What does this reference mean??

[Ravissant Markandey]

Hi everyone,
I came upon this piece of code in Kathy sierra, wasn't able to make much sence of it so thought of asking, could anyone tell me why SOP(1) gives an object while SOP(2) gives null, actually the piece of code:instance1.instance2 is not clear to me, the code is as follows:

public class Q6 {
 
Q6(){

}
 
Q6(Q6 qPar){

qInstance=qPar;
}
 
 Q6 qInstance;
 
  public static void main(String s[]){

Q6 q1=new Q6();
System.out.println("The object pointed by q1 is:"+q1);


Q6 q2=new Q6(q1);
q2.go();



Q6 q3=q2.qInstance;
       System.out.println("The object pointed by q3 is:"+q3);///SOP(1) 
q3.go();
    
    Q6 q4=q1.qInstance;
    System.out.println("the object pointed by q4 is: "+q4);///SOP(2)
    //q4.go();////troublesome code
 
}
 void go(){
 
 System.out.println("hi");
 }
 
}

Thanks in advance,
Ravissant Markenday

[Joanne Neal]

q1 was created with the no argument constructor of the Q6 class, which doesn't initialise its qInstance variable.

[Rob Spoor]

Q6 q1 = new Q6(); calls the first constructor, which does nothing. As such, q1.qInstance never gets anything assigned to it.

Now the rule is, any field (not local variable!) that never gets a value assigned will get a default value - null for objects. Since q1.qInstance never gets anything assigned to it, it will remain null.

[Ravissant Markandey]

gawd yea ..Thanks so much Joanne

[Ravissant Markandey]

Yes Rob I got it, thanks so much