Constructor

Constructor is a special method that is defined in the class and will be called automatically whenever there is an instance of an object. Constructor itself serves to initialize the value of the data contained in the relevant class. If we do not define a constructor in the class that we created, Java will make it for us automatically. This kind of constructor is called default constructor. The default constructor initializes all the data with zero values. However, once we define a new constructor for a class that we created, then the default constructor is not not used anymore. Just as method, constructor can have parameter and can also be overloaded.

Keep in mind that the constructor does not have a return value, not even void. This is because implicitly the actual type of the return of the constructor is the type of the class itself. Constructor name must exactly match with the class name. Here is a sample program that demonstrates a constructor for class Box:

class Box {

   double length;
   double width;
   double height;

   //Define constructor for class Box
   Box() {
      length = 4;
      width = 3;
      height = 2;
   }

   double calculateVolume() {
      return (length * width * height);
   }
}

class DemoConstructor1 {
   
   public static void main(String[] args) {

      Box b1, b2;

      b1 = new Box();
      b2 = new Box();

      System.out.println("Volume b1 = " + b1.calculateVolume());
      System.out.println("Volume b2 = " + b2.calculateVolume());
   }
}

In the above program, we set the initial value for the data length, width, and height each with value 4, 3 and 2. This would apply to any establishment of Box object. Here is the result that will be given by the program:

Volume b1 = 24.0
Volume b2 = 24.0

Initialization process using this way can still be said to be less dynamic. Because, every time the creation of the object box, the value of which is used as the initial value is always 4, 3, and 2. To make the constructor is more dynamic, then we can put parameter, as shown in the following program example:

class Box {

   double length;
   double width;
   double height;

   //Define constructor with parameter
   Box(double l, double w, double h) {
      length = l;
      width = w;
      height = h;
   }

   double calculateVolume() {
      return (length * width * height);
   }
}

class DemoConstructor2 {
   
   public static void main(String[] args) {

      Box b1, b2;

      b1 = new Box(4, 3, 2);
      b2 = new Box(6, 5, 4);

      System.out.println("Volume b1 = " + b1.calculateVolume());
      System.out.println("Volume b2 = " + b2.calculateVolume());
   }
}

This time, we created constructor that using parameter. This causes the initial value can be dynamic depending on the value ​​that are passed during the creation of the object. Consider the following code:

b1 = new Box (4, 3, 2);
b2 = new Box (6, 5, 4);

The first line shows that we do initialization length with value 4, width with value 3, and height with value 2. The second line we initialize the data length, width, and height of each with 6, 5 and 4.