Read a byte array from file with Java

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Hi everybody,
I have an assignment in Java, one of its part is read a byte array from file, start at a specified offset and have a given length.
My solution is using method

Java

read()

of class

Java

FileInputStream

First, I read all file as an array. Then I copy the sub-array, from specified offset with given length to new array and return the new array.

But, because the memory for a Java application is small, my solution does not work with a 200 MB file.

How c an I read a part of file that I want without reading the whole file?

Thanks for reading my question.

Posted 29-Oct-11 17:22pm

Duc Huy Nguyen

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Richard MacCutchan 30-Oct-11 4:56am

Why are you reading the entire file when your instructions specify you to read a fixed length amount from a certain offset? Reading 200Mb in the way you are doing it is not a sensible option.

Sergey Alexandrovich Kryukov 30-Oct-11 19:56pm

Exactly; as I say, readFully reads array of bytes of required length, from required offset, see my answer.
--SA

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Sergey Alexandrovich Kryukov · Answer 1 · 2011-10-29T17:44:00

See: http://download.oracle.com/javase/1.4.2/docs/api/java/io/RandomAccessFile.html#readFully%28byte[],%20int,%20int%29[^], the class is java.io.RandomAccessFile.

[EDIT]

As Richard expressed concern about your familiarity with the concept (or just the terminology) of random access, you can see these articles:
http://en.wikipedia.org/wiki/Random_access[^],
http://en.wikipedia.org/wiki/Random-access_memory[^].

Files also allow random access to any record. Even in the case of disk or even tape memory it's always possible to relocate the reading head to any position. For such devices, this can be very expensive. So, using buffered stream will highly facilitate random-access operations.

—SA