OOP Basics - Junior Level¶

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

    // Constructor
    public Dog(String name, int age, String breed) {
        this.name = name;   // 'this' refers to the current object
        this.age = age;
        this.breed = breed;
    }

    // Methods
    public void bark() {
        System.out.println(this.name + " says: Woof!");
    }

    public String describe() {
        return this.name + " (" + this.breed + ", " + this.age + " years)";
    }

    // Getters and Setters (encapsulation)
    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        if (name != null && !name.isEmpty()) {
            this.name = name;
        }
    }

    public int getAge() {
        return this.age;
    }

    public void setAge(int age) {
        if (age >= 0) {
            this.age = age;
        }
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        Dog myDog = new Dog("Rex", 3, "German Shepherd");
        myDog.bark();                      // Rex says: Woof!
        System.out.println(myDog.describe()); // Rex (German Shepherd, 3 years)

        myDog.setAge(4);
        // myDog.age = -5;  // ERROR: age is private, can't access directly
    }
}

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

    // Default constructor
    public Person() {
        this.name = "Unknown";
        this.age = 0;
    }

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

    // Constructor chaining
    public Person(String name) {
        this(name, 0);  // calls the 2-arg constructor
    }
}

// Usage
Person p1 = new Person();              // Default
Person p2 = new Person("Alice", 25);   // Parameterized
Person p3 = new Person("Bob");         // Chained → age = 0

public class BankAccount {
    // Fields
    private String owner;
    private double balance;
    private static int accountCount = 0; // Class-level field (shared)

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

    // Instance method
    public void deposit(double amount) {
        if (amount > 0) {
            this.balance += amount;
        }
    }

    public boolean withdraw(double amount) {
        if (amount > 0 && amount <= this.balance) {
            this.balance -= amount;
            return true;
        }
        return false;
    }

    // Static method (belongs to class, not instance)
    public static int getAccountCount() {
        return accountCount;
    }

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

public class Example {
    public String publicField;       // Accessible everywhere
    protected String protectedField; // Same package + subclasses
    String packageField;             // Same package only (default)
    private String privateField;     // This class only
}

public class Point {
    private int x, y;

    public Point(int x, int y) {
        this.x = x;  // 'this' disambiguates field from parameter
        this.y = y;
    }

    public double distanceTo(Point other) {
        int dx = this.x - other.x;
        int dy = this.y - other.y;
        return Math.sqrt(dx * dx + dy * dy);
    }

    // 'this' can be passed to other methods
    public void register(Registry registry) {
        registry.add(this);
    }
}

public class Rectangle {
    private double width;
    private double height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

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

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

    public void scale(double factor) {
        this.width *= factor;
        this.height *= factor;
    }

    @Override
    public String toString() {
        return String.format("Rectangle(%.1f x %.1f)", width, height);
    }
}

public class Circle {
    private double radius;

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

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

    public double perimeter() {
        return 2 * Math.PI * this.radius;
    }

    @Override
    public String toString() {
        return String.format("Circle(r=%.1f)", radius);
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        Rectangle rect = new Rectangle(5, 3);
        System.out.println(rect);             // Rectangle(5.0 x 3.0)
        System.out.println(rect.area());      // 15.0
        System.out.println(rect.perimeter()); // 16.0
        rect.scale(2);
        System.out.println(rect.area());      // 60.0

        Circle circle = new Circle(4);
        System.out.println(circle);           // Circle(r=4.0)
        System.out.println(circle.area());    // 50.265...
    }
}