Enthuware

There is only one variable named obj in the given code and that variable is declared at line //1. So, I am not sure what is the confusion about the variable.
Please remember that variable and the object to which it points are two different things.

It's clear now, thanks!

Suppose the question code was (I just added one println line):

public class NewClass {
private Object o;
void doSomething(Object s){
o = s;
//line bellow was added
System.out.println("the second time this line is printed is the exact moment "
+ "when the object created in line 1 can be Garbage Collected"); //line 7
}

public static void main(String args[]){
Object obj = new Object(); // 1
NewClass tc = new NewClass(); //2
tc.doSomething(obj); //3
obj = new Object(); //4
obj = null; //5
tc.doSomething(obj); //6
}
}

Am I right? I mean, the second time line 7 is evoked is the exact moment where the object created in line 1 is eligible for garbage collector, right?

Correct.

How do I know line5 makes obj(on line 4) null?,instead of line 1 obj?

I thought line 5 made line1 obj null.

The code given in the question is: obj is being reassigned a new Object at //4, that means the statement obj = null; at //5 will cause that Object to lose the reference.