Validating IPv4 string in Java

UPDATE: Commons-HttpClient and its successor HttpComponents-HttpClient have adopted this functionality. You can utilize this version of it like so: InetAddressUtils.isIPv4Address(Str).

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The development version of Apache Commons Validator has a InetAddressValidator class which has a isValidInet4Address(String) method to perform a check to see that an given String is a valid IPv4 address.

The source code can be viewed from the repository, so that might provide some ideas for improvements, if you feel there are any.

A quick glance at the provided code shows that your method is compiling a Pattern on each invocation of the method. I would move that Pattern class out to a static field to avoid the costly pattern compilation process on each call.

If you want to use a library with a released version suppporting both IPv4 and IPv6 support, you can use Guava

boolean isValid = InetAddresses.isInetAddress("1.2.3.4");

Here is an easier-to-read, slightly less efficient, way you could go about it.

public static boolean validIP (String ip) {
    try {
        if ( ip == null || ip.isEmpty() ) {
            return false;
        }

        String[] parts = ip.split( "\\." );
        if ( parts.length != 4 ) {
            return false;
        }

        for ( String s : parts ) {
            int i = Integer.parseInt( s );
            if ( (i < 0) || (i > 255) ) {
                return false;
            }
        }
        if ( ip.endsWith(".") ) {
            return false;
        }

        return true;
    } catch (NumberFormatException nfe) {
        return false;
    }
}