Learning Objectives

1. Explain the relationship between a class and an object.
   • An object is a specific instance of a class with defined attributes. A class is the formal implementation, or blueprint, of the attributes and behaviors of an object.
   • A class hierarchy can be developed by putting common attributes and behaviors of related classes into a single class called a superclass. Classes that extend a superclass, called subclasses, can draw upon the existing attributes and behaviors of the superclass without replacing these in the code. This creates an inheritance relationship from the subclasses to the superclass.
   • All classes in Java are subclasses of the Object class.
2. Develop code to declare variables to store reference types.
   • A variable of a reference type holds an object reference, which can be thought of as the memory address of that object.

Notes

Java is an Object Oriented Programming language. A fundamental building block of Java is objects. There are also four pillars of Object-Oriented Programming (OOP). Those are: Abstraction, Encapsulation, Inheritance, and Polymorphism.

What is an object?

It is a user-defined variable. They expand upon primitives to make more complex and malleable variables. There are two main aspects of an object: state and behavior; think nouns and verbs. In the below image, the state is described using the variables unique_id, name, age, city, and gender. The behaviors are described by the methods.

Classes are the templates for creating new objects. You can think of classes like blueprints for houses and the objects are the houses that are built. The blueprint lays out some things that each house will have in common, but as it is built, each house has its own address and other defining details. Java has some built-in classes, some of which have already been seen, such as Math and Scanner. When using Scanner, you also created a Scanner object.

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