CODEWITHSUNDEEP
javaconstructordefault-constructor
user1420042
user1420042

Reputation: 1709

What exactly happens when an object is instantiated in Java?

I know that when creating an object of a class the constructor builds that object. Say I had these two class:

class Vehicle {
    public int a = func();

    public int func() { 
        System.out.println("9"); 
        return 9;
    }
}

class Car extends Vehicle  {

    public static void main(String[] args) {
        Car c = new Car();
    }
}

The output of this project is "9". But why does that happen? What exactly happens when the Car constructor is called? I know that there is some type of default constructor but I am not sure how it exactly works.

Can anyone explain me the object construction with the above example?

Upvotes: 0

Views: 2794

Answers (5)

user2020911
user2020911

Reputation:

I think you are misunderstanding the meaning of Car c = new Car();.

That statement creates an object reference, and that object's reference is c. The statement new Car(); creates an object and the reference of that object is sent stored in c.

Upvotes: 1

Mohan Raj B
Mohan Raj B

Reputation: 1026

When an object is created it initiates all instance variables present in that object.(to its variable's default value if no assignment is given).

If a default constructor of a particular class(Car) is called, a call to the default constructor of its super class(Vehicle) is made first. And while creating Vehicle object its instant variable "a" is assigned to function func() and it ll be executed, Hence printing 9.

Upvotes: 0

OldCurmudgeon
OldCurmudgeon

Reputation: 65889

  1. The JVM is informed that a Car must be constructed.
  2. The JVM determines that it is a Vehicle and therefore triggers a Vehicle construction.
  3. The JVM understands that the construction of a Vehicle requires the initialisation of the int a instance variable.
  4. The int a variable must be initialised to be the result of the call to func so func is called.
  5. func has been called so the code prints "9".

Upvotes: 1

Jon Skeet
Jon Skeet

Reputation: 1504052

The compiler provides a default constructor for Car which is effectively:

Car() {
    super();
}

And likewise Vehicle as a default constructor of:

Vehicle() {
    super();
}

Fields are initialized as part of initialization, after the call to the superclass constructor. So it's as if the Vehicle class was actually written like this:

class Vehicle {
    public int a;

    Vehicle() {
        super();
        a = func();
    }

    public int func() { 
        System.out.println("9"); 
        return 9;
    }
}

Now does that makes sense?

See section 15.9.4 of the Java language specification for a much more detailed description.

Upvotes: 9

PermGenError
PermGenError

Reputation: 46438

When the Car constructor is called a default call to it's super constroctor is made by the compiler which then intializes all the Super class's instance fields. during the initialization of your a field as it invokes func() method which has a sysout thus it prints 9.

public Car() {
super();// this invokes parent class's default cons
}

public Vehical() {
super();// invokes Object constructor then initializes all the instance variables of class vehical
}

Upvotes: 2

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