以下哪个类具有默认构造函数？

Question

Which of these classes has a default constructor? I'm thinking just A but am I wrong?

class A {}
class B {B() {}}
class C {C(int j) {}}

Answer 1

You are correct. A default constructor is, by definition, created when no constructor is defined. Since A does not have a constructor defined it will have a default constructor created for it.

Technically speaking, you could say B has a default constructor since a parameter-less constructor is what would be created if none is defined.

Reference: http://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.8.9

Answer 2

Precisely.

Class A has a default constructor because you're not providing any constructor for the class. The compiler, therefore, automatically provides a no-argument, default constructor. This default constructor will call the no-argument constructor of the superclass (in this case the Object class). The second class, Class B , is providing a no-argument constructor, but it is not default as the compiler is not providing it for you.