Question

In response to my answer to a file-reading question, a commenter stated that FileInputStream.read(byte[]) is “not guaranteed to fill the buffer.”

File file = /* ... */  
long len = file.length();
byte[] buffer = new byte[(int)len];
FileInputStream in = new FileInputStream(file);
in.read(buffer);

(The code assumes that the file length does not exceed 2GB)

Apart from an IOException, what could cause the read method to not retrieve the entire file contents?

EDIT:

The idea of the code (and the goal of the OP of the question I answered) is to read the entire file into a chunk of memory in one swoop, that’s why buffer_size = file_size.

Answer 1

Apart from an IOException, what could cause the read method to not retrieve the entire file contents?

In my own API implementation, and on my home rolled file-system I simply choose to fill half the buffer…… just kidding.

My point is that even if I wasn’t kidding, technically speaking it wouldn’t be a bug. It is a matter of method contract. This is the contract (documentation) in this case is:

Reads up to b.length bytes of data from this input stream into an array of bytes.

i.e., it gives no guarantees for filling the buffer.

Depending on the API implementation, and perhaps on the file-system the read method may choose not to fill the buffer. It’s basically a question of what the contract of the method says.

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Bottom line: It probably works, but is not guaranteed to work.