Java this.classProperty versus classProperty incase argument variable name clashes?

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

score 6 · Accepted Answer · edited Jun 20 '20 at 09:12

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.

thanks but if i use public User(int Id){ userId = Id;}, It updates class property. — talktopk, Apr 15 '12 at 19:01
@talktopk: Yes, because in that case the parameter doesn't shadow the field. — Jon Skeet, Apr 15 '12 at 19:01
@JonSkeet I was about to comment regarding Sun/Oracle's use of hiding vs shadowing but you've edited your answer already :) — darrengorman, Apr 15 '12 at 19:09
@milkplusvellocet: Yes, I hadn't looked that in detail before. And now obscuring, too... — Jon Skeet, Apr 15 '12 at 19:09

score 4 · Answer 2 · edited May 23 '17 at 10:08

4

This is because of shadowing.

You're not assigning the parameter value userId to the member variable because it's being shadowed. Because int member variables are initialised to 0 that's why you're seeing that output without the this keyword.

edited May 23 '17 at 10:08

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answered Apr 15 '12 at 19:01

darrengorman

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Java this.classProperty versus classProperty incase argument variable name clashes?

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