeract (http://code.google.com/p/tesjeract/) or Tess4J (http://tess4j.sf.net/). That could work for you, but it's rather hard to set up and will require developing image-preprocessing and font training on your side.
One more solution could be a cloud service. It requires end-user application to have the internet connection, but it's independent from your programming language choice and resources limitations (which is importatnt on mobile devices, OCR proccess consumes rather big amount of recources). Have a look at www.ocrsdk.com for android ocr, it's a cloud-based OCR SDK that let you upload an image through web API and returns you the OCRed data.
This Web API based OCR SDK is not free, which may not be suitable for you, but i still recommend you try it out (it has a free 90 days trial without any upfront charges) as its pricing is really affordable in comparison with enterprise solutions while it provides enterprise-level OCR accuracy which is way better than open source.