Check if an IPv6 is a valid address or not.

IsValidAddress.java

public boolean IsValidAddress(String ipAddr) {
    int nDC = 0;
    int nC = 0;

    ipAddr = ipAddr.trim();
    String s = ipAddr;
    char[] chars = s.toCharArray();

    /* 0- Error: Empty */
    if (s.isEmpty()) {
        return false;
    }

    /* 1- Error: UNDEFINED '::' */
    if (s.equals("::")) {
        return false;
    }

    /* 2- Error: Triple or more columns entered */
    if ((s.length() <= 1) || (s.contains(":::"))) {
        return false;
    }

    /* 3- Error: Not valid hex */
    if (!Pattern.matches("^[0-9A-Fa-f:]+$", s)) {
        return false;
    }

    /* 4- Error: Cannot start or end with ':' */
    if (chars[0] == ':' && chars[1] != ':') {
        return false;
    }

    if (chars[s.length() - 1] == ':' && chars[s.length() - 2] != ':') {
        return false;
    }

    /* 5- Error: More than 2 Bytes */
    String[] sa = s.split(":", -1);
    for (int j = 0; j < sa.length; j++) {
        if (sa[j].length() > 4) {
            return false;
        }
    }

    /* 6- Error: Count of DoubleColumns and Columns */
    s = ipAddr;
    nDC = s.split("::", -1).length - 1;
    s = s.replace("::", "**");
    nC = s.split(":", -1).length - 1;

    /* Case I- DoubleColumn can only appear once - RFC4291 */
    if (nDC > 1) {
        return false;
    }

    /* Case II- No DoubleColumns means there must be 7 columns */
    if (nDC == 0 && nC != 7) {
        return false;
    }

    /* Case III- If DoubleColumn at start/end, max. columns must be 6 or less */
    s = ipAddr;
    int sL = s.length();
    if ((chars[0] == ':' && chars[1] == ':')
            || (chars[sL - 1] == ':' && chars[sL - 2] == ':')) {
        if (nDC == 1 && nC > 6) {
            return false;
        }
    }

    /* Case IV- If DoubleColumn in middle, max. columns must be 5 or less */ 
    else if (nDC == 1 && nC > 5) {
        return false;
    }

    /* End of Check */
    return true;
}