Unit 1.12: Objects — Instances of Classes

Scope: CS Awesome 2, Section 1.12

Learning Goals

By the end of this lesson, you should be able to:

1. Distinguish a class from an object.
2. Identify attributes and behaviors.
3. Explain what an object reference represents.
4. Recognize superclass and subclass relationships.
5. Model a real entity as an object.

Classes and Objects

A class defines a reference type. It is a blueprint describing the attributes and behaviors shared by objects of that type.

An object is a specific instance of a class.

Class	Example objects
String	"north", "status"
Scanner	a scanner connected to keyboard input
Turtle	individual drawing turtles with different positions

The class is the general definition; each object has its own identity and state.

Attributes and Behaviors

An attribute, also called an instance variable or field, is data an object knows about itself. A behavior is an action implemented by a method.

For a delivery robot:

Attributes	Behaviors
identifier	move forward
battery level	turn
location	report status
direction	recharge

Two robot objects can have the same available behaviors but different current battery levels and locations.

Reference Variables

A variable of a class type stores an object reference, which can be thought of as information locating an object in memory.

String first = "signal";
String second = first;

Both variables now refer to the same String object. Copying a reference does not automatically create a second object.

first  ──┐
         ├──> String object "signal"
second ──┘

A reference variable may also hold null, meaning it currently refers to no object.

String message = null;

Creating an Object: Preview

Objects are commonly created with new and a constructor call. Constructors are the focus of Section 1.13.

Scanner input = new Scanner(System.in);

Read this from right to left:

1. new Scanner(System.in) creates a Scanner object.
2. input is a reference variable of type Scanner.
3. the resulting reference is stored in input.

Declaration alone does not create an object:

Scanner input;

Class Hierarchies and Inheritance

Related classes can be organized in a class hierarchy. A superclass contains common attributes and behaviors. A subclass extends a superclass and inherits accessible behavior.

Object
└── Vehicle
    ├── Bicycle
    └── Bus

In this model, Bicycle and Bus are subclasses of Vehicle. Shared behavior such as move() can be defined in Vehicle.

Every Java class ultimately extends the Object class. Designing inheritance structures is beyond the AP CSA implementation scope, but recognizing the relationship is important.

Objects in a Program

String route = "Blue Line";
System.out.println(route.length());

route is a reference variable, "Blue Line" is a String object created from a literal, and length() is one of the object's behaviors.

The dot operator asks the object referenced by route to perform a method.

Practice Missions

Mission 1: Class or Object?

Classify each item and justify your answer:

1. Scanner;
2. the scanner referenced by input;
3. String;
4. the string object "Emergency";
5. Turtle;
6. a turtle named atlas at coordinate (40, 90).

Mission 2: Model a Smart Locker

Design a conceptual SmartLocker class. List at least four attributes and four behaviors. Then describe two different locker objects and give their differing state.

Mission 3: Reference Diagram

Draw a reference diagram after each statement:

String alpha = "ready";
String beta = alpha;
String gamma = null;
gamma = beta;

Count the variables, references, and distinct string objects.

Mission 4: Build a Hierarchy

Organize Device, Phone, Laptop, and Object into a hierarchy. Add one shared attribute and one shared behavior at the most appropriate level. Explain what the subclasses inherit.

Key Summary

Concept	Core idea
Class	Blueprint and reference-type definition.
Object	A specific instance of a class.
Attribute	Data describing object state.
Behavior	Action implemented by a method.
Reference	Value locating or identifying an object.
Superclass	General class containing shared features.
Subclass	More specific class that inherits features.

Source scope: CS Awesome 2, Unit 1.12