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Java: Scopping rules

Posted on 2009-05-15
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Last Modified: 2013-12-29
This is a question about scopping rules.   In the code sample I have purposfully done somthing goofy to as a test.  I expected that line 3, {int b = 3; int c = 4;} ,would result in redefinition of b and c and shadow the variables with the same names in line 2.  Instead it results in the compiler errors shown below.  My question is why is it that the inner scope of line 3 does not allow the variables in line 2 to be redefined?
------------------------------
src\Initializer.java:5: b is already defined in main(java.lang.String[])
                { int b = 3; int c = 4;}
                      ^
src\Initializer.java:5: c is already defined in main(java.lang.String[])
                { int b = 3; int c = 4;}
----------------------------
                                 ^
class Initializer{
 
	public static void main(String [] args){
		int b = 1, c = 2;
		{ int b = 3; int c = 4;}
		System.out.println("Initializer: " + b + " " + c);
	
	}
}

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Question by:ssackett
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7 Comments
 
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Expert Comment

by:emce
ID: 24399481
the {...} by itself doesn't produce a new visibility scope
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Expert Comment

by:zwei
ID: 24399598
emce is right.

For what you want to do you'd have to do something like this:
class Initializer{
        static int b = 1;
        static int c = 2;
 
        public static void main(String [] args){
                int b = 3;
                int c = 4;
                System.out.println("Initializer: " + b + " " + c);
        
        }
}

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by:ssackett
ID: 24399788
emce & Zwei,

Why does the attached code fail to compile?  It seems that the b and c declared inside the initializer can't be seen outside the block (what I would expect) but that the block doesn't behave like a nested scope in other ways.
...
  {int b = 2;}
  b = 1
...

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Expert Comment

by:Thomas4019
ID: 24399921
The point of a custom block decaration { }, is so that variables declared inside can be garbage collected and deleted outside of the block
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Expert Comment

by:zwei
ID: 24400021
The variables are both local variables and are both on the same memory stack within a single method. When you create a new scope, you move on top of the original stack, which is why you can't create a new variable with the same name:
int a = 2;
{int a = 3;} <-- illegal, a is still on the stack.

But when you create a variable within a new scope, when you leave that scope the variable dies which is why this works:

{int b = 2;} <-- b dies here
int  b = 1;
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Accepted Solution

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Thomas4019 earned 125 total points
ID: 24400062
According to this link,

http://java.sun.com/docs/books/jls/second_edition/html/statements.doc.html

Java throws a compile time error because such naming can lead to very obscure and hard to detect errors.
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Author Closing Comment

by:ssackett
ID: 31582049
Thanks for the help guys.  Yet another reason why it's dangerous to mix C and Java.
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