The state pattern, a behavior pattern, is used to change a behavior of a class based on its state. The changes of the class behavior is polymorphism.

We define these classes:

• abstract class (interface)State
• class Context
• concrete state classes inherited from the Context class

In a drawing software, the canvas is going to behave differently according to the current selected tools like pen, pencil, eraser and etc. Instead of using if-else statement for each possible tools, we use a state pattern on the canvas class. So that in the future, if more tools are supported, we can simply adding another class without modifying existing code.

• Canvas => Context
• abstract drawing tool => State
• concrete different kinds of drawing tools => states