Convert long to byte array and add it to another array

There are multiple ways to do it:

• Use a ByteBuffer (best option - concise and easy to read):

  byte[] bytes = ByteBuffer.allocate(Long.SIZE / Byte.SIZE).putLong(someLong).array();
  

• You can also use DataOutputStream (more verbose):

  ByteArrayOutputStream baos = new ByteArrayOutputStream();
  DataOutputStream dos = new DataOutputStream(baos);
  dos.writeLong(someLong);
  dos.close();
  byte[] longBytes = baos.toByteArray();
  

• Finally, you can do this manually (taken from the LongSerializer in Hector's code) (harder to read):

  byte[] b = new byte[8];
  for (int i = 0; i < size; ++i) {
    b[i] = (byte) (l >> (size - i - 1 << 3));
  }
  

Then you can append these bytes to your existing array by a simple loop:

// change this, if you want your long to start from 
// a different position in the array
int start = 0; 
for (int i = 0; i < longBytes.length; i ++) {
   bytes[start + i] = longBytes[i];
}

If you want to really get under the hood...

public byte[] longToByteArray(long value) {
    return new byte[] {
        (byte) (value >> 56),
        (byte) (value >> 48),
        (byte) (value >> 40),
        (byte) (value >> 32),
        (byte) (value >> 24),
        (byte) (value >> 16),
        (byte) (value >> 8),
        (byte) value
    };
}

For me ByteBuffer and other utils are expensive from time perspective. Here are 2 methods that you can use:

// first method (that is using the second method), it return the array allocated and fulfilled

public byte[] longToByteArray(long value) 
{
        byte[] array = new byte[8];

        longToByteArray(value,array,0);
        return array;
}

// this method is useful if you have already allocated the buffer and you want to write the long a specific location in the array.

public void longToByteArray(long value, byte[] array, int startFrom) 
{
    for (int i=7; i>=0; i--)
    {
        array[startFrom+7-i] = (byte) (value >> i*8);
    }
}