Java nested scopes and variables' name hiding

Question

I'm learning name look up in Java, and coming from C++ I found interesting that even if Java lets me nest however many blocks of code,I am allowed to hide a name only in the first nested scope:

// name hiding-shadowing: local variables hide names in class scope

class C {

  int a=11;

  {
    double a=0.2; 

  //{
  //  int a;             // error: a is already declared in instance initializer
  //}

  }

  void hide(short a) {  // local variable a,hides instance variable
    byte s;
    for (int s=0;;);    // error: s in block scope redeclares a
    {
      long a=100L;      // error: a is already declared in (method) local scope
      String s;         //error: s is alredy defined in (method) local scope 
    }                   
  }

}

this is weird from a C++ perspective,since there I can nest how many scopes I want,AND I'm able to hide variables as I like. Is this the normal behaviour of Java or am I missing something?

actually your paradigm coming from C++ is erroneous, you must adapt your mind to Java way of thinking — Jordi Castilla, Sep 28 '15 at 09:24
For future Googler: see also [*Block scope variables*](https://stackoverflow.com/questions/20499554/block-scope-variables). — Franklin Yu, Nov 20 '18 at 06:48

score 9 · Accepted Answer · answered Sep 28 '15 at 09:25

9

It's not about the "first nested scope" - it's a matter of Java allowing a local variable to hide a field, but not allowing it to hide another local variable. Presumably the designers of Java believed such hiding to be bad for readability.

Note that your example of a local variable in an instance initializer does not create an error - this code is valid:

class C {
  int a = 11;

  {
    // Local variable hiding a field. No problem.
    double a = 0.2;
  }
}

answered Sep 28 '15 at 09:25

Jon Skeet

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1

Nice to get an answer from you... `:)` – Praveen Kumar Purushothaman Sep 28 '15 at 09:28
Correct answer !! Classic case of someone trying to use a language before reading through the first chapter :-) – Geek Sep 28 '15 at 09:30
yes,thanks for the clarification about nesting.. I noticed that even if I declare a new name into a local scope ( initializer block,method or loop ) that doesn't hide a field I cannot hide that name,the compiler complains – Luca Sep 28 '15 at 09:32
1

@Geek the anwer is correct indeed and you're just guessing wrong since I've been studying C++ and Java for 6 months now, but I guess you learn everything on page one of your manual,good for you! – Luca Sep 28 '15 at 09:34
@Luca: Yes, exactly - you can't hide one local variable with another. – Jon Skeet Sep 28 '15 at 09:36
@Luca : No offense intended. Happy Learning !! – Geek Sep 28 '15 at 09:41

score 3 · Answer 2 · answered Sep 28 '15 at 09:26

I'm not a C++ guy but that really looks weird from C++ side and If I were the designer , I completely remove that behaviour. That really causes bugs and hard to read the code.

To lead a peaceful life in Java that behaviour is completely removed. Compiler shows you an error if you try to do that as you are seeing it now.

Java nested scopes and variables' name hiding

2 Answers2