Java inheritance - constructors

Question

While studying for my finals, I came across the following statement in the book from which I am currently studying. Considering the following code :

class A {
    public A(int x) {   }
}

class B extends A {
    public B(int x ) {   }
}

is it mandatory to call the constructor of class A in the constructor of class B(super(x)). The book states that it's not mandatory, because they have the exact number and type of parameters. But when I try this in a java compiler, the following error gets thrown :

constructor A in class A cannot be applied to given types; required: int found: no arguments reason: actual and formal argument lists differ in length

Bozho · Accepted Answer · 2011-06-06 07:27:31Z

17

The compiler automatically inserts super() in the beginning.

However, even constructors arguments, super() (without arguments) is added which invokes the default constructor of the superclass. And you don't have one, hence the error.

You have to specify super(x) (to invoke A(x)), or define a no-argument constructor.

By the way, Eclipse compiler gives a way better error message:

Implicit super constructor A() is undefined. Must explicitly invoke another constructor

edited Jun 6, 2011 at 7:27

answered Jun 6, 2011 at 7:15

Bozho

599k147 gold badges1.1k silver badges1.2k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Colin Dumitru Over a year ago

The error is thrown at this line : "public B(int x ) {}". The code that I posted in the question is all the code I am trying to compile (just the class definition, no instantiations at all).

MGwynne Over a year ago

It will only insert super(). It won't insert super(int). Hence the error here. In this case, he does need to call super(x). See download.oracle.com/javase/tutorial/java/IandI/super.html .

Andreas Dolk · Accepted Answer · 2011-06-06 07:49:22Z

It looks like the compiler tries to create a call to the superclasses default constructor with super(), which isn't avaliable:

required: int
found:    no arguments

But back to your book: I've never heard of a rule that you can skip the super statement in a constructor if the actual constructor has the exact same parameter list as a constructor in the direct superclass. Only a call to the superclass's default constructor is added implicitly (super()) but that requires that the superclass has a default constructor.

In contrast to what's written in your book (or in contrast to your understanding of the written text), here's a sentence from the language spec:

If a constructor body does not begin with an explicit constructor invocation and the constructor being declared is not part of the primordial class Object, then the constructor body is implicitly assumed by the compiler to begin with a super class constructor invocation “super();”, an invocation of the constructor of its direct superclass that takes no arguments.

MGwynne · Accepted Answer · 2011-06-06 07:52:50Z

4

If you have a the base class having a default constructor (no-arg constructor), when you extend B, you don't need to explicitly call super() because it is called any way.

But when you have a constructor with arguments, when making the contructor with parameters in B, you need to pass in super() a parameter for A

example :

class A {
    public A(int x) {   }
  }

  class B extends A {
    public B(int x ) 
    {
       super(x); // need to specify the parameter for class A
       //... 
    }
  }

edited Jun 6, 2011 at 7:52

MGwynne

3,5321 gold badge25 silver badges35 bronze badges

answered Jun 6, 2011 at 7:19

Andrei Sfat

8,6265 gold badges56 silver badges71 bronze badges

Comments

GuruKulki · Accepted Answer · 2011-06-06 07:16:44Z

1

This happens when you dont have a default constructor and you are creating an instance with default one. Because If you have any Parameterized constructor then compiler will not insert the default one for you, instead you have to define.

answered Jun 6, 2011 at 7:16

GuruKulki

26.6k55 gold badges148 silver badges206 bronze badges

Comments

Stuti · Accepted Answer · 2011-06-06 07:22:12Z

0

It is neccessary to call constructor of super class in case of Java. Therefore, whenever you generate constructor of sub class, super class' constructor is self created by IDE.

It is because whenever base class constructor demands arguments, compiler thinks they are to be filled by base class constructor. In case of default constructor, it is OK. No need to call super() in sub class.

answered Jun 6, 2011 at 7:22

Stuti

1,6361 gold badge16 silver badges33 bronze badges

Collectives™ on Stack Overflow

Java inheritance - constructors

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related