Read an integer from a binary file

Question

I need to read an integer from a non-text file in Java. Here is the equivalent code in C

unsigned int MagicNumber;
int fd = open("MyFile", O_RDONLY);
int rc = read(fd, &MagicNumber, 4);

What is the equivalent in Java?

This does NOT work for me (it produces different results than the C code):

FileInputStream fin = new FileInputStream("MyFile");
DataInputStream din = new DataInputStream(fin);
int MagicNumber = din.readInt();

https://docs.oracle.com/javase/7/docs/api/java/io/DataInputStream.html#readInt() — Konrad Rudolph, May 09 '20 at 19:24
I know if I read enough about Java then I can figure this out myself. But I did not find my answer in the links you listed. — JB_User, May 09 '20 at 19:30
False alarm. I though I had figured it out, but it's still wrong. — JB_User, May 09 '20 at 19:32
It looks like Java is flipping the byte order as compared to C. I think I should ask a new question. — JB_User, May 09 '20 at 19:48

score 0 · Answer 1 · answered May 09 '20 at 21:08

Java uses a machine independent representation of the interger as a byte sequence, if you write it with DataOutputStream and next try to read it with DataInputStream on a different machine even with different endian, it will read it correctly anyway. Java was designed exactly for that: i.e. to be machine independent.

Of course you cannot assume the byte sequence will be the same in the file you generated in C.

It happens that Java defaults to big-endian (see the DataOutput specs). If you do your test on any big-endian machine it will work, on a little-endian (like x86) it will fail.

You can try to use LittleEndianDataOutputStream to solve this.

Read an integer from a binary file

1 Answers1