Classes & Objects

Java is an object-oriented language. Almost everything revolves around classes and objects. A class is a blueprint; an object is an instance of that blueprint.

Your first class

public class Dog {
    // Fields (instance variables)
    String name;
    int age;

    // Method
    String bark() {
        return name + " says: Woof!";
    }
}

Creating and using objects:

public class Main {
    public static void main(String[] args) {
        Dog dog1 = new Dog();
        dog1.name = "Rex";
        dog1.age = 3;

        Dog dog2 = new Dog();
        dog2.name = "Bella";
        dog2.age = 5;

        System.out.println(dog1.bark());
        System.out.println(dog2.bark());
        System.out.println(dog1.age);
    }
}

Result:

Rex says: Woof!
Bella says: Woof!
3

new Dog() creates an object (instance) of the Dog class
Each object has its own copy of the fields (name, age)
Methods operate on the object's data

Constructors

A constructor initializes an object when it is created. It has the same name as the class and no return type:

public class User {
    String name;
    int age;

    // Constructor
    User(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

User user = new User("Ada", 36);
System.out.println(user.name + " is " + user.age);

Result:

Ada is 36

`this`

Inside a method or constructor, this refers to the current object. It is commonly used when parameter names match field names:

User(String name, int age) {
    this.name = name;   // this.name = field, name = parameter
    this.age = age;
}

Default constructor

If you define no constructors, Java provides a default no-argument constructor. If you define any constructor, the default is no longer generated:

public class Product {
    String name;
    double price;

    Product(String name, double price) {
        this.name = name;
        this.price = price;
    }
}

// Product p = new Product(); // compile error - no no-arg constructor
Product p = new Product("Laptop", 999.99); // works

Multiple constructors

public class User {
    String name;
    int age;
    String role;

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

    User(String name, int age) {
        this(name, age, "user"); // calls the other constructor
    }

    User(String name) {
        this(name, 0, "user");
    }
}

User u1 = new User("Ada", 36, "admin");
User u2 = new User("Bob", 25);
User u3 = new User("Charlie");

System.out.println(u1.name + " - " + u1.role);
System.out.println(u2.name + " - " + u2.role);
System.out.println(u3.name + " - " + u3.age);

Result:

Ada - admin
Bob - user
Charlie - 0

this(...) calls another constructor in the same class. It must be the first statement.

Access modifiers

Control who can see and use your fields, methods, and classes:

Modifier	Class	Package	Subclass outside package	World
`public`	Yes	Yes	Yes	Yes
`protected`	Yes	Yes	Yes	No
(none - package-private)	Yes	Yes	No	No
`private`	Yes	No	No	No

public class BankAccount {
    private double balance;        // only this class
    String accountHolder;          // package-private
    protected String accountType;  // this class + subclasses + same package
    public String bankName;        // everyone
}

Encapsulation

Encapsulation means hiding internal data and exposing it through methods. The pattern: private fields + public getters/setters.

public class BankAccount {
    private String owner;
    private double balance;

    public BankAccount(String owner, double initialBalance) {
        this.owner = owner;
        this.balance = initialBalance;
    }

    // Getter
    public double getBalance() {
        return balance;
    }

    public String getOwner() {
        return owner;
    }

    // Business logic instead of a raw setter
    public void deposit(double amount) {
        if (amount <= 0) {
            throw new IllegalArgumentException("Deposit must be positive");
        }
        balance += amount;
    }

    public void withdraw(double amount) {
        if (amount <= 0) {
            throw new IllegalArgumentException("Withdrawal must be positive");
        }
        if (amount > balance) {
            throw new IllegalArgumentException("Insufficient funds");
        }
        balance -= amount;
    }
}

BankAccount account = new BankAccount("Ada", 1000);
System.out.println(account.getOwner());
System.out.println(account.getBalance());

account.deposit(500);
System.out.println(account.getBalance());

account.withdraw(200);
System.out.println(account.getBalance());

// account.balance = 999999; // compile error - balance is private

Result:

Ada
0
0
0

Why encapsulation matters:

You control how data is modified (validation in deposit/withdraw)
You can change internal representation without breaking callers
The class enforces its own invariants (balance cannot go negative)

`toString`

By default, printing an object shows its class name and hash code. Override toString for a useful representation:

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

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

    @Override
    public String toString() {
        return "User{name='" + name + "', age=" + age + "}";
    }
}

User user = new User("Ada", 36);
System.out.println(user);

Result:

User{name='Ada', age=36}

The @Override annotation tells the compiler you intend to override a method from a parent class. If you misspell the method name, the compiler will flag it.

`equals` and `hashCode`

By default, == compares object references. And if a class does not override .equals(), it inherits Object.equals(), which also compares references. Override both equals and hashCode to compare by content:

import java.util.Objects;

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

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

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        User user = (User) o;
        return age == user.age && Objects.equals(name, user.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }

    @Override
    public String toString() {
        return "User{name='" + name + "', age=" + age + "}";
    }
}

User a = new User("Ada", 36);
User b = new User("Ada", 36);
User c = new User("Bob", 25);

System.out.println(a == b);       // false - different objects
System.out.println(a.equals(b));  // true - same content
System.out.println(a.equals(c));  // false - different content

Result:

false
true
false

Rule: if you override equals, you must also override hashCode. Collections like HashMap and HashSet depend on this contract.

`static` fields and methods

static members belong to the class, not to any instance:

public class Counter {
    private static int count = 0; // shared across all instances

    private String label;

    public Counter(String label) {
        this.label = label;
        count++;
    }

    public static int getCount() {
        return count;
    }

    public String getLabel() {
        return label;
    }
}

Counter a = new Counter("A");
Counter b = new Counter("B");
Counter c = new Counter("C");

System.out.println("Total counters: " + Counter.getCount());
System.out.println(a.getLabel());

Result:

Total counters: 3
A

Common uses of static:

Constants: static final double PI = 3.14159;
Utility methods: Math.abs(), Integer.parseInt()
Factory methods: creating instances with descriptive names

Static vs instance

	Static	Instance
Belongs to	The class	An object
Accessed via	`ClassName.method()`	`object.method()`
Can access instance fields?	No	Yes
Can access static fields?	Yes	Yes

Records (Java 16+)

Records are a concise way to create immutable data classes. They auto-generate the constructor, getters, toString, equals, and hashCode:

public record Point(int x, int y) {}

This single line generates the equivalent of:

A constructor Point(int x, int y)
Accessor methods x() and y() (not getX())
toString(): Point[x=3, y=4]
equals() and hashCode() based on all fields

Point p1 = new Point(3, 4);
Point p2 = new Point(3, 4);
Point p3 = new Point(0, 0);

System.out.println(p1);
System.out.println(p1.x());
System.out.println(p1.y());
System.out.println(p1.equals(p2));
System.out.println(p1.equals(p3));

Result:

Point[x=3, y=4]
3
4
true
false

Records with validation

public record User(String name, int age) {
    // Compact constructor for validation
    public User {
        if (name == null || name.isBlank()) {
            throw new IllegalArgumentException("Name cannot be blank");
        }
        if (age < 0) {
            throw new IllegalArgumentException("Age cannot be negative");
        }
    }
}

User valid = new User("Ada", 36);
System.out.println(valid);

try {
    User invalid = new User("", 25);
} catch (IllegalArgumentException e) {
    System.out.println("Error: " + e.getMessage());
}

Result:

User[name=Ada, age=36]
Error: Name cannot be blank

Records with methods

Records can have methods, static fields, and implement interfaces:

public record Rectangle(double width, double height) {

    public double area() {
        return width * height;
    }

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

Rectangle r = new Rectangle(5, 3);
System.out.println("Area: " + r.area());
System.out.println("Perimeter: " + r.perimeter());

Result:

Area: 15.0
Perimeter: 16.0

When to use records: whenever you need a simple data carrier - DTOs, value objects, configuration, return multiple values from a method.

Builder pattern (brief introduction)

For classes with many optional fields, a builder is cleaner than many constructors:

public class HttpRequest {
    private final String url;
    private final String method;
    private final String body;
    private final int timeout;

    private HttpRequest(Builder builder) {
        this.url = builder.url;
        this.method = builder.method;
        this.body = builder.body;
        this.timeout = builder.timeout;
    }

    @Override
    public String toString() {
        return method + " " + url + " (timeout=" + timeout + "ms)";
    }

    static class Builder {
        private final String url;
        private String method = "GET";
        private String body = "";
        private int timeout = 5000;

        Builder(String url) {
            this.url = url;
        }

        Builder method(String method) {
            this.method = method;
            return this;
        }

        Builder body(String body) {
            this.body = body;
            return this;
        }

        Builder timeout(int timeout) {
            this.timeout = timeout;
            return this;
        }

        HttpRequest build() {
            return new HttpRequest(this);
        }
    }
}

HttpRequest request = new HttpRequest.Builder("https://api.example.com/users")
        .method("POST")
        .body("{\"name\": \"Ada\"}")
        .timeout(3000)
        .build();

System.out.println(request);

Result:

POST https://api.example.com/users (timeout=3000ms)

Organizing files

Java convention:

One public class per file
Filename matches the class name: User.java → public class User
Package structure matches directory structure
Classes in lowercase package names: com.example.app

src/
├── Main.java
├── model/
│   └── User.java
└── util/
    └── StringUtils.java

Summary

A class is a blueprint; an object is an instance created with new.
Constructors initialize objects. this(...) chains to another constructor.
Access modifiers control visibility: private → package-private → protected → public.
Encapsulation: private fields + public methods. Control access, enforce invariants.
Override toString for readable output, equals/hashCode for content comparison.
static members belong to the class, not instances - used for shared state and utility methods.
Records (Java 16+) auto-generate constructors, accessors, toString, equals, hashCode.
The builder pattern handles objects with many optional parameters.

Next up: Inheritance & Interfaces - extending classes, implementing contracts, and polymorphism.

Your first class​

Constructors​

this​

Default constructor​

Multiple constructors​

Access modifiers​

Encapsulation​

toString​

equals and hashCode​

static fields and methods​

Static vs instance​

Records (Java 16+)​

Records with validation​

Records with methods​

Builder pattern (brief introduction)​

Organizing files​

Summary​

Your first class

Constructors

`this`

Default constructor

Multiple constructors

Access modifiers

Encapsulation

`toString`

`equals` and `hashCode`

`static` fields and methods

Static vs instance

Records (Java 16+)

Records with validation

Records with methods

Builder pattern (brief introduction)

Organizing files

Summary