Explain code from Java?

Question

Java

I didn’t understand the explanation of the site, explain why there is an error in line 3, why is the default constructor needed?

Answer 1

If no constructor is declared in class A, then by default it contains a constructor without parameters - A(), however, if at least one constructor with parameters is declared, then the constructor A() disappears. But it disappears only when it is not explicitly declared.
Examples

//содержит неявно 1 конструктор A()
class A{
}

//тоже содержит  конструктор A() но явно
class A{
    A(){
    }
}

//cодержит СОДЕРЖИТ ТОЛЬКО 1 конструктор, конструктора A() уже нет
class A{
    A(int x){
    }
}

//есть оба конструктора
class A{
    A(int x){
    }

    A(){}
}

In the example, class A contains one constructor with a parameter - A(int x), which means that it does not have an A() constructor, and you must (if necessary) define it yourself.
The inheritance mechanism is as follows:
- we want to create an object B by calling the constructor B(int b)
- the first line of the constructor should be the string super(...) - with or without parameters, this is necessary so that the parent is first constructed, because B is inherited from A
- if the first line is different, then the compiler substitutes the parent's constructor without parameters - super(), in this case, the creation of the object A using the constructor A() will be called.
- but since this constructor is not defined in object A - an error occurs.
You can fix it like this
Either add the A() constructor to object A, or add an explicit call to the parent super(int x) constructor to the object B constructor (in this case, the parent has only 1 constructor - A(int x)