Java Convert 4 bytes to int

Question

i was wondering if the solution for this documented here is still the solution or is there any other way getting an int from 4 bytes?

thank you.

EDIT: im getting the byte[] from sockets .read

EDIT: int recvMsgSize = in.read(Data, 0, BufferSize); if recvMsgSize is -1 i know the connection has been dropped.

how do i detect this when im using DataInputStream instead of InputStream?

thanks.

EDIT: apologies for being a yoyo regarding accepting the right answer. but after mihi's updated final response, it would appear that the method is solid and cuts down extended coding and in my opinion best practice.

when you get them from the socket input stream, I'd wrap that one into a DataInputStream and use it for reading all kinds of data. Especially since `InputStream.read(byte[])` will not guarantee to read the whole byte array... DataInputStream.readFully does. — mihi, May 15 '10 at 14:10
possible duplicate of [Convert 4 bytes to int ](http://stackoverflow.com/questions/2383265/convert-4-bytes-to-int) — finnw, Jan 22 '11 at 16:34

score 22 · Answer 1 · edited May 23 '17 at 12:18

22

You have to be very careful with any widening conversion and numeric promotion, but the code below converts 4 byte into int:

    byte b1 = -1;
    byte b2 = -2;
    byte b3 = -3;
    byte b4 = -4;
    int i = ((0xFF & b1) << 24) | ((0xFF & b2) << 16) |
            ((0xFF & b3) << 8) | (0xFF & b4);
    System.out.println(Integer.toHexString(i)); // prints "fffefdfc"

1

while it looks like it should work, are we sure it doesn't screw up when there's a signed bit around? – o0'. May 15 '10 at 13:19
2

i think you misunderstood. i would like 4 bytes to int. not int to 4 bytes. – iTEgg May 15 '10 at 13:19
1

@ikurtz: fixed. Sorry for misunderstanding. – polygenelubricants May 15 '10 at 13:22
2

@Lo'oris: feel free to check the latest revision. – polygenelubricants May 15 '10 at 13:28
And exactly that discussion is the reason you should better use the pre-existing methods in ByteBuffer or DataInput :-) They **do** work. – mihi May 15 '10 at 14:09
I think b1 doesnt need to be & 0xFF for this to work – Lunkle Aug 22 '21 at 19:35
this works, you may notice: big endian vs little endian – dotrinh PM Jun 16 '22 at 05:25

Cowan · Answer 2 · 2010-05-15T22:23:35.073

22

If you have them already in a byte[] array, you can use:

int result = ByteBuffer.wrap(bytes).getInt();

or, if you have Google's guava-libraries on your classpath, you have the shortcut:

int result = Ints.fromByteArray(array);

which has the advantage that you have similarly nice APIs for other types (Longs.fromByteArray, Shorts.fromByteArray, etc).

edited May 15 '10 at 22:23

answered May 15 '10 at 13:51

Cowan

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score 8 · Accepted Answer · edited Dec 12 '13 at 12:37

Depending on where you get those 4 bytes from:

http://docs.oracle.com/javase/7/docs/api/java/io/DataInput.html#readInt()

http://docs.oracle.com/javase/7/docs/api/java/nio/ByteBuffer.html#getInt(int)

You can of course still do it manually, but in most cases using one of those (if you have to convert a byte array with lots of bytes, you might want to use a DataInputStream around a ByteArrayInputStream for example) is easier.

Edit: If you need to change the endianness, you will have to use a ByteBuffer, or reverse the bytes yourself, or do the conversion yourself, as DataInput does not support changing the endianness.

Edit2: When you get them from the socket input stream, I'd wrap that one into a DataInputStream and use it for reading all kinds of data. Especially since InputStream.read(byte[]) will not guarantee to fill the whole byte array... DataInputStream.readFully does.

DataInputStream in = new DataInputStream(socket.getInputStream());
byte aByte = in.readByte();
int anInt = in.readInt();
int anotherInt = in.readInt();
short andAShort = in.readShort(); // 11 bytes read :-)
byte[] lotOfBytes = new byte[anInt];
in.readFully(lotOfBytes);

Edit3: When reading multiple times from a stream, they will continue reading where you stopped, i. e. aByte will be byte 0, anInt will be bytes 1 to 4, anotherInt will be bytes 5 to 8, etc. readFully will read on after all that and will block until it has read lotOfbytes.

When the stream stops (the connection drops) you will get EOFException instead of -1, so if you get -1, the int really was -1.

If you do not want to parse any bytes at all, you can skip() them. Parsing one byte in 2 different ways is not possible with DataInputStream (i. e. read first an int from byte 0 to 3, then one from byte 2 to 5), but usually not needed either.

Example:

// read messages (length + data) until the stream ends:
while (true) {
int messageLength;
try {
    messageLength = in.readInt(); // bytes 0 to 3
} catch (EOFException ex) {
    // connection dropped, so handle it, for example
    return;
}
byte[] message = new byte[messageLength];
in.readFully(message);
// do something with the message.
}
// all messages handled.

Hope this answers your additional questions.

@mihi: in.readFully(lotOfBytes); would this read completely after the int message length header? would it read from the beginning or start after the 4 byte int data holder? — iTEgg, May 15 '10 at 14:23
http://java.sun.com/j2se/1.4.2/docs/api/java/io/DataInputStream.html ok im studying now :) — iTEgg, May 15 '10 at 14:27
@mihi: how do i handle a closed/hungup connection using DataInputStream? i.e. -1? do i do a readInt and that would give -1? please advise. — iTEgg, May 15 '10 at 14:46
int recvMsgSize = in.readInt(); (using DataInputStream is not working in terms or detecting connection drop. please advise. — iTEgg, May 15 '10 at 15:04
int recvMsgSize = in.read(); solved it. thanks, you posting is the most informative. thanks to everyone else also. — iTEgg, May 15 '10 at 15:12
no, i get what you are saying but having implementation issues. how do i test for -1 (connection dropped) and then read int from position 0 to position 3? — iTEgg, May 15 '10 at 16:01
@mihi: Super. thank you for taking the time to update your answer. very helpful. — iTEgg, May 17 '10 at 18:37

score -1 · Answer 4 · answered May 15 '10 at 13:34

-1

A solution in functional style (just for variety, imho not very convinient in use):

private int addByte (int base, byte appendix) {
    return (base << 4) + appendix;
}

public void test() {
    byte b1 = 5, b2 = 5, byte b3 = 0, b4 = 1;
    int result = addByte (addByte (addByte (addByte (0, b1), b2), b3), b4);
}

answered May 15 '10 at 13:34

Roman

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2

Shouldn't the bit shift be 8, not 4? – Gigatron Feb 01 '13 at 22:34

bgw · Answer 5 · 2010-05-15T13:26:07.420

-4

As mihi said, it depends on where you are getting those bytes from, but this code might be of use:

int myNumber = (((int)byteOne) << 0) |
    (((int)byteTwo) << 8) |
    (((int)byteThree) << 16) |
    (((int)byteFour) << 24);

edited May 15 '10 at 13:26

answered May 15 '10 at 13:20

bgw

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1

`(int)` is superfluous and wrong, you need to undo sign extension with `& 0xFF` mask. – polygenelubricants May 15 '10 at 13:33
1

This one messes up on signed bits. Polygenelubricants wins. – bgw May 15 '10 at 13:34

Java Convert 4 bytes to int

5 Answers5

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