Title here
Summary here
Constructors and the `this` Keyword
These concepts are crucial for object creation and understanding how to manage object state properly.
🏗️ 1. What is a Constructor?
✅ Definition:
A constructor in Java is a special method that is automatically called when an object is created. It is used to initialize objects.
🔧 Syntax:
ClassName() {
// initialization code
}🔍 Key Rules:
- Constructor has no return type (not even
void) - The name of the constructor must match the class name
- It can be parameterized (to initialize values) or default (no parameters)
✅ Example: Default Constructor
public class Car {
String brand;
// Default constructor
Car() {
brand = "Tesla";
}
void showBrand() {
System.out.println("Brand: " + brand);
}
}public class Main {
public static void main(String[] args) {
Car c = new Car(); // constructor called automatically
c.showBrand(); // Output: Brand: Tesla
}
}✅ Example: Parameterized Constructor
public class Car {
String brand;
int speed;
// Parameterized constructor
Car(String brand, int speed) {
this.brand = brand;
this.speed = speed;
}
void showDetails() {
System.out.println("Brand: " + brand + ", Speed: " + speed + " km/h");
}
}public class Main {
public static void main(String[] args) {
Car car1 = new Car("Toyota", 120);
Car car2 = new Car("BMW", 180);
car1.showDetails(); // Output: Brand: Toyota, Speed: 120 km/h
car2.showDetails(); // Output: Brand: BMW, Speed: 180 km/h
}
}🔁 2. The this Keyword
✅ Definition:
this is a reference to the current object inside a class. It is used to:
- Refer to current object’s fields
- Invoke current class methods or constructors
- Pass the current object as a parameter
🧠 Why use this?
When parameter names and field names are the same, this helps distinguish between them.
🔧 Example:
public class Student {
String name;
int age;
Student(String name, int age) {
this.name = name; // distinguishes between parameter and instance variable
this.age = age;
}
void show() {
System.out.println("Name: " + name + ", Age: " + age);
}
}public class Main {
public static void main(String[] args) {
Student s1 = new Student("Alice", 22);
s1.show(); // Output: Name: Alice, Age: 22
}
}🔄 Constructor Overloading
Just like method overloading, Java allows multiple constructors in the same class with different parameters.
🔧 Example:
public class Book {
String title;
int pages;
// Default constructor
Book() {
this.title = "Untitled";
this.pages = 0;
}
// Parameterized constructor
Book(String title, int pages) {
this.title = title;
this.pages = pages;
}
void info() {
System.out.println("Title: " + title + ", Pages: " + pages);
}
}🧠 When to Use this() Constructor Call
You can use this() to call another constructor inside the same class.
public class Book {
String title;
int pages;
Book() {
this("Unknown", 0); // Calls the parameterized constructor
}
Book(String title, int pages) {
this.title = title;
this.pages = pages;
}
void info() {
System.out.println("Title: " + title + ", Pages: " + pages);
}
}📝 Summary Table
| Concept | Use |
|---|---|
| Constructor | Initializes objects |
| Default Constructor | No parameters, sets default values |
| Parameterized | Sets values via arguments |
| Constructor Overloading | Multiple ways to initialize |
this keyword | Refers to current object or constructor |
🎓 Homework for Students
Create a
Laptopclass with fields:brand(String)ram(int)- Constructors:
- Default constructor → sets brand = “Dell”, ram = 8
- Parameterized constructor → sets custom values
- Method:
display()to print details
Create 2 objects in
main:- One using the default constructor
- One using the parameterized constructor
Bonus: Use
this()to call one constructor from the other.
Would you like the next topic to be:
- Static Keyword & Static Blocks
- Method Overloading vs Overriding
- Access Modifiers (public, private, etc.)
You’re steering the ship, coach 🚢👨🏫
Next
Inheritance