Objects
Instances of Classes

Learn how classes serve as blueprints and how objects bring them to life as individual instances in Java.

AP Computer Science A

What Is an Object?

In Java, an object is a specific instance of a class. It exists in memory, holds its own data (state), and can perform actions (behavior). Think of it as a real, tangible thing built from a template.

What Is a Class?

A class is a blueprint or template that defines what data an object will store and what actions it can perform. It specifies instance variables (state) and methods (behavior), but it does not occupy memory as an object itself.

State

Instance variables that hold data unique to each object (e.g., name, age).

Behavior

Methods that define what an object can do (e.g., speak(), move()).

From Blueprint to Objects

One ticket office counter, many fixed ticket IDs

One ticket agent hands a new stub to a patron while another patron retains an issued stub.

Separate tickets provide an analogy for separate objects. In the Dog example, each instance has its own name and age fields.

Each object keeps its own state

Separate Dog object boxes contain name Milo with age 2 and name Luna with age 4.

Suppose two Dog instances store Milo, age 2, and Luna, age 4. Each name and age belongs to its own instance. Changing one object’s fields does not automatically change the other object.

Creating Objects with new

In Java, you create an object using the new keyword followed by a call to the class's constructor. This allocates memory and initializes the object.

// Declare and create
Dog myDog = new Dog("Buddy", "Golden", 3);

The Constructor

A constructor is a special method that runs when an object is created. It has the same name as the class and no return type. Its job is to initialize instance variables.

public class Dog {
  private String name;
  private int age;

  // Constructor
  public Dog(String n, int a) {
    name = n;
    age = a;
  }
}

Reference vs. Object

Class vs. Object: Key Differences

	Class	Object
Definition	Blueprint / template	Instance of a class
Memory	No memory allocated	Allocated on the heap
Created with	class keyword	new keyword
Quantity	Defined once	Many can exist
Holds data?	Defines structure only	Stores actual values

Understanding null References

A reference variable can be set to null, meaning it points to no object. Calling a method on a null reference causes a NullPointerException at runtime.

Dog myDog = null;      // no object created
myDog.bark();             // NullPointerException!

// Fix: create the object first
myDog = new Dog("Rex", 2);
myDog.bark();             // Works fine

How to Create and Use an Object

1

Define the class with instance variables and methods (or use an existing class).

2

Declare a reference variable of the class type: ClassName varName;

3

Instantiate with new: varName = new ClassName(args);

4

Call methods on the object using dot notation: varName.methodName();

5

Access return values or modify state through the object's public interface.

Example 1: Simple Object Creation

public class Student {
  private String name;
  private double gpa;

  public Student(String n, double g) {
    name = n;
    gpa = g;
  }

  public String getName() { return name; }
  public double getGpa()  { return gpa; }
  public void setName(String newName) { name = newName; }
  public void setGpa(double newGpa) { gpa = newGpa; }
}

// In main method:
Student s1 = new Student("Alice", 3.9);
System.out.println(s1.getName()); // Alice
System.out.println(s1.getGpa());  // 3.9

Example 2: Multiple Objects, One Class

Student s1 = new Student("Alice", 3.9);
Student s2 = new Student("Bob",   3.5);
Student s3 = new Student("Carol", 3.7);

System.out.println(s1.getName()); // Alice
System.out.println(s2.getName()); // Bob
System.out.println(s3.getGpa());  // 3.7

Example 3: Reference Aliasing

Student a = new Student("Alice", 3.9);
Student b = a;  // b and a point to the SAME object

b.setGpa(4.0);
System.out.println(a.getGpa()); // 4.0  (changed!)

Two collaborators work on one shared model

Two collaborators adjust one blue articulated model while a second model stands apart.

Two variables can provide access to one shared object. Copying a reference creates another access path; it does not duplicate the object.

Follow object identity

References d1, d2 and d3 are traced through two distinct Dog objects and a null value.

Two new expressions create Fido and Rex. After the assignments, d1 refers to Rex, d3 refers to Fido and d2 is null. Both objects remain reachable; == compares reference identity, while equals depends on the class’s implementation.

Calling Methods with Dot Notation

Once you have an object reference, you invoke its methods using dot notation: the reference name, a dot, then the method name with parentheses.

String greeting = new String("Hello, World!");

// Calling methods on the String object
int len = greeting.length();        // 13
String upper = greeting.toUpperCase(); // "HELLO, WORLD!"
char ch = greeting.charAt(0);        // 'H'

Common Mistakes to Avoid

✕ Forgetting new

Writing Dog d = Dog("Rex", 2); without new causes a compile error. The new keyword is required to instantiate.

✕ Calling on null

Declaring a reference without assigning an object, then calling a method: NullPointerException at runtime.

✕ Confusing class for object

Trying to use a class name to call non-static methods directly: Dog.bark() won't work unless bark is static.

Summary: Key Concepts

Concept	Definition	Example
Class	Blueprint defining state and behavior	class Dog { ... }
Object	Instance created from a class	new Dog("Rex", 2)
Constructor	Special method that initializes objects	public Dog(String n, int a)
Reference	Variable storing an object's address	Dog d = ...
null	Reference pointing to no object	Dog d = null;

Question 1 of 5

Which keyword is used to create a new object in Java?

The new keyword allocates memory on the heap and calls the constructor to create a new object instance.

Question 2 of 5

Given the Student class from the examples, what is the output of the following code?

Student a = new Student("Alice", 3.9);
Student b = a;
b.setName("Max");
System.out.println(a.getName());

Since b = a creates an alias (both reference the same object), calling b.setName("Max") changes the object that a also points to. So a.getName() returns "Max".

Question 3 of 5

What happens when you execute: Dog d = null; d.bark();?

Calling a method on a null reference causes a NullPointerException at runtime. The code compiles fine because the types are correct, but no object exists to receive the method call.

Question 4 of 5

Which statement about constructors is correct?

Constructors must have the exact same name as the class and declare no return type (not even void). A class can have multiple constructors through overloading.

Question 5 of 5

Consider:
Student s1 = new Student("A", 3.5);
Student s2 = new Student("A", 3.5);
How many objects exist in memory?

Each use of new creates a distinct object in memory, even if the arguments are identical. s1 and s2 reference two separate Student objects.

ObjectsInstances of Classes

What Is an Object?

What Is a Class?

From Blueprint to Objects

One ticket office counter, many fixed ticket IDs

Each object keeps its own state

Creating Objects with new

The Constructor

Reference vs. Object

Class vs. Object: Key Differences

Understanding null References

How to Create and Use an Object

Example 1: Simple Object Creation

Example 2: Multiple Objects, One Class

Example 3: Reference Aliasing

Two collaborators work on one shared model

Follow object identity

Calling Methods with Dot Notation

Common Mistakes to Avoid

Summary: Key Concepts

Question 1 of 5

Question 2 of 5

Question 3 of 5

Question 4 of 5

Question 5 of 5

Objects
Instances of Classes