Convert long to byte array and add it to another array

Question

I want to change a values in byte array to put a long timestamp value in in the MSBs. Can someone tell me whats the best way to do it. I do not want to insert values bit-by-bit which I believe is very inefficient.

long time = System.currentTimeMillis();
Long timeStamp = new Long(time);
byte[] bArray = new byte[128];

What I want is something like:

byte[0-63] = timeStamp.byteValue(); 

Is something like this possible . What is the best way to edit/insert values in this byte array. since byte is a primitive I dont think there are some direct implementations I can make use of?

Edit:
It seems that System.currentTimeMillis() is faster than Calendar.getTimeInMillis(), so replacing the above code by it.Please correct me if wrong.

Answer 1

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];
}

Answer 2

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
    };
}