A computer uses a class definition to find out how to construct an object in the same way that a builder uses a blueprint to find out how to build a house. A builder can build many houses from the same blueprint in the same way that a computer program can use a class definition to instantiate many objects of the same type.

In this lecture we are going to compare two blueprints for building an object that represents a rectangle.

• Look at, trace and compare the structure of two Java programs that develop and test a class representing a rectangle (handout):
  • The RectangleA example follows the pattern of the program in Figures 2.2 and 2.3.

  • The RectangleB is a more traditional object-oriented design.

  Notice that RectangleA is a blueprint for an object that computes properties of rectangles, while RectangleB is a blueprint for an object that represents a particular rectangle.

• Develop the programs under Java Workshop to review the process.