Constructors

What is a Constructor?

A constructor is a special method that runs when you create an object with new. It has the same name as the class and no return type. Its job is to put the new object into a valid starting state.

Default constructor: if you write no constructor, Java adds an empty no-argument one for you.
Parameterized constructor: takes values, e.g. new Student("Asha", 21).
As soon as you write any constructor, Java no longer adds the default one.
Constructors can be overloaded (several constructors with different parameter lists).

Default and Parameterized Constructors

Example

Two constructors in one class. Java picks the one that matches the arguments.

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student();
        Student s2 = new Student("Asha", 21);
        s1.show();
        s2.show();
    }
}

class Student {
    String name;
    int age;

    Student() {                       // no-argument constructor
        name = "Unknown";
        age = 18;
    }

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

    void show() {
        System.out.println(name + " is " + age + " years old");
    }
}

Output

Unknown is 18 years old
Asha is 21 years old

Constructor Chaining with this()

One constructor can call another constructor of the same class with this(...). This avoids repeating the same setup code. The this(...) call must be the first statement.

Example

public class Main {
    public static void main(String[] args) {
        System.out.println(new Pizza());
        System.out.println(new Pizza("Large"));
        System.out.println(new Pizza("Medium", "Paneer"));
    }
}

class Pizza {
    String size;
    String topping;

    Pizza() {
        this("Regular");                 // calls Pizza(String)
    }

    Pizza(String size) {
        this(size, "Cheese");            // calls Pizza(String, String)
    }

    Pizza(String size, String topping) {
        this.size = size;
        this.topping = topping;
    }

    public String toString() {
        return size + " pizza with " + topping;
    }
}

Output

Regular pizza with Cheese
Large pizza with Cheese
Medium pizza with Paneer

Practice

Exercise: Create a class BankAccount with fields holder and balance. Add a constructor that takes only the holder (balance starts at 0) and one that takes both. Use this(...) so the logic is written once.

Show answer

class BankAccount {
    String holder;
    double balance;

    BankAccount(String holder) {
        this(holder, 0);
    }

    BankAccount(String holder, double balance) {
        this.holder = holder;
        this.balance = balance;
    }
}