Comment the interface, implementation or both?

As a general rule, I use the same DRY (Don't Repeat Yourself) principle as with code:

• on interface, document the interface
• on implementation, document the implementation specifics

Java specific: when documenting the implementation, use {@inheritDoc} tag to "include" javadocs from the interface.

For more information:

• Official javadoc documentation
• Some unofficial advice.

C# usage:

Interface can look like this:

    /// <summary>
    /// Helper class to access various properties for the current site.
    /// </summary>
    public interface ISiteHelper
    {
        /// <summary>
        /// Gets the site id of the current site
        /// </summary>
        /// <returns>The site id.</returns>
        int GetSiteID();
    }
}

Implementation can look like this:

/// <inheritdoc />
public class SiteHelper: ISiteHelper
{
    /// <inheritdoc />
    public int GetSiteID()
    {
        return CommonRepository.GetSiteID();
    }
}