Topic 3.3 — Anatomy of a Class

Goal: identify the key parts of a Java class (fields, constructors, methods), and explain how a class defines the data and behavior of its objects.

The big idea

A class is a blueprint for creating objects. It bundles data (fields/instance variables) and behavior (methods).

Objects created from the class each get their own copy of the instance variables.

What’s inside a class?

Part	What it is	Example
Fields	store data (state)	`private int score;`
Constructors	initialize new objects	`public Student() { ... }`
Methods	actions the object can do	`public void addPoints(int p)`

Example: a simple class

public class Counter {
  private int value;          // field (instance variable)

  public Counter() {          // constructor
    value = 0;
  }

  public void increment() {   // method
    value++;
  }

  public int getValue() {     // method
    return value;
  }
}

This shows the three main building blocks you’ll see constantly in AP CSA.

Fields (instance variables)

Store the object’s state (its “data”).
Usually marked private to protect the data.
Each object has its own copy.

private String name;
private int gradeLevel;

Constructors

Run automatically when you create an object with new.
Have the same name as the class.
No return type (not even void).

public Counter() {
  value = 0;
}

Methods

Define what an object can do.
Often interact with fields (read/update state).
Can return a value or be void.

public void increment() { value++; }
public int getValue() { return value; }

Common confusion: class vs object

Class	Object
Blueprint/template	One specific instance made from the class
Defines fields + methods	Has actual stored field values
One class definition	Many objects can be created

Quick self-check

What are the three main parts of a class you should recognize?
When does a constructor run?
Why are fields often marked private?
What’s the difference between a class and an object?
In the example class, which method changes the object’s state?

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