Constructors in Java

Overview

Types of Constructors

• Default Constructors - These are default constructors who doesn't have any arguments

 Dog myDog = new Dog();

 public Dog() {
 // Write the code that will execute automatically when an object of Dog is created   
 }

• Parameterized Constructor

 Dog bravo = new Dog("Bravo");

 public Dog(String name) {
    this.name = name;
 }

 Dog newDog = new Dog(5);

 public Dog(int age) {
    this.age = age;
 }

 Dog jackie = new Dog("Jackie", 3);

 public Dog(String name, int age) {
    this.name = name;
    this.age = age;
 }

• Hello World

Do's and Dont's

• The purpose of new keyword is to create an object.
• The purpose of a constructor is to initialize an object.
• In the event of creating an object: first new keyword is executed to create an object and then the constructor method of a class is called to initialize the object. 
• If you have added a parameterized constructor in your class, then Java won't call the default constructor at all. Java will always call the constructor which you defined in the class.
• If you declare a constructor of a class as private then you have to define a parameterized constructor to create it's object, otherwise you won't be able to create it's object. But you can use it's static methods and properties.

 public class Cat {
    public static final int NUMBER_OF_MONTHS_IN_A_YEAR = 12;
    public final int NUMBER_OF_DAYS_IN_A_WEEK = 7;
    public String name;

    public void greetUser(String name) {
        System.out.println("Hello " + name);
    }

    private Cat() {
    }

    public Cat(String name) {
        this.name = name;
    }
 }

 Cat chinki = new Cat("Chinki");
 chinki.greetUser("Minki");
 System.out.println("Hello " + chinki.name);

• Hello World!!!

Reference