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javaclasssubclasssuperclass

Calling constructor of superclass when it doesn't exist

If I had following question on java theoretical test, what would be the right answer?

The question/task:

Create a Circle class inheriting from the Shape class and call the Shape class constructor in its constructor. Shape class:

public class Shape {
    private int size;
}

Choose the correct answer:

A:

class Circle extends Shape{

    Circle(){
        super();
    }
}

B:

"You can't call constructor of Shape class as it doesn't exist"

Some say that the right answer is B, but I don't understand why it can't be A? Won't Java create default constructor and call it anyway?

Solution

  • According to the official Java Language Specification (JLS), section 8.8.9:

    If a class contains no constructor declarations, then a default constructor is implicitly declared.

    Reading through that section shows that when Shape is compiled, it gets a constructor as if defined by

    public Shape() {}

    It is public because Shape is public and implicitly calls super(); because it is empty.

    So clearly you are correct that option A is the answer.

    But what about option B? As it happens, the very next section of the JLS (section 8.8.10) deals exactly with how to create a non-instantiatable class:

    A class can be designed to prevent code outside the class declaration from creating instances of the class by declaring at least one constructor, to prevent the creation of a default constructor, and by declaring all constructors to be private (§6.6.1).

    In practice, if you manually declared a private empty constructor to Shape, you would not be able to extend it, exactly because the call to super() in Circle would not resolve:

    private Shape() {}