How many objects are created due to inheritance in java?

Let's say I have three classes:

class A {
    A() {
        // super(); 
        System.out.println("class A");
    }
}
class B extends A {
    B() {
        // super(); 
        System.out.println("class B");
    }
}
class C extends B {
    public static void main(String args[]) {
        C c = new C(); //Parent constructor will get called
    }
}

When I create an instance of class C, it calls the constructor of super class. So, is there more than one object that is getting created? If only one object is created, then how is super() like another class' constructor? Does super() method internally create an object? What I know is, the constructor is also a method (I may be wrong).

My questions are:

1. How many number of Object is created in this case?

2. If one object is created then how does Super() internally call the parent class constructor?

Great question. What you are probing is how Java initializes objects - and there are a number of steps involved.

> i know is constructor is also a method (Maybe i am wrong).

Nearly right. The Constructor is a special method. If you decompile a class file, you'll see the constructors get renamed to <init>. <init> is treated differently from other methods and, for example, can't be called explicitly except through use of the keyword new or super. This is so fundamental that it is implemented in the JVM itself rather than being something defined in the Java language.

> How many number of Object is created in this case.

One object is created - an instance of C.

C is additionally and simultaneously an instance of B and an instance of A and also Object.

> If one object is created then how internally super() is calling Parent class Constructor . How Super is able to call parent class constructor.

This is where we get into initialization - initialization is how the JVM creates a new instance of an object and sets all the member values - those of the specific class and those of the superclasses. There are several stages involved:

• Load all the referenced classes and initialize those classes. Class initialization is itself non-trivial so I won't cover it here. It is well worth reading up.

• Allocate a chunk of memory for holding the members of the instance, which will include the all members of A, B and C. NOTE this explains one aspect of your question: how can the constructors of the base class and its subclasses update or refer to the same object - all the members of the instance from all classes are stored one after the other in the same chunk of memory.

• Initialize all the members to their default value. For example, int and float members will be set to 0 and 0.0f.

• Execute or calculate the member initializers, eg:

    private int a = 10;
    private int b = a * 5;
    private String c = Singleton.getInstance().getValue();
  

• Note (1) that member initialization occurs strictly in the order that members are declared in the class. This means that references to members later in the declaration are broken:

    private int a = b * 5; // Forward reference; won't compile
    private int b = 10;
  

• Note (2) that there is a under-used facility in Java to run arbitrary code to initialize values before the constructor is executed. These code blocks are executed at this time again strictly in order of declaration:

    private int a;
    private int b = 1;
    {
        // Initization occurs after b but before c.
        // c cannot be referenced here at all
        int i = SomeClass.getSomeStatic();
        a = i * 2;
    }
    private int c = 99;
  

• Execute the constructor of C. Constructors must either directly invoke a constructor from the superclass or the compiler will automatically add super() as the first line of the constructor. This means that the constructors are strictly executed in order:

  1. Object
  2. A
  3. B
  4. C

The object is now initialized and is ready for use. You can do some dangerous stuff if you initialize value using instance methods:

public class Wrong {
    int a = getB(); // Don't do this!
    int b = 10;
    public int getB() {
         return b;
    }
}

Here, a is initialized to 0. This is because, at the point getB() is invoked, Java has cleared the value of b to the default (0), but has not yet set it to 10 in the second phase of initialization.

In summary - there is only one object and it is created and initialized in a number in stages. During those stages, the object is, by definition, not completely defined.

1. There will be one and only one object will be created and ie. A object.

2. You can imagine like when class A extends B, then all methods and variables are copied to class A.

In Code only one object will be created and super call the parent class constructor .

Prove of object creation :

package one;

public class A {
	public static A super_var;

	public A() {
		super_var = this;
		System.out.println("Constrcutor of A invoked");
	}
}

package two;

public class B extends A {
	public static A sub_var;

	public B() {
		sub_var = this;
		System.out.println("Constructor of B invoked");
	}

	public void confirm() {
		if (sub_var == A.super_var)
			System.out.println("There is only one object is created");
		else
			System.out.println("There are more than one object created");
	}

	public static void main(String Args[]) {
		B x = new B();
		x.confirm();
	}
}

This will prove that there will be only one object created.

And about Super(). What I know that it call Parent class constructor . and each constructor ahve Super() as first statement like you mention in your code . so that you know

I don't know how it internally call super class constructor .

Hope this will make you understand there is only the instace you create in program

1. In your case only, 1 objects is getting created.
2. When subclasses constructor is called, it calls the constructor of super class internally to initailize super class's members.

Invoking constructor does not mean you are creating objects. Object is already created when invoking the constructor.The objects is created by the JVM first(i.e memory is allocated on heap and then constructor is called).

Constructor are meant for initializing the members of objects.

Your classes will be internally converted to something like this

class A
{
	A(){
		super(); 
		System.out.println("class A");
	}
}

class B extends A{
	B(){
		super(); 
		System.out.println("class B");
	}
}

public class C extends B
{
	public static void main(String args[])
	{
		C c  = new C(); //Parent constructor will get call 
	}
}

> How many number of Object is created in this case.

Only one, which is instance of C, calling super() just invokes the constructor of parent class and doesn't create object

> If one object is created then how internally Super() is calling Parent > class Constructor . How Super is able to call parent class > constructor.

When you create C's instance. C's constructor gets called, which first calls B's constructor, which in turn calls A's constructor

How many number of Object is created in this case.

When you create an instance of Class C by C cInstance = new C(); a single instance(Object) of Class C is creates(None of A and B). However since C extends B and B extends A, C will have all the methods of Class A and B(Actually depends on access modifiers used but lets say for this case they are public or default).

If one object is created then how internally Super() is calling Parent class Constructor
. How Super is able to call parent class constructor.

That is how inheritance works. When a new object is created it will call it's super class constructor and that super class will call it's super class constructor and so on. In other ordinary function you have to explicitly call super(). So calling super class constructor goes bottom-up fashion while execution goes top-down fashion of the inheritance hierarchy tree

If you look at dynamics of object allocation as per this SO answer, it must be clear that using new operator, you create only one object per statement. To further clarify the doubt that there is only one object which is being created, go thru this program:

public class A {
    public static int aInstanceCount=0;
    public static A aInstance;
    public String aInstanceVariable;
    A() {
//Super();
        aInstanceCount++;
        aInstanceVariable="aInstanceVar";
        System.out.println("class A");
        aInstance=this;
    }
}

class B extends A {
    public static int bInstanceCount=0;
    public static B bInstance;
    public String bInstanceVariable;
    B() {
//Super();
        bInstanceCount++;
        bInstanceVariable="bInstanceVar";
        System.out.println("class B");
        bInstance=this;
    }
}

class C extends B {
    public static void main(String args[]) {
        int instanceCount=0;
        C c = new C(); //Parent constructor will get call
        if(A.aInstance!=null){
            instanceCount++;
            System.out.println("Value of aInstanceVariable: "+A.aInstance.aInstanceVariable);

        }
        if(B.bInstance!=null){
            instanceCount++;
            System.out.println("Value of bInstanceVariable: "+B.bInstance.bInstanceVariable);
        }
        A a=A.aInstance;
        B b=B.bInstance;
        System.out.println("bInstanceVariable of B earlier: " + B.bInstance.bInstanceVariable);
        //Now we are changing the bInstanceVariable of c which is inherited from B
        c.bInstanceVariable="bInstance After modified by C";
        System.out.println("bInstanceVariable of B after: " + B.bInstance.bInstanceVariable);
        System.out.println("aInstanceVariable of A earlier: " + A.aInstance.aInstanceVariable);
        //Now we are changing the aInstanceVariable of c which is inherited from A
        c.aInstanceVariable="aInstance After modified by C";
        System.out.println("bInstanceVariable of A after: " + A.aInstance.aInstanceVariable);
    }
}

The output:

class A
class B
Value of aInstanceVariable: aInstanceVar
Value of bInstanceVariable: bInstanceVar
bInstanceVariable of B earlier: bInstanceVar
bInstanceVariable of B after: bInstance After modified by C
aInstanceVariable of A earlier: aInstanceVar
bInstanceVariable of A after: aInstance After modified by C

If you can notice, the super constructor is implicitly getting called each time if a subclass object is created, but since the new operator is used only once, there is only one object which is actually allocated the space. And by modifying the aInstanceVariable via C object c, we are actually changing the aInstanceVariable of aInstance. So it clearly proves that there is actually one object.

Steps of object creation when you call a constructor to create object:

1. Memory allocation using init is done. This init makes a system call to allocate memory for object creation.

2. Then your constructor is called to initialize the object's fields.

3. Then it calls super class constructor (If there's any super class) and Step 1 through 3 repeats.

What you see when you decompile a class file using javap shows different calls to be made. init makes system call to initialize memory allocation but object's field are initialized when constructor's code is run.

I am not sure how polymorphism/overriding works at the time of GC.

But it should be worth a try to override finalize method in all your classes and check when JVM exits main method.

• If only C object is created, it should call finalize for 'C'.
• If all A,B, C object is created, it should call finalize for A,B, C.

I think this is simplest check you can apply.

class A {
    A() {
        //Super(); 
        System.out.println("class A");
    }
    
    public void finalize(){
    System.out.println("Class A object destroyed");
    }
}
class B extends A {
    B() {
       //Super(); 
        System.out.println("class B");
    }

    public void finalize(){
    System.out.println("Class B object destroyed");
    }
}
class C extends B {
    public static void main(String args[]) {
        C c = new C(); //Parent constructor will get call 
    }
    
    public void finalize(){
    System.out.println("Class C object destroyed");
    } 
}

I agree with the previously posted answers, but want to add a reference to the ultimate authority on this issue, the Java Language Specification.

The expression new C() is a "Class Instance Creation Expression". Section 15.9.4 Run-time Evaluation of Class Instance Creation Expressions describes the run time steps involved in creating an object. Note that it refers to "the object", and only allocates space once, but states "Next, the selected constructor of the specified class type is invoked. This results in invoking at least one constructor for each superclass of the class type."

This all becomes much clearer by distinguishing between creating a new object, and invoking a constructor. Invoking a constructor only does part of object creation, the part that runs initializers, superclass constructors, and the body of the constructor. Because a C is also a B, the B constructor has to run during creation of a C.

The super keyword enables a subclass to call the methods and fields of its superclass. It is not an instance of the superclass object but a way to tell the compiler which methods or fields to reference. The effect is the same as if the subclass is calling one of its own methods. Examples:

Consider a subclass Employee that extends its superclass Person:

public class Employee extends Person{
 
   public Employee()
   {
     //reference the superclass constructor 
     super(); 
   }
 
   public String getName()
   {
     //reference superclass behaviors
     return super.getFirstName() + " " + super.getLastName();
   }
 } 

The super keyword can be used to reference the constructer of the Person class or any of the behaviors or fields that it has access to (e.g., getFirstName() and getLastName()).

1. In your Case One object is created

2. while doing the following, this super() will provided by Compiler Implicitly

   class A {
   A() {
       System.out.println("class A");
   }
   }
   class B extends A {
   B() {
       System.out.println("class B");
   }
   }
   class C extends B {
   public static void main(String args[]) {
       C c = new C(); //
   }
   }
   

It is similar to calling a super() inside your methods

    B() {
        super();
        System.out.println("class B");
    }

The super keyword can also be used when a method is overridden in the current class, but you want to invoke the super class method.

super() will makes the all constructors reference to one class. (For easy understanding: Its like all the member functions are comes under a same class.) Its going to call all the constructor methods only.

So its done the work of calling constructor only, so super() will not done any object creation. Its just referring the member functions.

If you add one more line of Code System.out.println(this.hashCode()) will remove your confusion.

Here in All Case hashCode() will print same hashCode all the time. It means there is one and only one unique Object is Created.

class A {
    A() {
        // super(); 
        System.out.println(this.hashCode()); // it will print 2430287
        System.out.println("class A");
    }
}
class B extends A {
    B() {
        // super(); 
        System.out.println(this.hashCode()); // it will print 2430287
        System.out.println("class B");
    }
}
class C extends B {
    public static void main(String args[]) {
        C c = new C(); //Parent constructor will get called
        System.out.println(this.hashCode()); // it will print 2430287
    }
}

but there is two Constructor is getting invoked to initialize the Parent member variable. I think if you know the concept of super() keyword which invokes the Constructor of parent Class and Initialize the member Variable of parent Class.

3 constructors will call

Code:

class A
{
	A()
	{
		System.out.println("In A");
	}
}

class B extends A
{
	B()
	{
		System.out.println("In B");
	}
}

class C extends B
{
	C()
	{
		System.out.println("In C");
	}
}

public class InheritanceTest {
	public static void main(String args[])
	
	
	
	{
		C c1=new C();
	}

}

Output:

In A

In B

In C