a question about memory pointers

what is the output of the this method

i think its
0
200

why its the opposite??

the answer is
200
0

i thought in a certain way
1st p,q have the (0,0) values
than we put them in some method which calls p >>a
q>>b
2nd it puts the values of (100,100) inside of a( which is p)
3rd it puts the values of a into temp
4th it puts the values of b=(0,0) inside a
5th it puts in b the values of temp which are(100,100)
so why its the opposote??
public static void main(String[] args) { Point p, q; p=new Point(0,0); q=new Point(0,0); tryFun(p,q); System.out.println(p.x + p.y ); System.out.println(q.x + q.y); } public static void tryFun(Point a, Point b) { Point temp; a.x=100; a.y=100; temp=a; a=b; b=temp; }

Don't forget the semantics of Java parameter passing is pass by value. Specifically, when objects are passed to a method parameter, a copy of a reference to the object is passed. In this case:

When the tryFun() method is called:
public static void tryFun(Point a, Point b) { //a and p both refer to the same Point object where x = 0, y = 0 //b and q both refer to the same Point object where x = 0, y = 0 Point temp; //temp is declared, but currently refers to no Point object a.x=100; //a and p both refer to the same Point object where x = 100, y = 0 a.y=100; //a and p both refer to the same Point object where x = 100, y = 100 temp=a; //temp refers to the same Point object that a and p refer to a=b; //a now refers to the same object that b and q refer to (note: the object that p refers to has not changed) b=temp; //b now refers to the same object that temp and p refer to (note: the object that q refers to has not changed }

Code examples like this can be tricky until you really get a firm grasp of pass by value semantics.

in the start you want to say that
a and p
b and q

are not the same object???
so why if we change a.x=100
a.y=100
then the value of p got to be(100,100)

in case we say the "he this change in b applies also to q"??

in what change in b we say that its stays in b and not applies to q??

you said temp "refers to the same Point object that a and p refer to"

so they are linked together and every chahge in one leads to the chnge in the other

but in some point you say that its not true anymore

why???/
[ February 16, 2008: Message edited by: donaldth smithts ]

The fact that you swap the references inside the tryFun method doesn't mean they are swapped outside it.

You pass a reference to p (maybe 1324abcd) to tryFun, and then get the object at 1234abcd which you are now calling a and change its fields.

You also pass a reference to another object which might (but probably isn't, but we'll stick with the numbers) at 2345bcde.

You call it b and don't change it.

Then you change it around so inside the method you swap a to be 2345bcde and b to be 1234abcd.

But p is still 1234abcd and q is still 2345bcde.

Then you print out p and get 200, and q gives 0.

Voila!

Read what Garrett Rowe wrote very slowly and carefully.

Try this change: Point temp; temp=a; a=b; b=temp; a.x=100; a.y=100;

so when we make
a.x=100
a.y=100

it doesnt count as change in reference??

[donaldth smithts]: so when we make
a.x=100
a.y=100

it doesnt count as change in reference??

No thats not a change in the reference. That is a change to the state of the object that the reference refers to.

Check out Cup Size -- a story about variables.

Originally posted by Garrett Rowe:
No thats not a change in the reference. That is a change to the state of the object that the reference refers to.[/URL].

Exactly. You are not changing "a" or "b." You are changing "x" or "y."

"a" and "b" are the object references, "x" and "y" are the fields of the object.

so after we change there reference we can say that
a and p
b and q are detached

if we say after that operation that
a.x=1000
a.y=1000
p is still will be equaled to(100,100)