Implicit super constructor Num() is undefined for default constructor. Must define an explicit constructor, what is the logic behind this

Question

class Num 
{
    Num(double x) 
    { 
        System.out.println( x ) ; 
    }
}
class Number extends Num 
{ 
    public static void main(String[] args)
    { 
        Num num = new Num(2) ; 
    } 
} 

In the above program, its show the error. Please help me out.

Answer 1

When you define your own constructor,the compiler does not provide a no-argument constructor for you. When you define a class with no constructor,the compiler inserts a no-arg constructor for you with a call to super().

class Example{
}

becomes

class Example{

Example(){
super();   // an accessible no-arg constructor must be present for the class to compile.
}

However,it is not the case with your class as Number class cannot find a no-arg constructor for Num class.You need to explicity define a constructor for you with a call to any of the super constructor

Solution:-

class Num 
{
    Num(double x) 
    { 
        System.out.println( x ) ; 
    }
}

class Number extends Num 
{ 


 Number(double x){
 super(x);
 }

 public static void main(String[] args)
    { 
        Num num = new Num(2) ; 
    } 
}