Constructors

Why this matters

Without constructors you have to set every field by hand after creating an object, which is long and easy to forget.

Syntax

class ClassName {
    ClassName() { }                 // default / no-arg constructor
    ClassName(int x) { }            // parameterised constructor
}

Example

class Student {
    String name;
    int rollNumber;

    Student() {                       // no-argument constructor
        name = "Unknown";
        rollNumber = 0;
    }

    Student(String n, int r) {        // parameterised constructor
        name = n;
        rollNumber = r;
    }

    void display() {
        System.out.println(rollNumber + " - " + name);
    }
}

public class Main {
    public static void main(String[] args) {
        Student a = new Student();
        Student b = new Student("Ananya Rao", 12);
        a.display();
        b.display();
    }
}

Step by step

1. 1A constructor has the same name as the class and no return type, not even void.
2. 2`new Student()` calls the no-argument constructor.
3. 3`new Student("Ananya Rao", 12)` calls the two-argument version — this is constructor overloading.
4. 4If you write no constructor at all, Java supplies a hidden default one.
5. 5The moment you write any constructor, the hidden default disappears.

Key points

• Same name as class, no return type
• Called automatically by new
• Can be overloaded with different parameter lists
• Writing a parameterised constructor removes the free default one

Common mistakes

• Adding `void` before the constructor name — it becomes an ordinary method
• Calling new ClassName() after defining only a parameterised constructor
• Misspelling the constructor name (case matters)

Try it yourself

• Give the Book class a constructor taking title, author and price.
• Add both a no-arg and a parameterised constructor to a Rectangle class.