Constructors

Constructor

In Java, a constructor is a block of codes similar to the method. It is called when an instance of the object is created, and memory is allocated for the object.

It is a special type of method which is used to initialize the object.

When is a constructor called?

When an object is created, compiler makes sure that constructors for all of its subobjects (its member and inherited objects) are called. If members have default constructors or constructor without parameter then these constructors are called automatically, otherwise parameterized constructors can be called using initializer list.

Note: It is called constructor because it constructs the values at the time of object creation. It is not necessary to write a constructor for a class. It is because java compiler creates a default constructor if your class doesn't have any.

Rules to remember while creating a constructor

There are three rules defined for the constructor.

  • Constructor name must be the same as its class name
  • A Constructor must have no explicit return type
  • A Java constructor cannot be abstract, static, final, and synchronized

Types of constructors

  • Default Constructor
  • Parameterized Constructor

Default Constructor

The default constructor is a no-args constructor that the Java compiler inserts on your behalf; it contains a default call to super(); which is the default behavior. If you implement any constructor then you no longer receive a default constructor.

Note : If there is no constructor in the class, the compiler adds a default constructor

No-Argument Constructor

public Bicycle() {
    gear = 1;
    cadence = 10;
    speed = 0;
}

Bicycle yourBike = new Bicycle(); invokes the no-argument constructor to create a new Bicycle object called yourBike.

Note : The diagram below describes that a default constructor is added by the compiler when there is no constructor declared in the class.

Parameterized Constructor

A constructor which has a specific number of parameters is called a parameterized constructor.

The parameterized constructor is used to provide different values to the distinct objects. However, you can provide the same values also. 

class Student4{  
    int id;  
    String name;  
    Student4(int i,String n){  
    id = i;  
    name = n;  
    }  
    void display(){System.out.println(id+" "+name);}  
   
    public static void main(String args[]){  
    Student4 s1 = new Student4(111,"Karan");  
    Student4 s2 = new Student4(222,"Aryan");   
    s1.display();  
    s2.display();  
   }  
}

In the above example, we have created the constructor of Student class that have two parameters. We can have any number of parameters in the constructor.

Constructor Overloading

In Java, a constructor is just like a method but without return type. It can also be overloaded like Java methods.

Constructor overloading in Java is a technique of having more than one constructor with different parameter lists. They are arranged in a way that each constructor performs a different task. They are differentiated by the compiler by the number of parameters in the list and their types. 

class Student5{  
    int id;  
    String name;  
    int age;  
    //creating two argument constructor  
    Student5(int i,String n){  
    id = i;  
    name = n;  
    }  
    //creating three argument constructor  
    Student5(int i,String n,int a){  
    id = i;  
    name = n;  
    age=a;  
    }  
    void display(){System.out.println(id+" "+name+" "+age);}  
   
    public static void main(String args[]){  
    Student5 s1 = new Student5(111,"Karan");  
    Student5 s2 = new Student5(222,"Aryan",25);  
    s1.display();  
    s2.display();  
   }  
}

Constructor vs Method

Constructor :

  • A constructor is used to initialize the state of an object.
  • A constructor must not have a return type.
  • The constructor is invoked implicitly.
  • The Java compiler provides a default constructor if you don't have any constructor in a class.
  • The constructor name must be same as the class name.

Method :

  • A method is used to expose the behavior of an object.
  • A method must have a return type.
  • The method is invoked explicitly.
  • The method is not provided by the compiler in any case.
  • The method name may or may not be same as class name.