A long time ago:
Call any of these
byte[] org.apache.commons.io.FileUtils.readFileToByteArray(File file)
byte[] org.apache.commons.io.IOUtils.toByteArray(InputStream input)
From
http://commons.apache.org/io/
If the library footprint is too big for your Android app, you can just use relevant classes from the commons-io library
Today (Java 7+ or Android API Level 26+)
Luckily, we now have a couple of convenience methods in the nio packages. For instance:
byte[] java.nio.file.Files.readAllBytes(Path path)
Javadoc here