Inheritance

Experience creating objects soon showed programmers that many objects shared features, e.g. several might have text blocks, or creation dates, or screen positions, and they realized that they could further reduce the amount of new code they had to write if they could reuse existing code by having one type of object inherit behaviour and properties from another. Then if the ancestor's behaviour or properties were updated the descendant would instantly inherit the improvements. This can lead to large hierarchies of object types; some define thousands of object types. To give you a flavour of a type hierarchy here is a medium-sized inheritance hierarchy for a Python-based internet application development platform:

Inheritance hierarchies are easiest to read bottom up. For example, if we begin at the lower right the chart above tells us that a FlexTable is a kind of HTMLTable which is in turn a kind of Panel which is a kind of Widget which is a kind of UIObject. What this means is that if we improve our code for UIObjects, then Widgets and Panels will instantly and transparently inherit these improvements as well without us having to modify their code at all.