Convert long to byte array and add it to another a

问题:

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.

回答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];
}

回答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
    };
}

回答3:

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);
    }
}

回答4:

It doesn't look like you can slice a byte array to insert something into a subset without doing it byte by byte. Look at Grab a segment of an array in Java without creating a new array on heap . Basically what I would do is set create a 64 byte array and set the time to it then append a blank 64 byte array to it. Or just do it byte by byte.

回答5:

I am updating this post because I have just announced a pre-release version of a library that will convert longs to byte arrays (and back again). The library is very small and will convert any java primitive to a byte array.

http://rschilling.wordpress.com/2013/09/26/pre-release-announcement-pend-oreille/ http://code.google.com/p/pend-oreille/

If you use it you can do things like convert long arrays to byte arrays:

Double[] doubles = new Double[1000];
for (int i = 2; i < 1002; i++) {
    doubles[i - 2] = (double) i;
}

byte[] resultBytes1 = (byte[]) new PrimitiveHelper(PrimitiveUtil.unbox(doubles))
        .asType(byte[].class);

You can also convert a single long value as well.

byte[] resultBytes1 = (byte[]) new PrimitiveHelper(1000l)
        .asType(byte[].class);

Feel free to provide some feedback.

Update on October 4, 2013: I've now released the production of the library http://rschilling.wordpress.com/2013/10/04/pend-oreille-official-1-0-release/