life, scope, and visibility... some fun.

[Les Morgan]

My son is learning to program and we were having a little discussion on "life, scope, and visibility", this the code we came up with to illustrate different questions he had. See if you can walk through it and say why the print is the print that comes out when it comes out. In programming, IMO, most problems come down to life, scope, and visibility.

public class Scopes {
  public static int i = 5;
  public Scopes (){
    System.out.println("This is the Constructor");
    System.out.println(i);
    i++;
    {int i = 795;
      System.out.println("inner i " + i);
    }
    System.out.println("i test " + i);
    i = 77;  
    int i = 4;
    i = 105;
    System.out.println("This is the strange i " + i);
  }
  public int geti (){
    return i;
  }
  public void seti(int eye){
    i = eye;
  }

  public static void main(String[] args) {
    int i = 9;
    Scopes red = new Scopes ();
    Scopes blue = new Scopes ();
    System.out.println(red.geti());
    System.out.println(blue.geti());
    blue.seti (95);
    System.out.println(red.geti());
    System.out.println(blue.geti());
    System.out.println(i);
  }
}

[Jeanne Boyarsky]

Fun!

[Marco Behler]

Crazy indeed Took a while to wrap my head around things written like that...

[Campbell Ritchie]

It is even worse in C/C++/* scopes.c */ #include <stdio.h> #include <stdlib.h> int i = 666; int main(int argc, char **argv) {    {        printf("i = %u\n", i);        int i = atoi(*(argv + 1));        printf("i = %u\n", i);        {            printf("i = %u\n", i);            int i = 999;            {                printf("i = %u\n", i);                int i = 987654321;                printf("i = %u\n", i);                {                    printf("i = %u\n", i);                    int i = 0;                    printf("i = %u\n", i);                }            }            printf("i = %u\n", i);        }        printf("i = %u\n", i);    }    printf("i = %u\n", i);    return 0; }

gcc -o scopes scopes.c
[critchie@campbellscomputer ~]$ ./scopes 123
. . .

[German Martinez]

Defining the variable i as static in Scopes causes misunderstandings. This variable is UNIQUE for all instances of Scope.

public class Scopes {
    public static int i = 5;    // this is a **UNIQUE** static variable used by all instances of Scopes
                                // Also you can access it using something as  a = Scopes.i, or  Scopes.i = a

    public Scopes (){
        System.out.println("This is the Constructor");
        
        // this is the UNIQUE static variable of Scopes class
            System.out.println(i);
        
        // this is the UNIQUE static variable of Scopes class
            i++;                                    

        {
            // these are a local variable inside the structure
            int i = 795;
            System.out.println("inner i " + i);
        }

        // these are the UNIQUE static variable of Scopes class
            System.out.println("i test " + i);
            i = 77;
        
        // these are a local variable in the constructor
        // static variable i in Scopes is hided
            int i = 4;
            i = 105;

            System.out.println("This is the strange i " + i);
    }
    
    public int geti (){
        // this is the UNIQUE static variable used by all instances of Scopes
        return i;
    }
    
    public void seti(int eye){
        // this is the UNIQUE static variable used by all instances of Scopes
        i = eye;
    }
    
    public static void main() {
        // this is a local variable in main(), it hides the static variable in Scope
            int i = 9;
        
        Scopes red  = new Scopes ();
        Scopes blue = new Scopes ();
        
        // these are the UNIQUE static variable i in Scopes
            System.out.println(  red.geti() );
            System.out.println( blue.geti() );
        
        // this set the UNIQUE static variable i in Scopes
            blue.seti (95);
        
        // these read the UNIQUE static variable i in Scopes
            System.out.println(  red.geti() );
            System.out.println( blue.geti() );
        
        // prints the local variable in main()
            System.out.println(i);              
    }
}

[Les Morgan]

German,

You are absolutely right, the static definition of i, in the body of the object makes i a Class variable, and it is very useful in some cases, but not if you do not expect what it does. The difference is one thing I wanted my son to see as an introduction fo use of static members.

Les