Problem
You are contacted by a company to build an application that will allow users to create shapes based on an input sequence of characters and then manipulate them. As a start, the application should support three shapes: circle, square, and rectangle. More will definitely come later so your design should allow easily adding those.
For example, the sequence "c s c r" corresponds to the user wanting to calculate a circle, a square, a circle, and a rectangle. The first shape will always be created with a default size (of your choice but can be changed by the user). The next will be created by doubling the default size. The third shape - by tripling the default size, etc. The sequence of shapes entered by the used should be stored in an appropriate data structure.
The user can manipulate the shapes in different ways. For instance, he should be able to Compute the area of a sequence of shapes, which is the cumulative area of all shapes.
Filter shapes on a specific criteria, such as the area of the shape. For example, the user might want to know what are the shapes whose area is less than or equal to a certain threshold.