Reputation: 1037

Why is constructor of super class invoked when we declare the object of sub class?

Consider this code:

class Test {
    Test() {
        System.out.println("In constructor of Superclass");
    }

    int adds(int n1, int n2) {
        return(n1+n2);
    }

    void print(int sum) {
        System.out.println("the sums are " + sum);
    }
}


class Test1 extends Test {
    Test1(int n1, int n2) {
        System.out.println("In constructor of Subclass");
        int sum = this.adds(n1,n2);
        this.print(sum);
    }

    public static void main(String[] args) {
        Test1 a=new Test1(13,12);
        Test c=new Test1(15,14);
    }
}

If we have a constructor in super class, it will be invoked by every object that we construct for the child class (ex. Object a for class Test1 calls Test1(int n1, int n2) and as well as its parent Test()).

Why does this happen?

The output of this program is:

In constructor of Superclass

In constructor of Subclass

the sums are 25

In constructor of Superclass

In constructor of Subclass

the sums are 29

Upvotes: 40

Answers (18)

Heisenbug

Reputation: 39194

Yes. A superclass must be constructed before a derived class could be constructed too, otherwise some fields that should be available in the derived class could be not initialized.

A little note: If you have to explicitly call the super class constructor and pass it some parameters:

baseClassConstructor(){
    super(someParams);
}

then the super constructor must be the first method call into derived constructor. For example this won't compile:

baseClassConstructor(){
     foo(); 
     super(someParams); // compilation error
}

Upvotes: 24

Shubhamhackz

Reputation: 7993

Parents Exits First!! And like real world Child Can't exist without the Parents.. So initialising parents(SuperClass) first is important in order to use thrm in the children(Subclass) Classes..

Upvotes: 0

Akhil Vijayan

Reputation: 119

here your extending Test to your test1 class meaning u can access all the methods and variable of test in your test1. keep in note that u can access a class methods or variable only if memory is allocated to it and for that it need some constructor either a default or parameterized ,so here wen the compiler finds that it is extending a class it will try to find the super class constructor so that u can access all its methods.

Upvotes: 1

maneeshp626

Reputation: 11

As we know that member variables(fields)of a class must be initialized before creating an object because these fields represent the state of object. If these fields are explicitely not initilized then compiler implicitely provides them default values by calling no-argument default constructor. Thats why subclass constructor invokes super class no-argument default constructor or implicitely invoked by compiler .Local variables are not provided default values by compiler.

Upvotes: 1

NL3294

Reputation: 1514

I'll try to answer this from a different perspective.

Suppose Java didn't call the super constructor for you automatically. If you inherit the class, you'd have to either call the super constructor implicitly, or rewrite it yourself. This would require you to have internal knowledge of how the super class works, which is bad. It would also require to to rewrite code, which is also not good.

I agree that calling the super constructor behind the scenes is a little unintuitive. On the other hand, I'm not sure how they could have done this in a more intuitive way.

Upvotes: 1

Sameer Khanal

Reputation: 1334

"If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem."
(source: https://docs.oracle.com/javase/tutorial/java/IandI/super.html)

Upvotes: 1

Jaimin Patel

Reputation: 4637

super() is added in each class constructor automatically by compiler.

As we know well that default constructor is provided by compiler automatically but it also adds super() for the first statement.If you are creating your own constructor and you don't have either this() or super() as the first statement, compiler will provide super() as the first statement of the constructor.

Upvotes: 11

Ashkan Aryan

Reputation: 3534

Because it will ensure that when a constructor is invoked, it can rely on all the fields in its superclass being initialised.

see 3.4.4 in here

Upvotes: 41

Bhaskar

Reputation: 337

There is a default super() call in your default constructors of sub classes.

 //Default constructor of subClass
    subClass() {
    super();
    }

Upvotes: 1

Isham

Reputation: 434

In simple words if super class has parameterized constructor, you need to explicitly call super(params) in the first line of your child class constructor else implicitly all super class constructors are called untill object class is reachead.

Upvotes: 2

Mukul Goyal

Reputation: 11

Constructor of Super class in called first because all the methods in the program firstly present in heap and after compilation they stores in to the stack,due to which super class constructor is called first.

Upvotes: 1

user1923551

Reputation: 4712

Since you are inheriting base class properties into derived class, there may be some situations where your derived class constructor requires some of the base class variables to initialize its variables. So first it has to initialize base class variables, and then derived class variables. That's why Java calls first base class constructor, and then derived class constructor.

And also it doesn't make any sens to initialize child class with out initializing parent class.

Upvotes: 1

Aniket Thakur

Reputation: 69025

When we create an object of subclass, it must take into consideration all the member functions and member variables defined in the superclass. A case might arise in which some member variable might be initialized in some of the superclass constructors. Hence when we create a subclass object, all the constructors in the corresponding inheritance tree are called in the top-bottom fashion.

Specifically when a variable is defined as protected it will always be accessible in the subclass irrespective of whether the subclass is in the same package or not. Now from the subclass if we call a superclass function to print the value of this protected variable(which may be initialized in the constructor of the superclass) we must get the correct initialized value.Hence all the superclass constructors are invoked.

Internally Java calls super() in each constructor. So each subclass constructor calls it's superclass constructor using super() and hence they are executed in top-bottom fashion.

Note : Functions can be overridden not the variables.

Upvotes: 1

Ravi

Reputation: 31427

The base class constructor will be called before the derived class constructor. This makes sense because it guarantees that the base class is properly constructed when the constructor for the derived class is executed. This allows you to use some of the data from the base class during construction of the derived class.

Upvotes: 1

HJW

Reputation: 23443

Java classes are instantiated in the following order:

(at classload time) 0. initializers for static members and static initializer blocks, in order of declaration.

(at each new object)

create local variables for constructor arguments
if constructor begins with invocation of another constructor for the class, evaluate the arguments and recurse to previous step. All steps are completed for that constructor, including further recursion of constructor calls, before continuing.
if the superclass hasn't been constructed by the above, construct the the superclass (using the no-arg constructor if not specified). Like #2, go through all of these steps for the superclass, including constructing IT'S superclass, before continuing.
initializers for instance variables and non-static initializer blocks, in order of declaration.
rest of the constructor.

Upvotes: 4

AlexR

Reputation: 115398

Constructor implements logic that makes the object ready to work. Object may hold state in private fields, so only its class' methods can access them. So if you wish instance of your subclass be really ready to work after calling constructor (i.e. all its functionality including inherited from base class is OK) the base class's constructor must be called.

This is why the system works this way.

Automatically the default constructor of base class is called. If you want to change this you have to explicitly call constructor of base class by writing super() in the first line of your subclass' constructor.

Upvotes: 1

Andreas Dolk

Reputation: 114817

The subclass inherits fields from it's superclass(es) and those fields have to get constructed/initialised (that's the usual purpose of a constructor: init the class members so that the instance works as required. We know that some people but a lot more functionality in those poor constructors...)

Upvotes: 1

anon

Reputation:

That´s how Java works. If you create a child object, the super constructor is (implicitly) called.

Upvotes: 2

Why is constructor of super class invoked when we declare the object of sub class?

Answers (18)

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