Java Constructors

Java Constructors

What's the point then? I should be able to store data in it right?

That's when we use either getter (e.g., getName()) / setter (e.g., setName()) methods or in this case constructors to initialize a class. Basically every Java Class has a constructor, which is the method which is called first when any object of the class is initialized. Think of it as a bit of starter code.

When you write a class without any constructor, then Java assumes it has a default constructor :

public class Car {
    private String name;
}

Car modelS = new Car();

This initializing with no parameters is a way of calling the default constructor. You can also have a default constructor written yourself this way :

public class Car {
    private String name;

    public Car() {
        name = "Tesla";
    }
}

Then, when calling new Car(), the variable name will get auto-initialized to "Tesla".

Clearly, constructors are exactly what they sound like: they are used to construct i.e., instantiate an object of a particular class. Constructors look similar to method declarations, but are slightly different in the sense that they:

1. Are named exactly the same as the class.
2. Don't have a return type.

Hence, the purpose of using constructors is to provide:

1. A way to instantiate an object.
2. Provide initial values to a object properties.
3. Control how an object is created.

Let's look at another example. Say, Honda (the car manufacturer), wants all its cars to be named Honda <something>. In order to enforce this, we might represent this using a class as follows:

public class Car {

    private String name;

    //Constructor.
    public Car(String model){
        this.name = "Honda " + model;
    }

    public String getName(){
        return this.name;
    }

    public static void main(String args[]){
        Car car = new Car("Civic");
        System.out.println( car.getName() );
    }
}

🚀 Run Code

Notice that when we write a constructor in this way i.e., providing a parameter, we are controlling (point no. 3) the way an instance of Car is created. In short, we are saying in this example that you MUST provide a model name in order to get an instance of Car class.

Why is this important? There are times when you'd want one and only one instance of a class which you'd want to use in your entire application. One way of achieving this is by using a private constructor.

Assume you need a class to represent a Bank. You wouldn't want people to create instance of Bank ever. So, you design your class:

public class Bank {

    private static Bank instance;

    private Bank(){

    }

    public static Bank getInstance(){
        if(null==instance){
            instance = new Bank();
        }
        return instance;
    }


}

🚀 Run Code

Notice that the constructor is private. This enforces the fact that no one else is allowed to create an instance of the Bank. In fact, if in another class, you try:

Bank account = new Bank(); //-> throws a compilation error: Bank() has private access in Bank.

So, the only way to gain access to the instance is by using Bank.getInstance(). Such instances are called Singleton since you get exactly one instance (per VM to be precise) throughout the life of your application.

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