public static String canonize(String ipv6Address) throws IllegalArgumentException {

        if (ipv6Address == null) {
            return null;
        }

        // Definitely not an IPv6, return untouched input.
        if (!mayBeIPv6Address(ipv6Address)) {
            return ipv6Address;
        }

        // Length without zone ID (%zone) or IPv4 address
        int ipv6AddressLength = ipv6Address.length();
        if (isIPv4AddressInIPv6(ipv6Address)) {
            // IPv4 in IPv6
            // e.g. 0:0:0:0:0:FFFF:127.0.0.1
            int lastColonPos = ipv6Address.lastIndexOf(":");
            int lastColonsPos = ipv6Address.lastIndexOf("::");
            if (lastColonsPos >= 0 && lastColonPos == lastColonsPos + 1) {
                // IPv6 part ends with two consecutive colons, last colon is part of IPv6 format.
                // e.g. ::127.0.0.1
                ipv6AddressLength = lastColonPos + 1;
            } else {
                // IPv6 part ends with only one colon, last colon is not part of IPv6 format.
                // e.g. ::FFFF:127.0.0.1
                ipv6AddressLength = lastColonPos;
            }
        } else if (ipv6Address.contains(":") && ipv6Address.contains("%")) {
            // Zone ID
            // e.g. fe80:0:0:0:f0f0:c0c0:1919:1234%4
            ipv6AddressLength = ipv6Address.lastIndexOf("%");
        }

        StringBuilder result = new StringBuilder();
        char [][] groups = new char[IPV6_LEN][MAX_GROUP_LENGTH];
        int groupCounter = 0;
        int charInGroupCounter = 0;

        // Index of the current zeroGroup, -1 means not found.
        int zeroGroupIndex = -1;
        int zeroGroupLength = 0;

        // maximum length zero group, if there is more then one, then first one
        int maxZeroGroupIndex = -1;
        int maxZeroGroupLength = 0;

        boolean isZero = true;
        boolean groupStart = true;

        /*
         *  Two consecutive colons, initial expansion.
         *  e.g. 2001:db8:0:0:1::1 -> 2001:db8:0:0:1:0:0:1
         */
        StringBuilder expanded = new StringBuilder(ipv6Address);
        int colonsPos = ipv6Address.indexOf("::");
        int length = ipv6AddressLength;
        int change = 0;

        if (colonsPos >= 0 && colonsPos < ipv6AddressLength - 2) {
            int colonCounter = 0;
            for (int i = 0; i < ipv6AddressLength; i++) {
                if (ipv6Address.charAt(i) == ':') {
                    colonCounter++;
                }
            }

            if (colonsPos == 0) {
                expanded.insert(0, "0");
                change = change + 1;
            }

            for (int i = 0; i < IPV6_LEN - colonCounter; i++) {
                expanded.insert(colonsPos + 1, "0:");
                change = change + 2;
            }


            if (colonsPos == ipv6AddressLength - 2) {
                expanded.setCharAt(colonsPos + change + 1, '0');
            } else {
                expanded.deleteCharAt(colonsPos + change + 1);
                change = change - 1;
            }
            length = length + change;
        }


        // Processing one char at the time
        for (int charCounter = 0; charCounter < length; charCounter++) {
            char c = expanded.charAt(charCounter);
            if (c >= 'A' && c <= 'F') {
                c = (char) (c + 32);
            }
            if (c != ':') {
                groups[groupCounter][charInGroupCounter] = c;
                if (!(groupStart && c == '0')) {
                    ++charInGroupCounter;
                    groupStart = false;
                }
                if (c != '0') {
                    isZero = false;
                }
            }
            if (c == ':' || charCounter == (length - 1)) {
                // We reached end of current group
                if (isZero) {
                    ++zeroGroupLength;
                    if (zeroGroupIndex == -1) {
                        zeroGroupIndex = groupCounter;
                    }
                }

                if (!isZero || charCounter == (length - 1)) {
                    // We reached end of zero group
                    if (zeroGroupLength > maxZeroGroupLength) {
                        maxZeroGroupLength = zeroGroupLength;
                        maxZeroGroupIndex = zeroGroupIndex;
                    }
                    zeroGroupLength = 0;
                    zeroGroupIndex = -1;
                }
                ++groupCounter;
                charInGroupCounter = 0;
                isZero = true;
                groupStart = true;
            }
        }

        int numberOfGroups = groupCounter;

        // Output results
        for (groupCounter = 0; groupCounter < numberOfGroups; groupCounter++) {
            if (maxZeroGroupLength <= 1 || groupCounter < maxZeroGroupIndex
                    || groupCounter >= maxZeroGroupIndex + maxZeroGroupLength) {
                for (int j = 0; j < MAX_GROUP_LENGTH; j++) {
                    if (groups[groupCounter][j] != 0) {
                        result.append(groups[groupCounter][j]);
                    }
                }
                if (groupCounter < (numberOfGroups - 1)
                        && (groupCounter != maxZeroGroupIndex - 1
                                || maxZeroGroupLength <= 1)) {
                    result.append(':');
                }
            } else if (groupCounter == maxZeroGroupIndex) {
                result.append("::");
            }
        }

        // Solve problem with three colons in IPv4 in IPv6 format
        // e.g. 0:0:0:0:0:0:127.0.0.1 -> :::127.0.0.1 -> ::127.0.0.1
        int resultLength = result.length();
        if (result.charAt(resultLength - 1) == ':' && ipv6AddressLength < ipv6Address.length()
                && ipv6Address.charAt(ipv6AddressLength) == ':') {
            result.delete(resultLength - 1, resultLength);
        }

        /*
         * Append IPv4 from IPv4-in-IPv6 format or Zone ID
         */
        for (int i = ipv6AddressLength; i < ipv6Address.length(); i++) {
            result.append(ipv6Address.charAt(i));
        }

        return result.toString();
    }
public static String canonize ( String ipv6Address ) throws IllegalArgumentException { if ( ipv6Address == null ) { return null ; } if ( ! mayBeIPv6Address ( ipv6Address ) ) { return ipv6Address ; } int ipv6AddressLength = ipv6Address . length ( ) ; if ( isIPv4AddressInIPv6 ( ipv6Address ) ) { int lastColonPos = ipv6Address . lastIndexOf ( ":" ) ; int lastColonsPos = ipv6Address . lastIndexOf ( "::" ) ; if ( lastColonsPos >= 0 && lastColonPos == lastColonsPos + 1 ) { ipv6AddressLength = lastColonPos + 1 ; } else { ipv6AddressLength = lastColonPos ; } } else if ( ipv6Address . contains ( ":" ) && ipv6Address . contains ( "%" ) ) { ipv6AddressLength = ipv6Address . lastIndexOf ( "%" ) ; } StringBuilder result = new StringBuilder ( ) ; char [ ] [ ] groups = new char [ IPV6_LEN ] [ MAX_GROUP_LENGTH ] ; int groupCounter = 0 ; int charInGroupCounter = 0 ; int zeroGroupIndex = - 1 ; int zeroGroupLength = 0 ; int maxZeroGroupIndex = - 1 ; int maxZeroGroupLength = 0 ; boolean isZero = true ; boolean groupStart = true ; StringBuilder expanded = new StringBuilder ( ipv6Address ) ; int colonsPos = ipv6Address . indexOf ( "::" ) ; int length = ipv6AddressLength ; int change = 0 ; if ( colonsPos >= 0 && colonsPos < ipv6AddressLength - 2 ) { int colonCounter = 0 ; for ( int i = 0 ; i < ipv6AddressLength ; i ++ ) { if ( ipv6Address . charAt ( i ) == ':' ) { colonCounter ++ ; } } if ( colonsPos == 0 ) { expanded . insert ( 0 , "0" ) ; change = change + 1 ; } for ( int i = 0 ; i < IPV6_LEN - colonCounter ; i ++ ) { expanded . insert ( colonsPos + 1 , "0:" ) ; change = change + 2 ; } if ( colonsPos == ipv6AddressLength - 2 ) { expanded . setCharAt ( colonsPos + change + 1 , '0' ) ; } else { expanded . deleteCharAt ( colonsPos + change + 1 ) ; change = change - 1 ; } length = length + change ; } for ( int charCounter = 0 ; charCounter < length ; charCounter ++ ) { char c = expanded . charAt ( charCounter ) ; if ( c >= 'A' && c <= 'F' ) { c = ( char ) ( c + 32 ) ; } if ( c != ':' ) { groups [ groupCounter ] [ charInGroupCounter ] = c ; if ( ! ( groupStart && c == '0' ) ) { ++ charInGroupCounter ; groupStart = false ; } if ( c != '0' ) { isZero = false ; } } if ( c == ':' || charCounter == ( length - 1 ) ) { if ( isZero ) { ++ zeroGroupLength ; if ( zeroGroupIndex == - 1 ) { zeroGroupIndex = groupCounter ; } } if ( ! isZero || charCounter == ( length - 1 ) ) { if ( zeroGroupLength > maxZeroGroupLength ) { maxZeroGroupLength = zeroGroupLength ; maxZeroGroupIndex = zeroGroupIndex ; } zeroGroupLength = 0 ; zeroGroupIndex = - 1 ; } ++ groupCounter ; charInGroupCounter = 0 ; isZero = true ; groupStart = true ; } } int numberOfGroups = groupCounter ; for ( groupCounter = 0 ; groupCounter < numberOfGroups ; groupCounter ++ ) { if ( maxZeroGroupLength <= 1 || groupCounter < maxZeroGroupIndex || groupCounter >= maxZeroGroupIndex + maxZeroGroupLength ) { for ( int j = 0 ; j < MAX_GROUP_LENGTH ; j ++ ) { if ( groups [ groupCounter ] [ j ] != 0 ) { result . append ( groups [ groupCounter ] [ j ] ) ; } } if ( groupCounter < ( numberOfGroups - 1 ) && ( groupCounter != maxZeroGroupIndex - 1 || maxZeroGroupLength <= 1 ) ) { result . append ( ':' ) ; } } else if ( groupCounter == maxZeroGroupIndex ) { result . append ( "::" ) ; } } int resultLength = result . length ( ) ; if ( result . charAt ( resultLength - 1 ) == ':' && ipv6AddressLength < ipv6Address . length ( ) && ipv6Address . charAt ( ipv6AddressLength ) == ':' ) { result . delete ( resultLength - 1 , resultLength ) ; } for ( int i = ipv6AddressLength ; i < ipv6Address . length ( ) ; i ++ ) { result . append ( ipv6Address . charAt ( i ) ) ; } return result . toString ( ) ; }
<p>Convert IPv6 adress into RFC 5952 form.
E.g. 2001:db8:0:1:0:0:0:1 -> 2001:db8:0:1::1</p>

<p>Method is null safe, and if IPv4 address or host name is passed to the
method it is returned wihout any processing.</p>

<p>Method also supports IPv4 in IPv6 (e.g. 0:0:0:0:0:ffff:192.0.2.1 ->
::ffff:192.0.2.1), and zone ID (e.g. fe80:0:0:0:f0f0:c0c0:1919:1234%4
-> fe80::f0f0:c0c0:1919:1234%4).</p>

@param ipv6Address String representing valid IPv6 address.
@return String representing IPv6 in canonical form.
@throws IllegalArgumentException if IPv6 format is unacceptable.