shadowing

[akash azal]

  class Foo {  
   Bar myBar = new Bar(); 
   void changeIt(Bar myBar) {
     myBar.barNum = 99;
     System.out.println("myBar.barNum in changeIt is " + myBar.barNum);
     myBar = new Bar();
     myBar.barNum = 420;
     System.out.println("myBar.barNum in changeIt is now " + myBar.barNum
   }
   public static void main (String [] args) {
     Foo f = new Foo();
     System.out.println("f.myBar.barNum is " + f.myBar.barNum);
     f.changeIt(f.myBar);
     System.out.println("f.myBar.barNum after changeIt is "  
                        + f.myBar.barNum);
   }
 }
   

this is the code from k&S .i am not able to understand how variables/objects behave in case of shadowing.

[Ravikanth kolli]

  # class Foo {    
    Bar myBar = new Bar();   
    void changeIt(Bar myBar) {  
      myBar.barNum = 99;  
      System.out.println("myBar.barNum in changeIt is " + myBar.barNum);  
      myBar = new Bar();  
      myBar.barNum = 420;  
      System.out.println("myBar.barNum in changeIt is now " + myBar.barNum  
    }  
    public static void main (String [] args) {  
      Foo f = new Foo();  
      System.out.println("f.myBar.barNum is " + f.myBar.barNum);  
      f.changeIt(f.myBar);  
      System.out.println("f.myBar.barNum after changeIt is "    
                         + f.myBar.barNum);  
    }  
  }    

well lets start from line 12. AS there is a new object of Foo is created the value of barNum for that object is '0' .
then we have

f.changeIt(f.myBar);

which passes the bar object as a reference.
In line 5 it prints the value of 99 as the value of barNum is changed to 99.
In line 6 a new object of Bar is created and thus the reference to the previous bar is lost for that variable, which now points to a new object.
In line 8 420 is printed since that is the value for the new bar object.

Now when we return to line 14 the value for barNum is going to be 99 since that is the last time the value has been changed for that object.

The object passed as an argument is shadowed by the creation of a new object in the method.

[Stephen Davies]

I'm assuming the Bar Object somewhere has a public variable barNum?

[Ravikanth kolli]

yap that is what i assumed..

[Stephen Davies]

Running the code with my own implementation of Bar:

class Foo {      
     Bar myBar = new Bar();     
     void changeIt(Bar myBar) {    
       myBar.barNum = 99;    
       System.out.println("myBar.barNum in changeIt is " + myBar.barNum);    
       myBar = new Bar();    
       myBar.barNum = 420;    
       System.out.println("myBar.barNum in changeIt is now " + myBar.barNum);   
     }    
     public static void main (String [] args) {    
       Foo f = new Foo();    
       System.out.println("f.myBar.barNum is " + f.myBar.barNum);    
       f.changeIt(f.myBar);    
       System.out.println("f.myBar.barNum after changeIt is "      
                          + f.myBar.barNum);    
     }    
  } 

class Bar{
	
	public int barNum;
}

I get the folowing output:

f.myBar.barNum is 0
myBar.barNum in changeIt is 99
myBar.barNum in changeIt is now 420
f.myBar.barNum after changeIt is 99

May I be so bold as to ask exactly what this has to do with shadowing?

[Ravikanth kolli]

well i think that creation of a new object in the method has actually shadowed the object that is passed as an argument. dont you think so?

[Mumtaz Khan]

It is just an example of shadowing an instance variable by declaring a local variable of the same name and the shadowed variable is an object reference. The local variable myBar in changeIt() method is affecting myBar instance variable that is referring to an object of type Bar. myBar parameter receives reference to same Bar object.

[Stephen Davies]

Yes, I see now, the Instance myBar object reference is shadowed by the myBar Bar reference of the argument to the changeIt() variable (local variable) So when we are acting upon the myInt variable we are actually acting upon the local variable, but it can seem as if we are working on the instance variable.

[Bert Porter]

"ZooKeeper" topic turns out to be an instance of shadowing.