Convert a byte array to integer in Java and vice versa

byte[] toByteArray(int value) {
     return  ByteBuffer.allocate(4).putInt(value).array();
}

byte[] toByteArray(int value) {
    return new byte[] { 
        (byte)(value >> 24),
        (byte)(value >> 16),
        (byte)(value >> 8),
        (byte)value };
}

int fromByteArray(byte[] bytes) {
     return ByteBuffer.wrap(bytes).getInt();
}
// packing an array of 4 bytes to an int, big endian, minimal parentheses
// operator precedence: <<, &, | 
// when operators of equal precedence (here bitwise OR) appear in the same expression, they are evaluated from left to right
int fromByteArray(byte[] bytes) {
     return bytes[0] << 24 | (bytes[1] & 0xFF) << 16 | (bytes[2] & 0xFF) << 8 | (bytes[3] & 0xFF);
}

// packing an array of 4 bytes to an int, big endian, clean code
int fromByteArray(byte[] bytes) {
     return ((bytes[0] & 0xFF) << 24) | 
            ((bytes[1] & 0xFF) << 16) | 
            ((bytes[2] & 0xFF) << 8 ) | 
            ((bytes[3] & 0xFF) << 0 );
}

When packing signed bytes into an int, each byte needs to be masked off because it is sign-extended to 32 bits (rather than zero-extended) due to the arithmetic promotion rule (described in JLS, Conversions and Promotions).

There's an interesting puzzle related to this described in Java Puzzlers ("A Big Delight in Every Byte") by Joshua Bloch and Neal Gafter . When comparing a byte value to an int value, the byte is sign-extended to an int and then this value is compared to the other int

byte[] bytes = (…)
if (bytes[0] == 0xFF) {
   // dead code, bytes[0] is in the range [-128,127] and thus never equal to 255
}

Note that all numeric types are signed in Java with exception to char being a 16-bit unsigned integer type.

You can also use BigInteger for variable length bytes. You can convert it to long, int or short, whichever suits your needs.

new BigInteger(bytes).intValue();

or to denote polarity:

new BigInteger(1, bytes).intValue();

To get bytes back just:

new BigInteger(bytes).toByteArray()

Although simple, I just wanted to point out that if you run this many times in a loop, this could lead to a lot of garbage collection. This may be a concern depending on your use case.

A basic implementation would be something like this:

public class Test {
    public static void main(String[] args) {
        int[] input = new int[] { 0x1234, 0x5678, 0x9abc };
        byte[] output = new byte[input.length * 2];

        for (int i = 0, j = 0; i < input.length; i++, j+=2) {
            output[j] = (byte)(input[i] & 0xff);
            output[j+1] = (byte)((input[i] >> 8) & 0xff);
        }

        for (int i = 0; i < output.length; i++)
            System.out.format("%02x\n",output[i]);
    }
}

In order to understand things you can read this WP article: http://en.wikipedia.org/wiki/Endianness

The above source code will output 34 12 78 56 bc 9a. The first 2 bytes (34 12) represent the first integer, etc. The above source code encodes integers in little endian format.

/** length should be less than 4 (for int) **/
public long byteToInt(byte[] bytes, int length) {
        int val = 0;
        if(length>4) throw new RuntimeException("Too big to fit in int");
        for (int i = 0; i < length; i++) {
            val=val<<8;
            val=val|(bytes[i] & 0xFF);
        }
        return val;
    }

As often, guava has what you need.

To go from byte array to int: Ints.fromBytesArray, doc here

To go from int to byte array: Ints.toByteArray, doc here

Without using any external or obscure library or applying bitwise magic (bit difficult to read), you can use ByteArrayOutputStream combined with DataOutputStream to write an int into the stream like this using it's writeInt(int) method:

// assuming num is of type int that we want to convert into byte[]
ByteArrayOutputStream bos = new ByteArrayOutputStream();
DataOutputStream dos = new DataOutputStream(bos);
dos.writeInt(num);
byte[] intBytes = bos.toByteArray();
bos.close();

Similarly use ByteArrayInputStream combined with DataInputStream to read an int from the stream like this using it's int readInt() method:

// assuming byteArr is of type byte[] that contains a number we need to read
ByteArrayInputStream bis = new ByteArrayInputStream(byteArr);
DataInputStream dis = new DataInputStream(bis);
int num = dis.readInt();
bis.close();

Update: If integer number is small enough to be fit into 2 bytes only then use writeShort and readShort methods instead of writeInt and readInt methods.

Someone with a requirement where they have to read from bits, lets say you have to read from only 3 bits but you need signed integer then use following:

data is of type: java.util.BitSet

new BigInteger(data.toByteArray).intValue() << 32 - 3 >> 32 - 3

The magic number 3 can be replaced with the number of bits (not bytes) you are using.