fwd or backward ,don't know which reference??

[R K Singh]

 class Class020122{
public static void main(String str[]){
A1 a1 = null;
B b1 = null;
a1 = new A1(new B(a1)); // LINE01
b1 = new B(a1);
System.out.println("a1: "+ a1 + " #   b1: " + b1);
System.out.println("a1.b: "+ a1.bb + " #   b1.a: " + b1.aa);
}
}

class A1{
B bb;
A1(B b){
bb = b;
}
public String toString(){
//System.out.println("A Object");
return "A";
}
}
class B{
A1 aa;
B(A1 a){
aa = a;
}
public String toString(){
//System.out.println("B Object");
return "B";
}
}
 

I am not able to get why at LINE01 member variable of class B is not being assigned null
?
2. is it possible for object obj1 to be accesible from object obj2 and object obj2 to be accesible form obj1 ??
TIA
[ January 22, 2002: Message edited by: ravish kumar ]
[ January 22, 2002: Message edited by: ravish kumar ]

[Dave Vick]

Ravish
At LINE 01 the member variable is being assigned null. Actually you dont get a reference to the object that is created so you cant even test it anyway.
You're creating a B object in the creation expression for a1 so you are just passing a value of null to the constructor - you have no reference to the B object created except through the the A1 objects bb member. But you're still creating a valid B object even though one of its members has a value of null. Look at this code here with comments exlaining what is happening (I commented out the toString methods so you can see that they are different objects being created):

class Test{
public static void main(String str[]){
A1 a1 = null;
B b1 = null;
a1 = new A1(new B(a1)); // LINE01
      // *** NullPointerException because the A1 object referenced by aa is null
      // System.out.println("b1.aa: " + b1.aa);

      // The next line is OK because the bb variable does reference a valid B object
      System.out.println("a1.bb: " + a1.bb);

      // However the value of the aa member variable is null
System.out.println("a1.bb.aa: " + a1.bb.aa);

      // Here you assign a valid A1 object to the aa member of new B object so
      //  when you try to access the A1 object later you dont get a NullPointerException
      b1 = new B(a1);

System.out.println("a1: "+ a1 + " #   b1: " + b1);
System.out.println("a1.b: "+ a1.bb + " #   b1.a: " + b1.aa);
}
}

class A1{
B bb;
A1(B b){
bb = b;
}
/*public String toString(){
//System.out.println("A Object");
return "A";
} */
}
class B{
A1 aa;
B(A1 a){
aa = a;
}
/*public String toString(){
//System.out.println("B Object");
return "B";
}*/
}

hope that helps

[R K Singh]

Hi Dave ..
Thanks a lot ..
now things are clear to me ....
[ January 22, 2002: Message edited by: ravish kumar ]