This module foreshadows the entire course, covering O-O Relationships, Inheritance, Polymorphism, Interfaces, Aggregation, and Design Guidelines. The following modules revisit selected topics in more detail.

This module goes into more detail on implementing class inheritance in Java, including overriding methods, invoking overridden parent methods using super, and more.

This module discusses Abstract classes, including what they are; where you might use them; how to implement them; and concludes with discussion of casting, which is the technique of taking a reference of a given type, and asserting a different type.

Interfaces are the key to polymorphism in Java, since Java requires inheritance in order to implement polymorphism, and classes are limited to the single inheritance graph. This module goes into more detail on interfaces and polymorphism, and includes discussion of Java 8 features, such as default and static methods, Functional Interfaces, Java Lambdas, and Method References.

Just as the first module served to foreshadow the entire content of this course, this module serves as a brief capstone, with a conceptual discussion of polymorphism mechanisms in Java, and practices for ensuring that polymorphism is an option in your programs.