Question

Can someone please explain output 0 in case of first constructor without this ?

If the argument variable name is same as class property name and i am using that property inside the method. What does java interpret that “class property” or “the argument variable” ?

Without this:

 public User(int userId){
    userId = userId;

 }

With this:

 public User(int userId){
    this.userId = userId;

 }


 public void PrintUserId(){
    System.out.println(this.userId);
}

 User firstUser = new User(123);
 firstUser.PrintUserId(); 

// 0 without this

//123 with this

Answer 1

Can someone please explain output 0 in case of first constructor without this ?

Sure – this statement is a no-op:

userId = userId;

It just assigns the variable of the userId parameter to itself. It doesn’t touch the field at all. Within the method, the parameter userId shadows the field called userId – so you have to explicitly say that you want to refer to the field, which is what the second version does:

this.userId = userId;

I’d expect any modern IDE to highlight the no-op assignment in the first version with a warning.

(It’s worth being clear about terminology, by the way – an argument is a value provided to a method; a parameter is the variable which is declared as part of the method signature. Likewise it’s a field rather than a property.)

EDIT: If the parameter has a different name, e.g.

public User(int id) {
    userId = id;
}

then the parameter doesn’t shadow the field, and the identifier userId still refers to the field. It’s all a matter of working out what the meaning of an identifier is – in your first example, the simple name userId refers to the parameter, which is what causes the problem.

EDIT: From section 6.4.1 of the JLS:

Some declarations may be shadowed in part of their scope by another declaration of the same name, in which case a simple name cannot be used to refer to the declared entity.

…

A declaration d of a field or formal parameter named n shadows, throughout the scope of d, the declarations of any other variables named n that are in scope at the point where d occurs.

So in this case d would be the declaration of the formal parameter userId, and the scope of d is the constructor – so throught the constructor, the parameter shadows the field.