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OOP
OOP Β· Prerequisite: previous lecture

06. Classes and Objects

Java is an object-oriented (OOP) language. You define data with a **class** and stamp out **objects (instances)** from that blueprint. This lecture covers defining a class and using its fields and methods through objects.

JavaOOPobject-orientedclass and object
Duration
⏱ ~1-1.5 hours
Level
πŸ“Š Beginner-Intermediate
Prerequisite
🎯 Previous lecture or equivalent knowledge
OUTCOME
Java is an object-oriented (OOP) language. You define data with a **class** and stamp out **objects (instances)** from that blueprint. This lecture covers defining a class and using its fields and methods through objects.

What you'll learn

  • 1Define a class with the `class` keyword
  • 2Distinguish fields, methods, and constructors
  • 3Understand what `this` refers to
  • 4See the effect of access modifiers (`public` / `private`)

Overview

Java is an object-oriented (OOP) language. You define data with a **class** and stamp out **objects (instances)** from that blueprint. This lecture covers defining a class and using its fields and methods through objects.

Core Concepts

1) Defining a class

java
public class Person {
    String name;
    int age;

    void hello() {
        System.out.println("Hi, I'm " + name);
    }
}

A class is a bundle of **state (fields)** and **behaviour (methods)**.

2) Creating an object

java
Person p = new Person();
p.name = "Jisoo";
p.age = 21;
p.hello();

`new` allocates the object in memory, and its reference is stored in `p`.

3) Constructors

java
public class Person {
    String name;
    int age;

    Person(String name, int age) {  // constructor
        this.name = name;
        this.age = age;
    }
}

A constructor is a special method **with no return type** that initializes a new object. If you don't define one, the compiler adds a parameterless **default constructor**.

4) The `this` keyword

`this` refers to **the current object itself**. It's used to disambiguate when a parameter and a field share a name.

java
this.name = name;  // left=field, right=parameter

Examples

Example 1 β€” `Person.java`: define a class and create an object

java
public class Person {
    String name;
    int age;

    void hello() {
        System.out.println("Hi, I'm " + name + " (" + age + ")");
    }

    public static void main(String[] args) {
        Person p = new Person();
        p.name = "Jisoo";
        p.age = 21;
        p.hello();
    }
}

**Output**

text
Hi, I'm Jisoo (21)

**Note:** uninitialized fields take the default value (0, null, false).

Example 2 β€” `PersonWithCtor.java`: constructor and `this`

java
public class PersonWithCtor {
    String name;
    int age;

    PersonWithCtor(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public static void main(String[] args) {
        PersonWithCtor a = new PersonWithCtor("Jisoo", 21);
        PersonWithCtor b = new PersonWithCtor("Minsu", 25);
        System.out.println(a.name + ", " + a.age);
        System.out.println(b.name + ", " + b.age);
    }
}

**Output**

text
Jisoo, 21
Minsu, 25

**Note:** a constructor is only called via `new`.

Example 3 β€” `Counter.java`: per-instance state

java
public class Counter {
    int count;

    void increment() {
        count++;
    }

    public static void main(String[] args) {
        Counter c1 = new Counter();
        Counter c2 = new Counter();
        c1.increment();
        c1.increment();
        c2.increment();
        System.out.println("c1=" + c1.count + ", c2=" + c2.count);
    }
}

**Output**

text
c1=2, c2=1

**Note:** each instance has its **own** copy of the fields.

Example 4 β€” `Rectangle.java`: behaviour via methods

java
public class Rectangle {
    int width;
    int height;

    Rectangle(int w, int h) {
        this.width = w;
        this.height = h;
    }

    int area() {
        return width * height;
    }

    int perimeter() {
        return 2 * (width + height);
    }

    public static void main(String[] args) {
        Rectangle r = new Rectangle(3, 4);
        System.out.println("area=" + r.area());
        System.out.println("perimeter=" + r.perimeter());
    }
}

**Output**

text
area=12
perimeter=14

**Note:** bundling "data + methods that act on that data" into one class is the first step of OOP.

Full example code (src/)

src/Counter.java

java
public class Counter {
    int count;

    void increment() {
        count++;
    }

    public static void main(String[] args) {
        Counter c1 = new Counter();
        Counter c2 = new Counter();
        c1.increment();
        c1.increment();
        c2.increment();
        System.out.println("c1=" + c1.count + ", c2=" + c2.count);
    }
}

src/Person.java

java
public class Person {
    String name;
    int age;

    void hello() {
        System.out.println("Hi, I'm " + name + " (" + age + ")");
    }

    public static void main(String[] args) {
        Person p = new Person();
        p.name = "Jisoo";
        p.age = 21;
        p.hello();
    }
}

src/PersonWithCtor.java

java
public class PersonWithCtor {
    String name;
    int age;

    PersonWithCtor(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public static void main(String[] args) {
        PersonWithCtor a = new PersonWithCtor("Jisoo", 21);
        PersonWithCtor b = new PersonWithCtor("Minsu", 25);
        System.out.println(a.name + ", " + a.age);
        System.out.println(b.name + ", " + b.age);
    }
}

src/Rectangle.java

java
public class Rectangle {
    int width;
    int height;

    Rectangle(int w, int h) {
        this.width = w;
        this.height = h;
    }

    int area() {
        return width * height;
    }

    int perimeter() {
        return 2 * (width + height);
    }

    public static void main(String[] args) {
        Rectangle r = new Rectangle(3, 4);
        System.out.println("area=" + r.area());
        System.out.println("perimeter=" + r.perimeter());
    }
}

Common Mistakes

  1. Calling `Person.hello()` as if it were static β€” instance methods need an object
  2. Forgetting `this.` when a field and parameter collide β†’ you assign to yourself
  3. Naming a constructor wrong (it must have no return type and match the class name)
  4. Using a variable without `new` β†’ `NullPointerException`
  5. Comparing object references with `==` (usually use `.equals()`)

Summary

  • A class is the **state + behaviour** blueprint
  • An object is an instance stamped out via `new`
  • A constructor exists only to initialize an object
  • `this` refers to the current object

Practice

# Practice - 06. Classes and Objects

## Exercise 1 β€” `Book` class

  • File: `Homework01.java`
  • Key concepts: class, constructor, method

Requirements

  • Define a `Book` class (in `Homework01` or alongside it) with fields `title`, `author`, `pages`.
  • Initialize all three in the constructor.
  • An `info()` method returns `[title] author(pages pages)`.
  • In main create two books and print each `info()`.

Expected output

text
[Effective Java] Joshua Bloch(384 pages)
[Clean Code] Robert C. Martin(464 pages)

## Exercise 2 β€” `Circle` area

  • File: `Homework02.java`
  • Key concepts: instance method, `Math.PI`

Requirements

  • Class `Circle` with `double radius`, a constructor, and an `area()` method.
  • Print the area for radii 1, 2.5, 3 to 3 decimal places.

Expected output

text
r=1.000 area=3.142
r=2.500 area=19.635
r=3.000 area=28.274

## Solutions After trying it yourself, compare with [`answer/`](./answer/).

Solution code (homework/answer/)

answer/Homework01.java

java
/** Define a Book class and print info(). */
public class Homework01 {
    public static void main(String[] args) {
        Book a = new Book("Effective Java", "Joshua Bloch", 384);
        Book b = new Book("Clean Code", "Robert C. Martin", 464);
        System.out.println(a.info());
        System.out.println(b.info());
    }
}

class Book {
    String title;
    String author;
    int pages;

    Book(String title, String author, int pages) {
        this.title = title;
        this.author = author;
        this.pages = pages;
    }

    String info() {
        return "[" + title + "] " + author + "(" + pages + " pages)";
    }
}

answer/Homework02.java

java
/** Circle area. */
public class Homework02 {
    public static void main(String[] args) {
        double[] radii = {1.0, 2.5, 3.0};
        for (double r : radii) {
            Circle c = new Circle(r);
            System.out.printf("r=%.3f area=%.3f%n", r, c.area());
        }
    }
}

class Circle {
    double radius;

    Circle(double radius) {
        this.radius = radius;
    }

    double area() {
        return Math.PI * radius * radius;
    }
}

Try It Yourself

bash
cd 02_oop/06_class/src
javac Rectangle.java
java Rectangle

Next Lecture

[07_Encapsulation](../07_μΊ‘μŠν™”/) β€” `private` fields, getters/setters, intro to `record`.

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub β†—