Java Oops Ch2 Constructors

01 Sep 2022

Constructors

Definition:

A constructor initializes an object immediately upon creation.

Important points:

• It is a tedious task to initialize the variables everytime the object is created.
• Java allows objects to be automatically initialized with the help of constructors.
• Constructor has the same name as the class where it resides and syntactically similar to methods.
• Constructor is automatically called when objects are created, before new operator is completed.
• Constructors has no return type not even void.
• Implicit return type of class constructor is class type itself.

Types of constructors: Default(No arguments) and Parameterized.

Default Constructors:

Constructors with no arguments.

Example:

class Rectangle
{
   double width;
   double height;
  Rectangle()
  {
    width=10;
    height=20;
  }
  double perimeter()
  {
    double p= 2*(width+height);
    return p;
  }
 }
 class Main
 {
 public static void main(String args[])
 {
   Rectangle rec1=new Rectangle();
   
   double vol;
   
    vol=rec1.width * rec1.height; 
   System.out.println("Rectangle 1 area:  "+vol);
   System.out.println("Rectangle 1 perimeter method:  "+rec1.perimeter());
  
  
      }
 }
 

### Result: Rectangle 1 area: 200.0
Rectangle 1 perimeter method: 60.0 ## Parameterized Constructors

• Default constructors are not very useful as they does not initialze the constructors.
• Solution: use parameterized constructors when we initialize the instance variables while creation of the class. ### Example

 class Rectangle
{
   double width;
   double height;
  Rectangle(double w, double h)
  {
    width=w;
    height=h;
  }
  double perimeter()
  {
    double p= 2*(width+height);
    return p;
  }
 }
 class Main
 {
 public static void main(String args[])
 {
   Rectangle rec1=new Rectangle(10,20);
   
   double vol;
   
    vol=rec1.width * rec1.height; 
   System.out.println("Rectangle 1 area:  "+vol);
   System.out.println("Rectangle 1 perimeter method:  "+rec1.perimeter());
  
  
      }
 }
 

### Result Rectangle 1 area: 200.0
Rectangle 1 perimeter method: 60.0