jdk.internal.jrtfs.JrtPath does this:

        StringBuilder sb = new StringBuilder(path.length() + o.path.length());
        sb.append(path);
        if (path.charAt(path.length() - 1) != '/')
            sb.append('/');
        sb.append(o.path);
        return new JrtPath(jrtfs, sb.toString(), true);

Since path is always normalized to not have a trailing slash, the pre-sizing is insufficient (which means we'll always resize the array on the sb.append(o.path) step) and the if unnecessary.