inheritance in java (compile error)

Question

this is my program:

class A{
    A(int a){
    }

class B extends A{
    B(){    
    }
}

}

when I compile I have an error:

C:\Users\Public\Documents\AB.java:6: error: constructor A in class A cannot be applied to given types;
    B(){
       ^
  required: int
  found: no arguments
  reason: actual and formal argument lists differ in length

and it fixed by this change in my code:

    class A{
        A(int a){
        }
            A(){
        }

    class B extends A{
        B(){
        }
    }
}

but I don't understand why?

thanks for any help.

Read about default constructor, and also in what situation the compiler adds it to your class. — Rohit Jain
– Rohit Jain, Commented Nov 24, 2013 at 18:50

Sotirios Delimanolis · Accepted Answer · 2013-11-24 18:50:49Z

Since your A class declares a constructor

A(int a){
}

then any code that wants to instantiate A needs to do so with that constructor. Something like

A a = new A(42);

You also declare a child class, B. Since B is an A, in addition to its constructor, it must call the parent class' constructor. This is done implicitly by the compiler. Say you had

class A{
    A(){
    }

class B extends A{
    B(){
        // super(); injected by compiler
    }
}

If you don't have a no-arg constructor, then the compiler doesn't know which constructor call to inject. You need to explicitly declare the super(...) call.

class A{
    A(int a){
    }

class B extends A{
    B(){    
        super(42);
    }
}

Think of it this way: you can't construct an A without the constructor call. When you construct a B, you are, through inheritance, also constructing an A, so you need that same constructor call.

Kon · Accepted Answer · 2013-11-24 18:49:50Z

0

You need to have a default constructor (no-argument) in A. The first implicit call of a constructor of any class is super(). But there is not super() method in the superclass. You can either explicitly call super(5) or some value, or create a default constructor in A:

A() {}

answered Nov 24, 2013 at 18:49

Kon

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Comments

svz · Accepted Answer · 2013-11-24 18:50:18Z

0

When instance of class B is created a default call to super() constructor with no parameters is made. Each class has a default constructor with no parameters, unless a different constructor is provided explicitly.

As you have explicitly provided class A with a constructor that takes one parameter, it no longer has default constructor with no params, so you need to define it.

answered Nov 24, 2013 at 18:50

svz

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Comments

surajitk · Accepted Answer · 2013-11-24 19:14:51Z

0

Whenever we extend a class, by default it calls super() (it has no params). In your case , it dint get the matching A() , hence it resulted in compiler error. Whenever u write this, what happens internally is :

 class A
  {
   A(int a)
   {
   }
 class B extends A
 {
   B()
   {   
    //Internally compiler calls super()
    super();//In your case, it dint get the matching constructor
   }
 }

 }

answered Nov 24, 2013 at 19:14

surajitk

1474 silver badges22 bronze badges

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