Variable scoping

[Hareendra Reddy]

Hello all,
In the following code compiler is showing errors(Indicated in comments...)

class Testing {
  int i=0;            // 1       
  public static void main(String[] args) {
     for(int i=0;i<=0;i++)   //2
     ;
     int i=0;       //3
     {
       int i=0;     //4--  ERROR
     }
     int x=0;
     switch(x)
       {
          case 0:
                  {
                    int i=0;         //5--ERROR
                  }
       }
    // int i=0;            if we declare the variable here instead at (3), then there is no error... 
  }
}

Can any one explain these errors or suggest me some references ,
I think this behavior is different from other languages....

Thanks in advance...

[Tom Reilly]

Here's your code formatted better. All the errors are "Duplicate local variable i". The int i declared on line 3 is a class instance variable. The semicolon on line 8 ends the for loop. Was this intentional? The int i declared on line 9 is a method local variable. The int i declared on line 12 is declared in a code block. Java does not allow variables to be re-declared in code blocks because this is more often than not a bug. That is, the developer does not intend to re-declare the variable. The same rule applies for the int i declared on line 20. The int i declared on line 23 that was commented out also causes a compiler error, despite the comment that says it does not. This one is trying to declare an int i as a class instance variable but one is already declared on line 3

class Testing
{
    int i=0; // 1
    
    public static void main(String[] args)
    {
        for(int i=0;i<=0;i++) //2
;
        int i=0; //3

        {
            int i=0; //4-- ERROR
        }
        
        int x=0;
        switch(x)
        {
            case 0:
            {
                int i=0; //5--ERROR
            }
        }
        int i=0; //if we declare the variable here, then there is no error...
    }
}

[Hareendra Reddy]

hi Tom Reilly ,
Thank you for reply....
I am going to make code neater..

Semicolon on line 3 is intentional, as the for loop uses another variable i,
It just mean do-nothing statement...

variable declared at 23 , i mean to say if we declare the variable instead at (3) then there is no error...

[Deepak Chopra]

Continuing to what Tom has already mentioned - If you have access to any particular variable in any method or any block of data, you can not declare a variable with same properties. so form the code you have mentioned -

int i = 0 ;

{

int i = 7;  // Here we already have access to i defined previously, so you cannot declare another i

}

if you think over it, you will find that this is correct behavior...

[Hareendra Reddy]

Hello Sunny Jain ,
Can i confirm that if we change the code in the following manner ,then there wont be error..

  
 {  
   
 int i = 7; 
   
 } 

 int i = 0 ;  

[Deepak Chopra]

hi Hareendra ,

Yes If you write following piece of code, you will not get any error-

public class A{

{
int i = 0;
}
int i = 7;
}

You can also consider the way an object is initialized in java -

1) Static block of super class executes

2) Static block of Subclass executes

3) Inside constructor of subclass

4) call to super constructor

5) call of Object Class constructor

6) Instance Variable of super class executes

7) init blocks of super class executes,in the order in which they appear

8) Super class constructor executes

9) Instance Variable of Subclass are initialized

10) Init blocks of subclass executes, in the order in which they appear

11) Subclass constructor executes

Source - Object Initialization in JAVA