One of the main functions of classes is to be able to create objects from the class. This is where constructors come in. As the name suggests, constructors define how to construct objects from the class.

Constructors don't need a return type.

It is good practice to define them as the first method of the class.

Note: A constructor must have the same name as the class.

There are 2 types of constructors:

• Default constructors

• Parameterized constructors

Default constructors

Default constructors simply define the default values for objects created from the class. Hence, whenever you create instances from the class, the values defined in the default constructors will be the default values.

Example:

class Car {

  private String manufacturer;
  private int engineSize;
  private int powerSupply;


  // Default constructor
  public Car() {
    // We define the default values for manufacturer, engineSize, and powerSupply
    manufacturer = "Toyota";
    engineSize = 3000;
    powerSupply = 450;
  }

  // A simple print function
  public void printCarDetails(){ 
    System.out.println("Manufacturer: " + manufacturer);
    System.out.println("Engine Size: " + engineSize); 
    System.out.println("Power Supply: " + powerSupply);
  }

}

class CarDemo {

  public static void main(String args[]) {
    // Call the Car constructor to create its object
    Car car1 = new Car(); // Object created with default values
    car1.printCarDetails();
  }

}

Note: Notice that when we created a Car object in, we don’t treat the constructor as a method and write this:

car.Car();

We create the object just like we create any other object:

Car car1 = new Car();

In addition, even if we don’t define a default constructor, the JVM will call a default constructor and set data members to null or 0.

Parameterized constructor

In a parameterized constructor, we pass arguments to the constructor and set them as the values of our member variables. It's basically overloading the default constructor with our preferred values for the member variables.

class Car {

  private String manufacturer;
  private int engineSize;
  private int powerSupply;


  // Default constructor
  public Car() {
    // We define the default values for manufacturer, engineSize, and powerSupply
    manufacturer = "Toyota";
    engineSize = 3000;
    powerSupply = 450;
  }

  // Parameterized constructor
  public Car(String make, int eSize, int power){
    // The arguments are used as values
    manufacturer = make;
    engineSize = eSize;
    powerSupply = power;
  }

  // A simple print function
  public void printCarDetails(){ 
    System.out.println("Manufacturer: " + manufacturer);
    System.out.println("Engine Size: " + engineSize); 
    System.out.println("Power Supply: " + powerSupply);
  }

}

class CarDemo {

  public static void main(String args[]) {
    // Call the Car constructor to create its object
    Car car1 = new Car("Honda", 3300, 250); // Object created with default values
    car1.printCarDetails();
  }

}

this keyword

The this keyword exists for every class. It refers to the class object itself. We use this when we have an argument that has the same name as a member variable. this.memberVariable specifies that we are accessing the memberName variable of the particular class.

class Car {

  private String manufacturer;
  private int engineSize;
  private int powerSupply;


  // Default constructor
  public Car() {
    // We define the default values for manufacturer, engineSize, and powerSupply
    this.manufacturer = "Toyota";
    this.engineSize = 3000;
    this.powerSupply = 450;
  }

  // Parameterized constructor
  public Car(String manufacturer, int engineSize, int powerSupply){
    // The arguments are used as values
    this.manufacturer = manufacturer;
    this.engineSize = engineSize;
    this.powerSupply = powerSupply;
  }

  // A simple print function
  public void printCarDetails(){ 
    System.out.println("Manufacturer: " + manufacturer);
    System.out.println("Engine Size: " + engineSize); 
    System.out.println("Power Supply: " + powerSupply);
  }

}

class CarDemo {

  public static void main(String args[]) {
    // Call the Car constructor to create its object
    Car car1 = new Car("Honda", 3300, 250); // Object created with default values
    car1.printCarDetails();
  }

}

We can also use it when calling a constructor from another constructor. The this keyword followed by parentheses means that another constructor in the same Java class is being called.

Example:

class Car {

  private String manufacturer;
  private int engineSize;
  private int powerSupply;


  // Default constructor
  public Car() {
    // We define the default values for manufacturer, engineSize, and powerSupply
    this.manufacturer = "Toyota";
    this.engineSize = 3000;
    this.powerSupply = 450;
  }

  // Parameterized constructor
  public Car(String manufacturer, int engineSize, int powerSupply){
    // The arguments are used as values
    this.manufacturer = manufacturer;
    this.engineSize = engineSize;
    this.powerSupply = powerSupply;
  }

  // Parameterized constructor
  public Car(String manufacturer, int engineSize, int powerSupply, int fuelCapacity){
    this(manufacturer, engineSize, powerSupply); // calling the constructor
    this.fuelCapacity = fuelCapacity;
  }
  // A simple print function
  public void printCarDetails(){ 
    System.out.println("Manufacturer: " + manufacturer);
    System.out.println("Engine Size: " + engineSize); 
    System.out.println("Power Supply: " + powerSupply);
  }

}

class CarDemo {

  public static void main(String args[]) {
    // Call the Car constructor to create its object
    Car car1 = new Car("Honda", 3300, 250); // Object created with default values
    car1.printCarDetails();
  }

}

Conclusion

Constructors are very important when creating objects. All classes need them to create instances of the class.