Hidden algebra
It also handled object-oriented features like classes , subclasses ( inheritance ) , attributes , and methods . 
<triplet> classes <subj> object-oriented <obj> use <triplet> subclasses <subj> object-oriented <obj> part of <triplet> inheritance <subj> object-oriented <obj> part of <triplet> attributes <subj> classes <obj> part of <triplet> methods <subj> object-oriented <obj> part of <subj> classes <obj> part of