Java Programming Lesson 9 | Classes, Objects & Constructors — the first full OOP lesson in Java

Java Programming Lesson 9

✅ We are entering Lesson-9: Classes, Objects & Constructors — the first full OOP lesson in Java. This is very important as it forms the foundation of all advanced Java programming.

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📘 Lesson-9: Classes, Objects & Constructors

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✅ 1️⃣ What is a Class?

A class is a blueprint for creating objects (real-world entities).

Syntax:

class ClassName {
    // variables (attributes)
    // methods (functions)
}

Example:

class Car {
    String color;
    String model;

    void drive() {
        System.out.println("Car is driving");
    }
}

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✅ 2️⃣ What is an Object?

An object is an instance of a class.

Example:

Car myCar = new Car();  // Object created
myCar.color = "Red";
myCar.model = "Toyota";
myCar.drive();  // Output: Car is driving

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✅ 3️⃣ Constructor

A constructor is a special method used to initialize objects.

• Name = class name

• No return type

Example:

class Car {
    String color;
    String model;

    // Constructor
    Car(String c, String m) {
        color = c;
        model = m;
    }

    void display() {
        System.out.println("Color: " + color + ", Model: " + model);
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car("Red", "Toyota");
        myCar.display();  // Output: Color: Red, Model: Toyota
    }
}

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✅ 4️⃣ Key Points About Objects & Classes

Feature	Description
Class	Blueprint/Template
Object	Instance of Class
Constructor	Initializes Objects
Method	Defines Behavior
Attribute	Defines State/Property

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✅ 5️⃣ Example: Student Class

class Student {
    String name;
    int age;

    // Constructor
    Student(String n, int a){
        name = n;
        age = a;
    }

    void info() {
        System.out.println("Name: " + name + ", Age: " + age);
    }
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student("JITU", 21);
        Student s2 = new Student("ALICE", 20);

        s1.info();  // Name: JITU, Age: 21
        s2.info();  // Name: ALICE, Age: 20
    }
}

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✅ 6️⃣ Default Constructor vs Parameterized Constructor

class Example {
    Example() {   // Default Constructor
        System.out.println("Default Constructor Called");
    }
    
    Example(String msg) {  // Parameterized Constructor
        System.out.println("Message: " + msg);
    }
}

public class Main {
    public static void main(String[] args) {
        Example obj1 = new Example();          // Default
        Example obj2 = new Example("Hello");   // Parameterized
    }
}

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✅ 7️⃣ This Keyword

• Refers to the current object

• Useful when variable names shadow parameter names

class Car {
    String model;
    Car(String model) {
        this.model = model;  // distinguish class variable from parameter
    }
}

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✅ 8️⃣ Practice Tasks

1️⃣ Create a Book class:

• Attributes: title, author, price

• Constructor: initialize attributes

• Method: display info

• Create 2 books and display info

2️⃣ Create BankAccount class:

• Attributes: accountNumber, balance

• Methods: deposit(), withdraw(), displayBalance()

• Test with 2 accounts

3️⃣ Create Rectangle class:

• Attributes: length, width

• Method: area(), perimeter()

• Initialize rectangles and display results

Exception Handling (try, catch, finally)

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✅ What is an Exception?

An exception is a problem that occurs during program execution.

Examples:

• Dividing by zero

• Accessing invalid array index

• File not found

If not handled, program crashes.

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✅ Java Exception Handling Keywords

Keyword	Purpose
try	Code block to test for exceptions
catch	Handle exception if occurs
finally	Executes always (optional)
throw	Manually throw exception
throws	Declare exception in method

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✅ Basic Syntax

try {
    // risky code
} catch(ExceptionType e) {
    // handle error
} finally {
    // always executes
}

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✅ Example 1 — Divide by Zero

public class ExceptionExample {
    public static void main(String[] args) {
        int a = 10, b = 0;
        try {
            int c = a / b;
            System.out.println("Result: " + c);
        } catch (ArithmeticException e) {
            System.out.println("Error: Cannot divide by zero!");
        } finally {
            System.out.println("Execution finished.");
        }
    }
}

✅ Output:

Error: Cannot divide by zero!
Execution finished.

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✅ Example 2 — Array Index Out of Bounds

public class ArrayException {
    public static void main(String[] args) {
        int arr[] = {1,2,3};
        try {
            System.out.println(arr[5]);
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("Error: Invalid index!");
        } finally {
            System.out.println("Done checking array.");
        }
    }
}

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✅ Multiple Catch Blocks

try {
    int arr[] = {1,2,3};
    int x = arr[5];
    int y = 10/0;
} catch(ArrayIndexOutOfBoundsException e){
    System.out.println("Array index problem!");
} catch(ArithmeticException e){
    System.out.println("Division problem!");
} finally{
    System.out.println("Always executes.");
}

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✅ Why Use Exception Handling?

✔ Prevent program crash
✔ Show friendly error message
✔ Debugging easier
✔ Required in professional apps

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📝 Exercise for You ✅

Create a program:

1️⃣ Take two integers from user
2️⃣ Divide them → Handle division by zero
3️⃣ Access an array element → Handle invalid index
4️⃣ Print finally message

Example Output:

Enter numbers: 10 0
Cannot divide by zero!
Invalid array index!
Execution Completed

Reply with:
✅ Code
✅ Output

I will check & grade it ✅🔥

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🎯 Quick Quiz

1️⃣ Which block always executes?
A) try
B) catch
C) finally

2️⃣ Which exception occurs for invalid array index?
A) ArithmeticException
B) ArrayIndexOutOfBoundsException
C) NullPointerException

Reply like:
1 → ?, 2 → ?

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✅ Lesson-9 Completed!

✅ NEXT PART - Lesson-10: Advanced Scanner Input & Parsing / Reading Strings & Numbers