How do I convert Long to byte and back in java

Question

How do I convert a long to a byte   and back in Java   I m trying convert a long to a byte   so that I will be able to send the byte   over a TCP connection  On the other side I want to take that byte   and convert it back into a double

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Why do you need the byte    why not just write it to the socket   I assume you mean long rather than Long  the latter needs to allow for null values   DataOutputStream dos   new DataOutputStream       new BufferedOutputStream socket getOutputStream      dos writeLong longValue    DataInputStream dis   new DataInputStream       new BufferedInputStream socket getInputStream      long longValue   dis readLong

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I will add another answer which is the fastest one possible   yes  even more than the accepted answer   BUT it will not work for every single case  HOWEVER  it WILL work for every conceivable scenario   You can simply use String as intermediate  Note  this WILL give you the correct result even though it seems like using String might yield the wrong results AS LONG AS YOU KNOW YOU RE WORKING WITH  NORMAL  STRINGS  This is a method to increase effectiveness and make the code simpler which in return must use some assumptions on the data strings it operates on   Con of using this method  If you re working with some ASCII characters like these symbols in the beginning of the ASCII table  the following lines might fail  but let s face it - you probably will never use them anyway   Pro of using this method  Remember that most people usually work with some normal strings without any unusual characters and then the method is the simplest and fastest way to go   from Long to byte     byte   arr   String valueOf longVar  getBytes      from byte   to Long   long longVar   Long valueOf new String byteArr   longValue

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I tested the ByteBuffer method against plain bitwise operations but the latter is significantly faster  public static byte   longToBytes long l        byte   result   new byte 8       for  int i   7  i  gt   0  i--            result i     byte  l  amp  0xFF           l  gt  gt   8            return result     public static long bytesToLong final byte   b        long result   0      for  int i   0  i  lt  8  i              result  lt  lt   8          result     b i   amp  0xFF             return result     For Java 8  we can use the static variables that were added  public static byte   longToBytes long l        byte   result   new byte Long BYTES       for  int i   Long BYTES - 1  i  gt   0  i--            result i     byte  l  amp  0xFF           l  gt  gt   Byte SIZE            return result     public static long bytesToLong final byte   b        long result   0      for  int i   0  i  lt  Long BYTES  i              result  lt  lt   Byte SIZE          result     b i   amp  0xFF             return result

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Kotlin extensions for Long and ByteArray types   fun Long toByteArray     numberToByteArray Long SIZE BYTES    putLong this toByteArray     private inline fun numberToByteArray size  Int  bufferFun  ByteBuffer    - gt  ByteBuffer   ByteArray       ByteBuffer allocate size  bufferFun   array     Throws NumberFormatException  class  fun ByteArray toLong    Long   toNumeric Long SIZE BYTES    long     Throws NumberFormatException  class  private inline fun  lt reified T  Number gt  ByteArray toNumeric size  Int  bufferFun  ByteBuffer    - gt  T   T       if  this size    size  throw NumberFormatException    T  class java simpleName  value must contains  size bytes        return ByteBuffer wrap this  bufferFun       You can see full code in my library https   github com ArtemBotnev low-level-extensions

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You could use the implementation in org apache hadoop hbase util Bytes http   hbase apache org apidocs org apache hadoop hbase util Bytes html   The source code is here   http   grepcode com file repository cloudera com content repositories releases com cloudera hbase hbase 0 89 20100924-28 org apache hadoop hbase util Bytes java Bytes toBytes 28long 29  Look for the toLong and toBytes methods   I believe the software license allows you to take parts of the code and use it but please verify that

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You could use the Byte conversion methods from Google Guava   Example   byte   bytes   Longs toByteArray 12345L

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I find this method to be most friendly  var b   BigInteger valueOf x  toByteArray     var l   new BigInteger b

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public static long bytesToLong byte   bytes            if  bytes length  gt  8                throw new IllegalMethodParameterException  byte should not be more than 8 bytes                       long r   0          for  int i   0  i  lt  bytes length  i                  r   r  lt  lt  8              r    bytes i                      return r           public static byte   longToBytes long l            ArrayList lt Byte gt  bytes   new ArrayList lt Byte gt             while  l    0                bytes add  byte   l    0xff   1                 l   l  gt  gt  8                    byte   bytesp   new byte bytes size             for  int i   bytes size   - 1  j   0  i  gt   0  i--  j                  bytesp j    bytes get i                     return bytesp

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If you are looking for a fast unrolled version  this should do the trick  assuming a byte array called  b  with a length of 8   byte   -  long  long l     long  b 7   lt  lt  56             long  b 6   amp  0xff   lt  lt  48            long  b 5   amp  0xff   lt  lt  40            long  b 4   amp  0xff   lt  lt  32            long  b 3   amp  0xff   lt  lt  24            long  b 2   amp  0xff   lt  lt  16            long  b 1   amp  0xff   lt  lt  8            long  b 0   amp  0xff     long -  byte   as an exact counterpart to the above  byte   b   new byte             byte  lng          byte   lng  gt  gt  8           byte   lng  gt  gt  16           byte   lng  gt  gt  24           byte   lng  gt  gt  32           byte   lng  gt  gt  40           byte   lng  gt  gt  48           byte   lng  gt  gt  56

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Here s another way to convert byte   to long using Java 8 or newer   private static int bytesToInt final byte   bytes  final int offset        assert offset   Integer BYTES  lt   bytes length       return  bytes offset   Integer BYTES - 1   amp  0xFF                 bytes offset   Integer BYTES - 2   amp  0xFF   lt  lt  Byte SIZE                bytes offset   Integer BYTES - 3   amp  0xFF   lt  lt  Byte SIZE   2                bytes offset   Integer BYTES - 4   amp  0xFF   lt  lt  Byte SIZE   3     private static long bytesToLong final byte   bytes  final int offset        return toUnsignedLong bytesToInt bytes  offset    lt  lt  Integer SIZE               toUnsignedLong bytesToInt bytes  offset   Integer BYTES        Converting a long can be expressed as the high- and low-order bits of two integer values subject to a bitwise-OR  Note that the toUnsignedLong is from the Integer class and the first call to toUnsignedLong may be superfluous   The opposite conversion can be unrolled as well  as others have mentioned

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Just write the long to a DataOutputStream with an underlying ByteArrayOutputStream  From the ByteArrayOutputStream you can get the byte-array via toByteArray     class Main            public static byte   long2byte long l  throws IOException                   ByteArrayOutputStream baos new ByteArrayOutputStream Long SIZE 8           DataOutputStream dos new DataOutputStream baos           dos writeLong l           byte   result baos toByteArray            dos close                return result                      public static long byte2long byte   b  throws IOException                   ByteArrayInputStream baos new ByteArrayInputStream b           DataInputStream dos new DataInputStream baos           long result dos readLong            dos close            return result                      public static void main  String   args  throws java lang Exception                     long l 123456L           byte   b long2byte l            System out println l      byte2long b                         Works for other primitives accordingly    Hint  For TCP you do not need the byte   manually  You will use a Socket socket and its streams  OutputStream os socket getOutputStream     DataOutputStream dos new DataOutputStream os   dos writeLong l     etc      instead

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If you are already using an OutputStream to write to the socket  then DataOutputStream might be a good fit   Here is an example      Assumes you are currently working with a SocketOutputStream   SocketOutputStream outputStream       long longValue        DataOutputStream dataOutputStream   new DataOutputStream outputStream    dataOutputStream writeLong longValue   dataOutputStream flush      There are similar methods for short  int  float  etc   You can then use DataInputStream on the receiving side

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public byte   longToBytes long x        ByteBuffer buffer   ByteBuffer allocate Long BYTES       buffer putLong x       return buffer array       public long bytesToLong byte   bytes        ByteBuffer buffer   ByteBuffer allocate Long BYTES       buffer put bytes       buffer flip     need flip      return buffer getLong        Or wrapped in a class to avoid repeatedly creating ByteBuffers   public class ByteUtils       private static ByteBuffer buffer   ByteBuffer allocate Long BYTES            public static byte   longToBytes long x            buffer putLong 0  x           return buffer array               public static long bytesToLong byte   bytes            buffer put bytes  0  bytes length           buffer flip     need flip          return buffer getLong                Since this is getting so popular  I just want to mention that I think you re better off using a library like Guava in the vast majority of cases  And if you have some strange opposition to libraries  you should probably consider this answer first for native java solutions  I think the main thing my answer really has going for it is that you don t have to worry about the endian-ness of the system yourself

[java] How do I convert Long to byte[] and back in java

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