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11.1. Object-Oriented Programming Concepts

Object-oriented programming has three main features: objects, inheritance, and polymorphism.

Objects have data (fields) and behavior (methods) and do the work in an object-oriented program. Objects are created by classes. A class defines the data (fields) and behavior (methods) for all objects of that class. You can create many objects from the same class. In the video below hundreds of Ant objects are created in Greenfoot. Greenfoot is a free Java development environment that makes it easy to create 2D simulations and games. For more information about Greenfoot see

The following video is also on YouTube at It introduces the concepts of objects and classes using Greenfoot.

Inheritance allows for cleaner code since a class can inherit fields and behavior from another class instead of copying code from class to class. The parent class is specified using the extends keyword in the class declaration. The class that is extending the parent class is called the child class. In the ants scenario the Ant class inherits from the Actor class. The Ant class is the child class and the Actor class is the parent class. The Ant class inherits the act method from the Actor class, but overrides it by creating a method with the same signature that will be executed instead of the parent’s method. This allows the Ant class to modify what an Ant object does when it acts.

The following video is also on YouTube at It introduces the concept of inheritance using Greenfoot.

Polymorphism allows for specialized behavior based on the run-time type. It also removes the need for conditional execution based on the type. Java uses inheritance-based polymorphism where a parent class has a method that the children classes override to provide specialized behavior. In the Ant scenario the Balloon and Bomb classes inherit from the Actor class and both override the act method. The world contains a list of all Actor objects in the world and tells each to act. What happens when an Actor object acts depends on the class that created it (the run-time type).

The following video is also on YouTube at It introduces the concept of polymorphism using Greenfoot.

The following sections will explain the ideas of inheritance and polymorphism in more depth.

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