Hierarchy

Hierarchy is a process of ranking or ordering of abstraction.

Hierarchy is decided by the help of 'divide and conquer' principle. Complex objects can be described in terms of simpler objects. This is the characteristic of object oriented analysis (OOA) which enables you to reuse the code.

You can place an existing class directly into a new class. The new class can be made up of any type and number of other objects and in any combination to achieve the desired functionality. This concept is known as composition (or more generally, aggregation). Composition is often indicated as a "part-of" relationship or "has-a" relationship, for example, in "automobile has an engine" or "engine is part of the automobile."