Make fields private (and final) if possible

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1477400 13f79535-47bb-0310-9956-ffa450edef68
Field might as well be final

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1477401 13f79535-47bb-0310-9956-ffa450edef68