Classes and Objects

1// Defining a Class
2public class Car {
3    // Fields (instance variables) - state
4    String brand;
5    String color;
6    int speed;
7    
8    // Method - behavior
9    public void accelerate() {
10        speed += 10;
11        System.out.println("Speed: " + speed);
12    }
13    
14    public void brake() {
15        speed = Math.max(0, speed - 10);
16        System.out.println("Speed: " + speed);
17    }
18    
19    public void displayInfo() {
20        System.out.println(brand + " " + color + " at " + speed + " mph");
21    }
22}

1// Creating Objects (Instantiation)
2public class Main {
3    public static void main(String[] args) {
4        // Create object using 'new' keyword
5        Car myCar = new Car();
6        
7        // Set field values
8        myCar.brand = "Toyota";
9        myCar.color = "Red";
10        myCar.speed = 0;
11        
12        // Call methods
13        myCar.accelerate();     // Speed: 10
14        myCar.accelerate();     // Speed: 20
15        myCar.brake();          // Speed: 10
16        myCar.displayInfo();    // Toyota Red at 10 mph
17        
18        // Create another object - independent instance
19        Car otherCar = new Car();
20        otherCar.brand = "Honda";
21        otherCar.color = "Blue";
22        otherCar.speed = 50;
23        otherCar.displayInfo(); // Honda Blue at 50 mph
24        
25        // Each object has its own state
26        System.out.println(myCar.speed);    // 10
27        System.out.println(otherCar.speed); // 50
28    }
29}

1// Object Reference vs Object
2Car car1 = new Car();  // car1 references an object in heap
3car1.brand = "Tesla";
4
5Car car2 = car1;       // car2 references SAME object!
6car2.brand = "BMW";
7
8System.out.println(car1.brand);  // BMW (same object!)
9
10// Creating new object
11Car car3 = new Car();
12car3.brand = "Audi";
13
14System.out.println(car1.brand);  // BMW
15System.out.println(car3.brand);  // Audi (different object)
16
17// Null reference
18Car car4 = null;       // Points to nothing
19// car4.brand = "X";   // NullPointerException!
20
21// Checking for null
22if (car4 != null) {
23    car4.accelerate();
24}

1// Complete Class Example with Best Practices
2public class BankAccount {
3    // Fields (encapsulated)
4    private String accountNumber;
5    private String holderName;
6    private double balance;
7    
8    // Constructor
9    public BankAccount(String accountNumber, String holderName) {
10        this.accountNumber = accountNumber;
11        this.holderName = holderName;
12        this.balance = 0.0;
13    }
14    
15    // Getters and Setters
16    public String getAccountNumber() { return accountNumber; }
17    public String getHolderName() { return holderName; }
18    public double getBalance() { return balance; }
19    
20    // Business methods
21    public void deposit(double amount) {
22        if (amount > 0) {
23            balance += amount;
24            System.out.println("Deposited: $" + amount);
25        }
26    }
27    
28    public boolean withdraw(double amount) {
29        if (amount > 0 && balance >= amount) {
30            balance -= amount;
31            System.out.println("Withdrawn: $" + amount);
32            return true;
33        }
34        System.out.println("Insufficient funds");
35        return false;
36    }
37    
38    @Override
39    public String toString() {
40        return "Account: " + accountNumber + ", Holder: " + holderName + ", Balance: $" + balance;
41    }
42}