Develop Reference

int array to byte array with one integer per byte

All related questions I've found here on SO describe the conversion between a byte array and an int array where 4 bytes are converted to a single integer and vice versa.
What I am looking for instead is converting each integer to a single byte and vice versa, knowing that none of the values of the integer array exceeds the range of an unsigned byte.
Is there a library that does that (preferably Guava or Apache commons)? Essentially, I am looking for something like this:
int[] -> byte[]
for (int i = 0; i < intArray.length; i++){
byteArray[i] = (byte) intArray[i];
}
byte[] -> int[]
for (int i = 0; i < byteArray.length; i++){
intArray[i] = 0xff & byteArray[i];
}

You cannot cast arrays in Java in that way. AFAIK, the VM just does not let you reinterpret the bytes in an array as a different type. I assume that you could use some platform-dependent JNI magic to make it so, but that would be extremely hacky.
Edit (removed sample code; added following)
If you are going to reinterpret the same bytes as different primitive types, use a ByteBuffer.
Once declared, you get direct access operations to reinterpreted bytes, such as
myByteBuffer.getInt(1); // reads bytes 4, 5, 6, 7 as an int
myByteBuffer.getByte(5); // reads byte 5 as a byte
You can also extract primitive arrays from there, but there will be extra allocations involved.

Try:
int number = 54353, divisor = 256;
byte[] byteArray = new byte[4];
byteArray[3] = number%divisor;
number = number/divisor
byteArray[2] = number%divisor;
number = number/divisor
byteArray[1] = number%divisor;
number = number/divisor
byteArray[0] = number;

Converting ints and strings into byte array and the reverse

I would like to convert some ints and some strings into a single byte array and then back again. I've done a bit of research on how to do converting, but I'm not sure if its all correct.
Converting a string to a byte array is easy:
byte[] bytes = string.getBytes();
Converting it back again via Arrays.toString() because that just creates a string of the bytes.
Does this work: String s = new String(bytes);?
Converting ints to byte array is like this:
int[] data = { int1, int2, int3 };
ByteBuffer byteBuffer = ByteBuffer.allocate(data.length * 4);
IntBuffer intBuffer = byteBuffer.asIntBuffer();
intBuffer.put(data);
byte[] my_app_state = byteBuffer.array();
But I don't know how to convert it back again.
My aim is to have say 4 ints and 2 strings converted to a single byte array and then convert them back again.
For example. I have these objects and would like them to converted to the same byte array.
int int1 = 1;
int int2 = 2;
int int3 = 3;
int int4 = 4;
String s1 = "mystring1"
String s2 = "mystring2"
Update: Removed code where I thought there was a problem. There wasn't.

For each operation, you need to determine the reverse operation, not just any operation which returns the right type. for example, the reverse of n * 2 is m / 2 not m - 2 even though the type is right.
Arrays.toString("Hi".getBytes()) => "{ 72, 105 }"
So you can do
text.getBytes() => new String(bytes) // if the same character encoding is used.
a better option is
text.getBytes("UTF-8") => new String(bytes, "UTF-8");
The problem with an array is you have two pieces of information a length and some bytes If you just write the bytes, you no longer know the length and so you can't easily decode it (perhaps impossible)
In your case, the simplest option is to use a Data Stream
// buffer which grows as needed.
ByteArrayOutputStream boas = new ByteArrayOutputStream();
// supports basic data types
DataOutputStream dos = new DataOutputStream(baos);
dos.writeInt(data.length);
for(int i: data) dow.writeInt(i);
// write the length of the string + the UTF-8 encoding of the text.
dos.writeUTF(s1);
dos.writeUTF(s2);
byte[] bytes = bytes.toByteArray();
To do the reverse, you use the InputStream and the readXxxx instead of writeXxxx methods.

Java makes it very simple to achieve this, as this is a very common use case. What you need looks very much like Serialization.
Serialization works like this: A single object can be converted to a set of bytes and stored in a byte array (usually for writing to a file or sending over a network).
The good things is that any object can become serializable by just implementing a marker interface (just 1 line of code). Also, all Wrapper datatypes and String and Collections objects like ArrayList are serializable.
Coming to your question: Put all your data in a single object and serialize that object. 3 options come to my mind:
1. An Object[] or ArrayList (if you know the order for sure, so that you can access based on position)
2. A HashMap, (if you can assign a name to each of them instead of relying on position)
3. Create your own data type with fields like int1, int2 or even more meaningful names. (Your class should implement Serializable).
Now, all your data is added into a single object. Convert this one object to a byte array and your job is done.
Check this link for how to convert a single object to byte array:
Java Serializable Object to Byte Array
Object[] payload = new Object[]{int1, int2, int3, int4, string1, string2};
ByteArrayOutputStream out = new ByteArrayOutputStream();
ObjectOutputStream os = new ObjectOutputStream(out);
os.writeObject(payload);
byte[] result = out.toByteArray(); //Done

for store Strings as well you have to use some thing like
IntBuffer intBuffer = byteBuffer.asIntBuffer();
CharBuffer stringBuffer = byteBuffer.asCharBuffer();
then you have to traverse the loop on char[][] string = {s1.toCharArray(),s2.toCharArray()};
to put the each character in stringBuffer and more thing you have to do is you to make your byteBuffer to sufficient to hold these values my friend

Parse C byte array into Java ByteBuffer.

I'm parsing a byte array which contains variables of different types. I'm getting this array from HID connected to my phone. Array was made by C programmer. I'm trying to parse it using ByteBuffer class:
byte[] buffer = new byte[64];
if(connection.bulkTransfer(endpoint, buffer, 64, 1000) >= 0)
{
ByteBuffer byteBuffer = ByteBuffer.wrap(buffer);
char mId = byteBuffer.getChar();
short rId = byteBuffer.getShort();
// ............................
}
But the values of this variables are not correct. Can anyone please tell me what i'm doing wrong?

There are systems with LitteEndian Byte order and BigEndian.
java uses BigEndian.
If the c programmer wrote the byte array in Little endian, you could use DataInputStream based on an Appache LittleEndianInputStream:
LittleEndianInputStream leis = new LittleEndianInputStream(is);
DataInputStream dis = new DataInputStream(leis);
int i1 = dis.readInt();
short s2 = dis.readShort();
If you and your colleague define a binary interface (file, or byte array) you always should force a speciifc byte order (Either little or big endian).

If byte order (little vs big endian) is the issue, you can set the byte order for the ByteBuffer to native without changing all of the program:
ByteBuffer byteBuffer = ByteBuffer.wrap(buffer);
byteBuffer.order(ByteOrder.nativeOrder()); // Set native byte order
char mId = byteBuffer.getChar();
short rId = byteBuffer.getShort();
On the other hand, if you find ByteBuffer objects more convenient than byte arrays, tell the C programmer to return you a direct byte buffer instead of an array: easier for all parties and probably more efficient.

Get single bytes from multi-byte variable in java

How can I split a variable into single bytes in java? I have for example following snippet in C++:
unsigned long someVar;
byte *p = (byte*)(void*) someVar; // byte being typedef unsigned char (from 0 to 255)
byte *bytes = new byte[sizeof(someVar)];
for(byte i = 0;i<sizeof(someVar);i++)
{
bytes[i] = *p++;
}
.... //do something with bytes
I want to accomplish the same under java, but I can't seem to find an obvious workaround.

There are two ways to do it with the ByteBuffer class. One is to create a new byte array dynamically.
long value = 123;
byte[] bytes = ByteBuffer.allocate(8).putLong(value).array();
Another is to write to an existing array.
long value = 123;
byte[] bytes = new byte[8];
ByteBuffer.wrap(bytes).putLong(value);
// bytes now contains the byte representation of 123.

If you use Guava, there is a convenience Longs.toByteArray. It is simply a wrapper for John's ByteBuffer answer above, but if you already use Guava, it's slightly "nicer" to read.

How to convert int [] to Big Integer?

I would like to convert an integer array of values, which was original were bytes.

First, make sure you know in which format your int[] is meant to be interpreted.
Each int can be seen as consisting of four bytes, and these bytes together can be converted to an BigInteger. The details are the byte order - which byte is the most and which one the least significant?
Also, do you have a signed or unsigned number?
A simple way to convert your ints to bytes (for latter use in a BigInteger constructor) would be to use ByteBuffer and wrap an IntBuffer around it.
public BigInteger toBigInteger(int[] data) {
byte[] array = new byte[data.length * 4];
ByteBuffer bbuf = ByteBuffer.wrap(array);
IntBuffer ibuf = bbuf.asIntBuffer();
ibuf.put(data);
return new BigInteger(array);
}
Obvious adaptions would be to set the byte order of bbuf, or use another BigInteger constructor (for unsigned).

Well, what about new BigInteger(byte[] val)?
To quote the API docs I linked to:
Translates a byte array containing the two's-complement binary representation of a BigInteger into a BigInteger. The input array is assumed to be in big-endian byte-order: the most significant byte is in the zeroth element.