Constructors in Java

When a bank opens an account, it doesn't hand over an empty form and hope you fill it in later. It takes your name and opening deposit at the counter, checks them, and only then gives you a working account. A constructor is that counter for your objects: special code that runs once, when the object is created with new, to put it into a valid starting state.

What new Does

public class Accounts {
    static class BankAccount {
        String holder;
        long balance;
 
        BankAccount(String holder, long balance) {
            if (balance < 0) {
                throw new IllegalArgumentException("Negative opening balance");
            }
            this.holder = holder;
            this.balance = balance;
            System.out.println("Account opened for " + holder);
        }
    }
 
    public static void main(String[] args) {
        BankAccount a = new BankAccount("Navika", 5000);
        System.out.println(a.holder + ": Rs. " + a.balance);
    }
}

Account opened for Navika
Navika: Rs. 5000

How it works

1. Lines 6 to 13 are a constructor: same name as the class, and no return type, not even void.
2. Line 17: new BankAccount("Navika", 5000) allocates the object, then runs the constructor with those arguments.
3. Lines 7 to 9 validate the input. A negative opening balance throws an exception (exceptions are covered later in the course), so no invalid account ever exists.
4. Lines 10 and 11 copy the parameters into the fields. this.holder means "this object's field holder", as opposed to the parameter with the same name (see "Java This Keyword").

Try it yourself: create an account with -100. What happens?

Three Kinds of Constructor

The Default Constructor Disappears

If you write any constructor, the compiler stops adding the default one. Code that used new BankAccount() then breaks:

NoArg.java:2: error: constructor Account in class Account cannot be applied to given types;
public class NoArg { public static void main(String[] a) { Account x = new Account(); } }
                                                                       ^
  required: String
  found:    no arguments

If you still want new BankAccount(), write a no-arg constructor yourself, often by calling the other one (see "Constructor Overloading and Constructor Chaining").

public class Defaults {
    static class Ticket {
        String name;
        int berth;
 
        Ticket() {
            this("Unassigned", 0);
        }
 
        Ticket(String name, int berth) {
            this.name = name;
            this.berth = berth;
        }
    }
 
    public static void main(String[] args) {
        Ticket blank = new Ticket();
        Ticket booked = new Ticket("Srihitha", 24);
        System.out.println(blank.name + " " + blank.berth);
        System.out.println(booked.name + " " + booked.berth);
    }
}

Unassigned 0
Srihitha 24

Constructor Rules

Common Mistakes

Adding void. void BankAccount(...) is a method, not a constructor; new BankAccount("Navika", 5000) then fails.

Forgetting this. when names match. holder = holder; assigns the parameter to itself and leaves the field null.

Expecting the default constructor after writing your own. It's only added when there's no constructor at all.

Interview Corner

Practice

1. Easy: Write a Student class with a parameterized constructor for name and roll number, and create two students.
2. Harder: Make the Student constructor reject an empty name and a roll number below 1, and test it with good and bad values.

Key Takeaways