Java default constructor

Neither of them. If you define it, it's not the default.

The default constructor is the no-argument constructor automatically generated unless you define another constructor. Any uninitialised fields will be set to their default values. For your example, it would look like this assuming that the types are String, int and int, and that the class itself is public:

public Module(){  super();  this.name = null;  this.credits = 0;  this.hours = 0;}

This is exactly the same as

public Module(){}

And exactly the same as having no constructors at all. However, if you define at least one constructor, the default constructor is not generated.

Reference: Java Language Specification

If a class contains no constructor declarations, then a default constructor with no formal parameters and no throws clause is implicitly declared.

Clarification

Technically it is not the constructor (default or otherwise) that default-initialises the fields. However, I am leaving it the answer because

• the question got the defaults wrong, and
• the constructor has exactly the same effect whether they are included or not.

A default constructor is created if you don't define any constructors in your class. It simply is a no argument constructor which does nothing. Edit: Except call super()

public Module(){}

A default constructor is automatically generated by the compiler if you do not explicitly define at least one constructor in your class. You've defined two, so your class does not have a default constructor.

Per The Java Language Specification Third Edition:

8.8.9 Default Constructor

If a class contains no constructor declarations, then a default constructor that takes no parameters is automatically provided...