TheAlgorithms/Java · error · UnknownHostException

IPv4 address is empty.

Error message

IPv4 address is empty.

What it means

Thrown by IPv6Converter.ipv4ToIpv6 as an UnknownHostException when ipv4Address is null or an empty string. Note: the Javadoc mentions @throws IllegalArgumentException, but the actual code throws UnknownHostException, so callers must catch UnknownHostException specifically. The guard fires before any DNS resolution attempt.

Solutions

1. Check for null or empty before calling ipv4ToIpv6 and provide a meaningful error or default.
2. Use Objects.requireNonNull(ipv4Address, "ipv4Address") if null should be a programming error.
3. If reading from a config file, validate the property is present and non-empty at startup.

Example fix

// before
String ipv6 = IPv6Converter.ipv4ToIpv6(ipv4Str); // ipv4Str may be null or empty

// after
if (ipv4Str == null || ipv4Str.isEmpty()) {
    throw new IllegalArgumentException("IPv4 address must not be null or empty");
}
String ipv6 = IPv6Converter.ipv4ToIpv6(ipv4Str);

if (ipv4Address == null || ipv4Address.isEmpty()) {
    throw new IllegalArgumentException("IPv4 address must not be null or empty");
}
String result = IPv6Converter.ipv4ToIpv6(ipv4Address);

static boolean isValidIpv4String(String addr) {
    return addr != null && !addr.isEmpty() && addr.matches("^\\d{1,3}(\\.\\d{1,3}){3}$");
}

try {
    String ipv6 = IPv6Converter.ipv4ToIpv6(ipv4Address);
} catch (UnknownHostException e) {
    // null, empty, or unresolvable address; handle gracefully
    logger.warn("Invalid IPv4 address: {}", ipv4Address);
}

When it happens

Trigger: Calling ipv4ToIpv6(null) or ipv4ToIpv6(""). Passing a String variable that was initialized to null or not populated from a config/API response. Supplying an empty field from a parsed CSV or properties file.

Common situations: A network configuration key is missing from a properties file, yielding null or empty. A JSON field is absent and mapped to null by the deserializer. The IPv4 string is conditionally set but the condition was not met.

Related errors

• Not a valid IPv6-mapped IPv4 address.
• Adjacency list must not be null
• Adjacency list must not contain null sets
• Array cannot be null
• Cannot enqueue null data

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/1a7c668969eb9880. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/com/thealgorithms/conversions/IPv6Converter.java:31

 *
 * @author Hardvan
 */
public final class IPv6Converter {
    private IPv6Converter() {
    }

    /**
     * Converts an IPv4 address (e.g., "192.0.2.128") to an IPv6-mapped IPv6 address.
     * Example: IPv4 "192.0.2.128" -> IPv6 "::ffff:192.0.2.128"
     *
     * @param ipv4Address The IPv4 address in string format.
     * @return The corresponding IPv6-mapped IPv6 address.
     * @throws UnknownHostException If the IPv4 address is invalid.
     * @throws IllegalArgumentException If the IPv6 address is not a mapped IPv4 address.
     */
    public static String ipv4ToIpv6(String ipv4Address) throws UnknownHostException {
        if (ipv4Address == null || ipv4Address.isEmpty()) {
            throw new UnknownHostException("IPv4 address is empty.");
        }

        InetAddress ipv4 = InetAddress.getByName(ipv4Address);
        byte[] ipv4Bytes = ipv4.getAddress();

        // Create IPv6-mapped IPv6 address (starts with ::ffff:)
        byte[] ipv6Bytes = new byte[16];
        ipv6Bytes[10] = (byte) 0xff;
        ipv6Bytes[11] = (byte) 0xff;
        System.arraycopy(ipv4Bytes, 0, ipv6Bytes, 12, 4);

        // Manually format to "::ffff:x.x.x.x" format
        StringBuilder ipv6String = new StringBuilder("::ffff:");
        for (int i = 12; i < 16; i++) {
            ipv6String.append(ipv6Bytes[i] & 0xFF);
            if (i < 15) {
                ipv6String.append('.');
            }

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