How to read one stream into another?

Write one method to do this, and call it from everywhere which needs the functionality. Guava already has code for this, in ByteStreams.copy. I'm sure just about any other library with "general" IO functionality has it too, but Guava's my first "go-to" library where possible. It rocks :)

Java 9 (and later) answer (docs):

in.transferTo(out);

Seems they finally realized that this functionality is so commonly needed that it’d better be built in. The method returns the number of bytes copied in case you need to know.

In Apache Commons / IO, you can do it using IOUtils.copy(in, out):

InputStream in = new FileInputStream(myFile);
OutputStream out = new ByteArrayOutputStream();
IOUtils.copy(in, out);

But I agree with Jon Skeet, I'd rather use Guava's ByteStreams.copy(in, out)