In Java how do I convert a byte array to a string of hex digits while keeping leading zeros

Question

I m working with some example java code for making md5 hashes  One part converts the results from bytes to a string of hex digits   byte messageDigest     algorithm digest         StringBuffer hexString   new StringBuffer    for  int i 0 i lt messageDigest length i          hexString append Integer toHexString 0xFF  amp  messageDigest i             However  it doesn t quite work since toHexString apparently drops off leading zeros  So  what s the simplest way to go from byte array to hex string that maintains the leading zeros

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The method javax xml bind DatatypeConverter printHexBinary    part of the Java Architecture for XML Binding  JAXB   was a convenient way to convert a byte   to a hex string  The DatatypeConverter class also included many other useful data-manipulation methods   In Java 8 and earlier  JAXB was part of the Java standard library  It was deprecated with Java 9 and removed with Java 11  as part of an effort to move all Java EE packages into their own libraries  It s a long story  Now  javax xml bind doesn t exist  and if you want to use JAXB  which contains DatatypeConverter  you ll need to install the JAXB API and JAXB Runtime from Maven   Example usage   byte bytes       byte 0   byte 0   byte 134   byte 0   byte 61   String hex   javax xml bind DatatypeConverter printHexBinary bytes     Will result in   000086003D

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Check out Hex encodeHexString from Apache Commons Codec   import org apache commons codec binary Hex   String hex   Hex encodeHexString bytes

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Guava makes it pretty simple too   BaseEncoding base16   encode  bytes      It s a nice alternative when Apache Commons is not available  It also has some nice controls of the output like   byte   bytes   new byte     0xa  0xb  0xc  0xd  0xe  0xf    BaseEncoding base16   lowerCase   withSeparator       2   encode  bytes        0a 0b 0c 0d 0e 0f

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I would use something like this for fixed length  like hashes   md5sum   String format   032x   new BigInteger 1  md digest        The 0 in the mask does the padding

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You can get it writing less without external libraries   String hex    new HexBinaryAdapter    marshal md5 digest YOUR STRING getBytes

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You can use the one below   I tested this with leading zero bytes and with initial negative bytes as well  public static String toHex byte   bytes        BigInteger bi   new BigInteger 1  bytes       return String format   0     bytes length  lt  lt  1     X   bi       If you want lowercase hex digits  use  x  in the format String

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This solution is a little older school  and should be memory efficient   public static String toHexString byte bytes          if  bytes    null            return null             StringBuffer sb   new StringBuffer        for  int iter   0  iter  lt  bytes length  iter              byte high    byte     bytes iter   amp  0xf0   gt  gt  4           byte low     byte     bytes iter   amp  0x0f           sb append nibble2char high            sb append nibble2char low               return sb toString       private static char nibble2char byte b        byte nibble    byte   b  amp  0x0f       if  nibble  lt  10            return  char    0    nibble             return  char    a    nibble - 10

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I would use something like this for fixed length  like hashes   md5sum   String format   032x   new BigInteger 1  md digest        The 0 in the mask does the padding

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String result   String format   0    messageDigest length    s   hexString toString      That s the shortest solution given what you already have   If you could convert the byte array to a numeric value  String format can convert it to a hex string at the same time

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Check out Hex encodeHexString from Apache Commons Codec   import org apache commons codec binary Hex   String hex   Hex encodeHexString bytes

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IMHO all the solutions above that provide snippets to remove the leading zeroes are wrong   byte messageDigest     algorithm digest    for  int i   0  i  lt  messageDigest length  i          hexString append Integer toHexString 0xFF  amp  messageDigest i             According to this snippet  8 bits are taken from the byte array in an iteration  converted into an integer  since Integer toHexString function takes int as argument  and then that integer is converted to the corresponding hash value  So  for example if you have 00000001 00000001 in binary  according to the code  the hexString variable would have 0x11 as the hex value whereas correct value should be 0x0101  Thus  while calculating MD5 we may get hashes of length  lt 32 bytes because of missing zeroes  which may not satisfy the cryptographically unique properties that MD5 hash does    The solution to the problem is replacing the above code snippet by the following snippet   byte messageDigest     algorithm digest    for  int i   0  i  lt  messageDigest length  i          int temp 0xFF  amp  messageDigest i       String s Integer toHexString temp       if temp lt  0x0F           s  0  s            hexString append s

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It appears concat and append functions can be really slow  The following was MUCH faster for me  than my previous post    Changing to a char array in building the output was the key factor to speed it up   I have not compared to Hex encodeHex suggested by Brandon DuRette   public static String toHexString byte   bytes        char   hexArray     0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F        char   hexChars   new char 10000000       int c   0      int v      for   j   0  j  lt  bytes length  j               v   bytes j   amp  0xFF          hexChars c    hexArray v 16           c            hexChars c    hexArray v 16           c              return new String hexChars  0  c

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static String toHex byte   digest        StringBuilder sb   new StringBuilder        for  byte b   digest            sb append String format   1 02X   b               return sb toString

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Is that a faulty solution   android java          Create MD5 Hash     MessageDigest digest   java security MessageDigest getInstance  MD5        digest update s getBytes         byte   md5sum   digest digest        BigInteger bigInt   new BigInteger 1  md5sum       String stringMD5   bigInt toString 16          Fill to 32 chars     stringMD5   String format   32s   stringMD5  replace       0        return stringMD5    So basically it replaces spaces with 0

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This solution requires no bit-shifting or -masking  lookup tables  or external libraries  and is about as short as I can get   byte   digest   new byte 16           Formatter fmt   new Formatter        for  byte b   digest       fmt format   02X   b          fmt toString

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byte messageDigest     algorithm digest    StringBuffer hexString   new StringBuffer    for  int i   0  i  lt  messageDigest length  i          String hexByte   Integer toHexString 0xFF  amp  messageDigest i        int numDigits   2 - hexByte length        while  numDigits--  gt  0            hexString append  0              hexString append hexByte

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byte messageDigest     algorithm digest    StringBuffer hexString   new StringBuffer    for  int i   0  i  lt  messageDigest length  i          String hexByte   Integer toHexString 0xFF  amp  messageDigest i        int numDigits   2 - hexByte length        while  numDigits--  gt  0            hexString append  0              hexString append hexByte

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This what I am using for MD5 hashes   public static String getMD5 String filename          throws NoSuchAlgorithmException  IOException       MessageDigest messageDigest            java security MessageDigest getInstance  MD5         InputStream in   new FileInputStream filename        byte    buffer   new byte 8192       int len   in read buffer  0  buffer length        while  len  gt  0            messageDigest update buffer  0  len           len   in read buffer  0  buffer length             in close         return new BigInteger 1  messageDigest digest    toString 16       EDIT  I ve tested and I ve noticed that with this also trailing zeros are cut  But this can only happen in the beginning  so you can compare with the expected length and pad accordingly

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You can use the one below   I tested this with leading zero bytes and with initial negative bytes as well  public static String toHex byte   bytes        BigInteger bi   new BigInteger 1  bytes       return String format   0     bytes length  lt  lt  1     X   bi       If you want lowercase hex digits  use  x  in the format String

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This will give two-char long string for a byte   public String toString byte b       final char   Hex   new String  0123456789ABCDEF   toCharArray        return   0x   Hex  b  amp  0xF0   gt  gt  4   Hex  b  amp  0x0F

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Is that a faulty solution   android java          Create MD5 Hash     MessageDigest digest   java security MessageDigest getInstance  MD5        digest update s getBytes         byte   md5sum   digest digest        BigInteger bigInt   new BigInteger 1  md5sum       String stringMD5   bigInt toString 16          Fill to 32 chars     stringMD5   String format   32s   stringMD5  replace       0        return stringMD5    So basically it replaces spaces with 0

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I m surprised that no one came up with the following solution   StringWriter sw   new StringWriter    com sun corba se impl orbutil HexOutputStream hex   new com sun corba se impl orbutil HexOutputStream sw   hex write byteArray   System out println sw toString

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This is also equivalent but more concise using Apache util HexBin where the code reduces to  HexBin encode messageDigest  toLowerCase

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String result   String format   0    messageDigest length    s   hexString toString      That s the shortest solution given what you already have   If you could convert the byte array to a numeric value  String format can convert it to a hex string at the same time

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The method javax xml bind DatatypeConverter printHexBinary    part of the Java Architecture for XML Binding  JAXB   was a convenient way to convert a byte   to a hex string  The DatatypeConverter class also included many other useful data-manipulation methods   In Java 8 and earlier  JAXB was part of the Java standard library  It was deprecated with Java 9 and removed with Java 11  as part of an effort to move all Java EE packages into their own libraries  It s a long story  Now  javax xml bind doesn t exist  and if you want to use JAXB  which contains DatatypeConverter  you ll need to install the JAXB API and JAXB Runtime from Maven   Example usage   byte bytes       byte 0   byte 0   byte 134   byte 0   byte 61   String hex   javax xml bind DatatypeConverter printHexBinary bytes     Will result in   000086003D

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I found Integer toHexString to be a little slow   If you are converting many bytes  you may want to consider building an array of Strings containing  00    FF  and use the integer as the index   I e   hexString append hexArray 0xFF  amp  messageDigest i       This is faster and ensures the correct length   Just requires the array of strings   String   hexArray      00   01   02   03   04   05   06   07   08   09   0A   0B   0C   0D   0E   0F    10   11   12   13   14   15   16   17   18   19   1A   1B   1C   1D   1E   1F    20   21   22   23   24   25   26   27   28   29   2A   2B   2C   2D   2E   2F    30   31   32   33   34   35   36   37   38   39   3A   3B   3C   3D   3E   3F    40   41   42   43   44   45   46   47   48   49   4A   4B   4C   4D   4E   4F    50   51   52   53   54   55   56   57   58   59   5A   5B   5C   5D   5E   5F    60   61   62   63   64   65   66   67   68   69   6A   6B   6C   6D   6E   6F    70   71   72   73   74   75   76   77   78   79   7A   7B   7C   7D   7E   7F    80   81   82   83   84   85   86   87   88   89   8A   8B   8C   8D   8E   8F    90   91   92   93   94   95   96   97   98   99   9A   9B   9C   9D   9E   9F    A0   A1   A2   A3   A4   A5   A6   A7   A8   A9   AA   AB   AC   AD   AE   AF    B0   B1   B2   B3   B4   B5   B6   B7   B8   B9   BA   BB   BC   BD   BE   BF    C0   C1   C2   C3   C4   C5   C6   C7   C8   C9   CA   CB   CC   CD   CE   CF    D0   D1   D2   D3   D4   D5   D6   D7   D8   D9   DA   DB   DC   DD   DE   DF    E0   E1   E2   E3   E4   E5   E6   E7   E8   E9   EA   EB   EC   ED   EE   EF    F0   F1   F2   F3   F4   F5   F6   F7   F8   F9   FA   FB   FC   FD   FE   FF

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byte messageDigest     algorithm digest    StringBuffer hexString   new StringBuffer    for  int i   0  i  lt  messageDigest length  i          String hexByte   Integer toHexString 0xFF  amp  messageDigest i        int numDigits   2 - hexByte length        while  numDigits--  gt  0            hexString append  0              hexString append hexByte

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A simple approach would be to check how many digits are output by Integer toHexString   and add a leading zero to each byte if needed  Something like this   public static String toHexString byte   bytes        StringBuilder hexString   new StringBuilder         for  int i   0  i  lt  bytes length  i              String hex   Integer toHexString 0xFF  amp  bytes i            if  hex length      1                hexString append  0                      hexString append hex              return hexString toString

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Or you can do this   byte   digest   algorithm digest    StringBuilder byteContet   new StringBuilder    for byte b  digest    byteContent   String format   02x  b    byteContent append byteContent       Its Short  simple and basically just a format change

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This what I am using for MD5 hashes   public static String getMD5 String filename          throws NoSuchAlgorithmException  IOException       MessageDigest messageDigest            java security MessageDigest getInstance  MD5         InputStream in   new FileInputStream filename        byte    buffer   new byte 8192       int len   in read buffer  0  buffer length        while  len  gt  0            messageDigest update buffer  0  len           len   in read buffer  0  buffer length             in close         return new BigInteger 1  messageDigest digest    toString 16       EDIT  I ve tested and I ve noticed that with this also trailing zeros are cut  But this can only happen in the beginning  so you can compare with the expected length and pad accordingly

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In order to keep leading zeroes  here is a small variation on what has Paul suggested  eg md5 hash    public static String MD5hash String text  throws NoSuchAlgorithmException       byte   hash   MessageDigest getInstance  MD5   digest text getBytes         return String format   032x  new BigInteger 1  hash        Oops  this looks poorer than what s Ayman proposed  sorry for that

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static String toHex byte   digest        StringBuilder sb   new StringBuilder        for  byte b   digest            sb append String format   1 02X   b               return sb toString

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This what I am using for MD5 hashes   public static String getMD5 String filename          throws NoSuchAlgorithmException  IOException       MessageDigest messageDigest            java security MessageDigest getInstance  MD5         InputStream in   new FileInputStream filename        byte    buffer   new byte 8192       int len   in read buffer  0  buffer length        while  len  gt  0            messageDigest update buffer  0  len           len   in read buffer  0  buffer length             in close         return new BigInteger 1  messageDigest digest    toString 16       EDIT  I ve tested and I ve noticed that with this also trailing zeros are cut  But this can only happen in the beginning  so you can compare with the expected length and pad accordingly

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And how can you convert back again from ascii to byte array    i followed following code to convert to ascii given by Jemenake     public static String toHexString byte   bytes        char   hexArray     0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F        char   hexChars   new char bytes length   2       int v      for   int j   0  j  lt  bytes length  j               v   bytes j   amp  0xFF          hexChars j 2    hexArray v 16           hexChars j 2   1    hexArray v 16             return new String hexChars

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static String toHex byte   digest        String digits    0123456789abcdef       StringBuilder sb   new StringBuilder digest length   2       for  byte b   digest            int bi   b  amp  0xff          sb append digits charAt bi  gt  gt  4            sb append digits charAt bi  amp  0xf              return sb toString

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This is also equivalent but more concise using Apache util HexBin where the code reduces to  HexBin encode messageDigest  toLowerCase

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String result   String format   0    messageDigest length    s   hexString toString      That s the shortest solution given what you already have   If you could convert the byte array to a numeric value  String format can convert it to a hex string at the same time

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A simple approach would be to check how many digits are output by Integer toHexString   and add a leading zero to each byte if needed  Something like this   public static String toHexString byte   bytes        StringBuilder hexString   new StringBuilder         for  int i   0  i  lt  bytes length  i              String hex   Integer toHexString 0xFF  amp  bytes i            if  hex length      1                hexString append  0                      hexString append hex              return hexString toString

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This solution requires no bit-shifting or -masking  lookup tables  or external libraries  and is about as short as I can get   byte   digest   new byte 16           Formatter fmt   new Formatter        for  byte b   digest       fmt format   02X   b          fmt toString

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static String toHex byte   digest        String digits    0123456789abcdef       StringBuilder sb   new StringBuilder digest length   2       for  byte b   digest            int bi   b  amp  0xff          sb append digits charAt bi  gt  gt  4            sb append digits charAt bi  amp  0xf              return sb toString

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I found Integer toHexString to be a little slow   If you are converting many bytes  you may want to consider building an array of Strings containing  00    FF  and use the integer as the index   I e   hexString append hexArray 0xFF  amp  messageDigest i       This is faster and ensures the correct length   Just requires the array of strings   String   hexArray      00   01   02   03   04   05   06   07   08   09   0A   0B   0C   0D   0E   0F    10   11   12   13   14   15   16   17   18   19   1A   1B   1C   1D   1E   1F    20   21   22   23   24   25   26   27   28   29   2A   2B   2C   2D   2E   2F    30   31   32   33   34   35   36   37   38   39   3A   3B   3C   3D   3E   3F    40   41   42   43   44   45   46   47   48   49   4A   4B   4C   4D   4E   4F    50   51   52   53   54   55   56   57   58   59   5A   5B   5C   5D   5E   5F    60   61   62   63   64   65   66   67   68   69   6A   6B   6C   6D   6E   6F    70   71   72   73   74   75   76   77   78   79   7A   7B   7C   7D   7E   7F    80   81   82   83   84   85   86   87   88   89   8A   8B   8C   8D   8E   8F    90   91   92   93   94   95   96   97   98   99   9A   9B   9C   9D   9E   9F    A0   A1   A2   A3   A4   A5   A6   A7   A8   A9   AA   AB   AC   AD   AE   AF    B0   B1   B2   B3   B4   B5   B6   B7   B8   B9   BA   BB   BC   BD   BE   BF    C0   C1   C2   C3   C4   C5   C6   C7   C8   C9   CA   CB   CC   CD   CE   CF    D0   D1   D2   D3   D4   D5   D6   D7   D8   D9   DA   DB   DC   DD   DE   DF    E0   E1   E2   E3   E4   E5   E6   E7   E8   E9   EA   EB   EC   ED   EE   EF    F0   F1   F2   F3   F4   F5   F6   F7   F8   F9   FA   FB   FC   FD   FE   FF

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This solution is a little older school  and should be memory efficient   public static String toHexString byte bytes          if  bytes    null            return null             StringBuffer sb   new StringBuffer        for  int iter   0  iter  lt  bytes length  iter              byte high    byte     bytes iter   amp  0xf0   gt  gt  4           byte low     byte     bytes iter   amp  0x0f           sb append nibble2char high            sb append nibble2char low               return sb toString       private static char nibble2char byte b        byte nibble    byte   b  amp  0x0f       if  nibble  lt  10            return  char    0    nibble             return  char    a    nibble - 10

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my variant      StringBuilder builder   new StringBuilder        for  byte b   bytes                builder append Character forDigit b 16  16            builder append Character forDigit b   16  16              System out println builder toString       it works for me

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String result   String format   0    messageDigest length    s   hexString toString      That s the shortest solution given what you already have   If you could convert the byte array to a numeric value  String format can convert it to a hex string at the same time

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byte messageDigest     algorithm digest    StringBuffer hexString   new StringBuffer    for  int i   0  i  lt  messageDigest length  i          String hexByte   Integer toHexString 0xFF  amp  messageDigest i        int numDigits   2 - hexByte length        while  numDigits--  gt  0            hexString append  0              hexString append hexByte

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This will give two-char long string for a byte   public String toString byte b       final char   Hex   new String  0123456789ABCDEF   toCharArray        return   0x   Hex  b  amp  0xF0   gt  gt  4   Hex  b  amp  0x0F

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A simple approach would be to check how many digits are output by Integer toHexString   and add a leading zero to each byte if needed  Something like this   public static String toHexString byte   bytes        StringBuilder hexString   new StringBuilder         for  int i   0  i  lt  bytes length  i              String hex   Integer toHexString 0xFF  amp  bytes i            if  hex length      1                hexString append  0                      hexString append hex              return hexString toString

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I liked Steve s submissions  but he could have done without a couple of variables and saved several lines in the process   public static String toHexString byte   bytes        char   hexArray     0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F        char   hexChars   new char bytes length   2       int v      for   int j   0  j  lt  bytes length  j               v   bytes j   amp  0xFF          hexChars j 2    hexArray v 16           hexChars j 2   1    hexArray v 16             return new String hexChars       What I like about this is that it s easy to see exactly what it s doing  instead of relying on some magic BigInteger black box conversion  and you re also free from having to worry about corner cases like leading-zeroes and stuff  This routine takes every 4-bit nibble and turns it into a hex char  And it s using a table lookup  so it s probably fast  It could probably be faster if you replace v 16 and v 16 with bitwise shifts and AND s  but I m too lazy to test it right now

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IMHO all the solutions above that provide snippets to remove the leading zeroes are wrong   byte messageDigest     algorithm digest    for  int i   0  i  lt  messageDigest length  i          hexString append Integer toHexString 0xFF  amp  messageDigest i             According to this snippet  8 bits are taken from the byte array in an iteration  converted into an integer  since Integer toHexString function takes int as argument  and then that integer is converted to the corresponding hash value  So  for example if you have 00000001 00000001 in binary  according to the code  the hexString variable would have 0x11 as the hex value whereas correct value should be 0x0101  Thus  while calculating MD5 we may get hashes of length  lt 32 bytes because of missing zeroes  which may not satisfy the cryptographically unique properties that MD5 hash does    The solution to the problem is replacing the above code snippet by the following snippet   byte messageDigest     algorithm digest    for  int i   0  i  lt  messageDigest length  i          int temp 0xFF  amp  messageDigest i       String s Integer toHexString temp       if temp lt  0x0F           s  0  s            hexString append s

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I liked Steve s submissions  but he could have done without a couple of variables and saved several lines in the process   public static String toHexString byte   bytes        char   hexArray     0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F        char   hexChars   new char bytes length   2       int v      for   int j   0  j  lt  bytes length  j               v   bytes j   amp  0xFF          hexChars j 2    hexArray v 16           hexChars j 2   1    hexArray v 16             return new String hexChars       What I like about this is that it s easy to see exactly what it s doing  instead of relying on some magic BigInteger black box conversion  and you re also free from having to worry about corner cases like leading-zeroes and stuff  This routine takes every 4-bit nibble and turns it into a hex char  And it s using a table lookup  so it s probably fast  It could probably be faster if you replace v 16 and v 16 with bitwise shifts and AND s  but I m too lazy to test it right now

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You can get it writing less without external libraries   String hex    new HexBinaryAdapter    marshal md5 digest YOUR STRING getBytes

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This solution is a little older school  and should be memory efficient   public static String toHexString byte bytes          if  bytes    null            return null             StringBuffer sb   new StringBuffer        for  int iter   0  iter  lt  bytes length  iter              byte high    byte     bytes iter   amp  0xf0   gt  gt  4           byte low     byte     bytes iter   amp  0x0f           sb append nibble2char high            sb append nibble2char low               return sb toString       private static char nibble2char byte b        byte nibble    byte   b  amp  0x0f       if  nibble  lt  10            return  char    0    nibble             return  char    a    nibble - 10

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This solution is a little older school  and should be memory efficient   public static String toHexString byte bytes          if  bytes    null            return null             StringBuffer sb   new StringBuffer        for  int iter   0  iter  lt  bytes length  iter              byte high    byte     bytes iter   amp  0xf0   gt  gt  4           byte low     byte     bytes iter   amp  0x0f           sb append nibble2char high            sb append nibble2char low               return sb toString       private static char nibble2char byte b        byte nibble    byte   b  amp  0x0f       if  nibble  lt  10            return  char    0    nibble             return  char    a    nibble - 10

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Another option  public static String toHexString byte  bytes        StringBuilder sb   new StringBuilder bytes length 2       for byte b  bytes        sb append Integer toHexString b 0x800  substring 1        return sb toString

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It appears concat and append functions can be really slow  The following was MUCH faster for me  than my previous post    Changing to a char array in building the output was the key factor to speed it up   I have not compared to Hex encodeHex suggested by Brandon DuRette   public static String toHexString byte   bytes        char   hexArray     0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F        char   hexChars   new char 10000000       int c   0      int v      for   j   0  j  lt  bytes length  j               v   bytes j   amp  0xFF          hexChars c    hexArray v 16           c            hexChars c    hexArray v 16           c              return new String hexChars  0  c

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Or you can do this   byte   digest   algorithm digest    StringBuilder byteContet   new StringBuilder    for byte b  digest    byteContent   String format   02x  b    byteContent append byteContent       Its Short  simple and basically just a format change

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I ve been looking for the same thing     some good ideas here  but I ran a few micro benchmarks  I found the following to be the fastest  modified from Ayman s above and about 2x as fast  and about 50  faster than Steve s just above this one    public static String hash String text  String algorithm          throws NoSuchAlgorithmException       byte   hash   MessageDigest getInstance algorithm  digest text getBytes         return new BigInteger 1  hash  toString 16       Edit  Oops - missed that this is essentially the same as kgiannakakis s and so may strip off a leading 0  Still  modifying this to the following  it s still the fastest   public static String hash String text  String algorithm          throws NoSuchAlgorithmException       byte   hash   MessageDigest getInstance algorithm  digest text getBytes         BigInteger bi   new BigInteger 1  hash       String result   bi toString 16       if  result length     2    0            return  0    result            return result

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my variant      StringBuilder builder   new StringBuilder        for  byte b   bytes                builder append Character forDigit b 16  16            builder append Character forDigit b   16  16              System out println builder toString       it works for me

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I ve been looking for the same thing     some good ideas here  but I ran a few micro benchmarks  I found the following to be the fastest  modified from Ayman s above and about 2x as fast  and about 50  faster than Steve s just above this one    public static String hash String text  String algorithm          throws NoSuchAlgorithmException       byte   hash   MessageDigest getInstance algorithm  digest text getBytes         return new BigInteger 1  hash  toString 16       Edit  Oops - missed that this is essentially the same as kgiannakakis s and so may strip off a leading 0  Still  modifying this to the following  it s still the fastest   public static String hash String text  String algorithm          throws NoSuchAlgorithmException       byte   hash   MessageDigest getInstance algorithm  digest text getBytes         BigInteger bi   new BigInteger 1  hash       String result   bi toString 16       if  result length     2    0            return  0    result            return result

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Check out Hex encodeHexString from Apache Commons Codec   import org apache commons codec binary Hex   String hex   Hex encodeHexString bytes

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And how can you convert back again from ascii to byte array    i followed following code to convert to ascii given by Jemenake     public static String toHexString byte   bytes        char   hexArray     0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F        char   hexChars   new char bytes length   2       int v      for   int j   0  j  lt  bytes length  j               v   bytes j   amp  0xFF          hexChars j 2    hexArray v 16           hexChars j 2   1    hexArray v 16             return new String hexChars

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Another option  public static String toHexString byte  bytes        StringBuilder sb   new StringBuilder bytes length 2       for byte b  bytes        sb append Integer toHexString b 0x800  substring 1        return sb toString

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Guava makes it pretty simple too   BaseEncoding base16   encode  bytes      It s a nice alternative when Apache Commons is not available  It also has some nice controls of the output like   byte   bytes   new byte     0xa  0xb  0xc  0xd  0xe  0xf    BaseEncoding base16   lowerCase   withSeparator       2   encode  bytes        0a 0b 0c 0d 0e 0f

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In order to keep leading zeroes  here is a small variation on what has Paul suggested  eg md5 hash    public static String MD5hash String text  throws NoSuchAlgorithmException       byte   hash   MessageDigest getInstance  MD5   digest text getBytes         return String format   032x  new BigInteger 1  hash        Oops  this looks poorer than what s Ayman proposed  sorry for that

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I m surprised that no one came up with the following solution   StringWriter sw   new StringWriter    com sun corba se impl orbutil HexOutputStream hex   new com sun corba se impl orbutil HexOutputStream sw   hex write byteArray   System out println sw toString

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A simple approach would be to check how many digits are output by Integer toHexString   and add a leading zero to each byte if needed  Something like this   public static String toHexString byte   bytes        StringBuilder hexString   new StringBuilder         for  int i   0  i  lt  bytes length  i              String hex   Integer toHexString 0xFF  amp  bytes i            if  hex length      1                hexString append  0                      hexString append hex              return hexString toString

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This what I am using for MD5 hashes   public static String getMD5 String filename          throws NoSuchAlgorithmException  IOException       MessageDigest messageDigest            java security MessageDigest getInstance  MD5         InputStream in   new FileInputStream filename        byte    buffer   new byte 8192       int len   in read buffer  0  buffer length        while  len  gt  0            messageDigest update buffer  0  len           len   in read buffer  0  buffer length             in close         return new BigInteger 1  messageDigest digest    toString 16       EDIT  I ve tested and I ve noticed that with this also trailing zeros are cut  But this can only happen in the beginning  so you can compare with the expected length and pad accordingly

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Check out Hex encodeHexString from Apache Commons Codec   import org apache commons codec binary Hex   String hex   Hex encodeHexString bytes

[java] In Java, how do I convert a byte array to a string of hex digits while keeping leading zeros?

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