Java Constructors Made Simple: A Beginner’s Guide with Easy Examples

 

What is a Constructor?

A constructor is a special method in a class that automatically runs when you create an object.

Think of it like this:

🧱 When you build a LEGO house (object), a constructor is like the instruction manual that tells how the pieces should be placed right from the start.

🧃 Why Do We Use Constructors?

Instead of writing code to manually set values every time we create an object, we let the constructor do it for us automatically.

🧪 Without Constructor Example

class Student {
    String name;
    int age;
}
 
public class Main {
    public static void main(String[] args) {
        Student s = new Student();  // create object
 
        // manually set values
        s.name = "Sanvi";
        s.age = 20;
 
        System.out.println(s.name + " is " + s.age + " years old.");
    }
}

Output:

Sanvi is 20 years old.

✅ With Constructor Example

class Student {
    String name;
    int age;
 
    // 👇 This is the constructor
    Student(String n, int a) {
        name = n;
        age = a;
    }
}
 
public class Main {
    public static void main(String[] args) {
        Student s = new Student("Jatasya", 22); // set values during object creation
 
        System.out.println(s.name + " is " + s.age + " years old.");
    }
}

Output:

Jatasya is 22 years old.

📌 Key Points About Constructors

Feature	What It Means
Same name as class	The constructor must have the same name as the class
No return type	Constructors do not return anything (not even void)
Runs automatically	It runs as soon as the object is created
Can have parameters	You can pass values into the constructor, just like methods

🧰 Types of Constructors

• Default Constructor (no parameters)
• Parameterized Constructor (takes values when creating object)

🧪 Default Constructor Example

class Animal {
    Animal() {
        System.out.println("An animal is created.");
    }
}
 
public class Main {
    public static void main(String[] args) {
        Animal a = new Animal(); // no values passed
    }
}

Output:

An animal is created.

🧪 Parameterized Constructor Example

class Animal {
    String type;
 
    Animal(String t) {
        type = t;
    }
 
    void show() {
        System.out.println("This is a " + type);
    }
}
 
public class Main {
    public static void main(String[] args) {
        Animal dog = new Animal("Dog");
        dog.show();
    }
}

Output:

This is a Dog

🤔 When Should You Use a Constructor?

• When you want to automatically set values when an object is created.
• When you want every object to have some initial setup.

🎯 Simple Analogy

🍼 Constructor is like a baby delivery checklist:

When a baby (object) is born (created), it automatically gets:

• Name tag
• ID
• Baby clothes

You don’t manually do this every time — the constructor handles it!

✅ Summary

• A constructor is a special method that runs when you create an object.
• It sets up the object with initial values.
• It makes code cleaner and faster.
• Java will give you a default constructor if you don’t write one.
• You can also make your own constructor with parameters.