why don't we change the arraylist element by reference?

[ahmet oguz]

Hi,
in java, primitives types move with pass by value but other types(Object Types) with pass by value of their references. There is a question at this point.
if we have got an arraylist with element of strings why don't we change the arraylist elements? a sample code:
ArrayList a = new ArrayList();
a.add("istabul");
a.add("Ankara");
a.add("Denizli");

if we take a new arraylist we pass the value of reference to the k variable.
ArrayList k = a;
This is ok now k and a is the same reference to the arraylist

but if we take the element of arraylist which elements are String
why don't we chage elements of arraylist?
String string = (String)a.get(2); --this is "Denizli"
string = "İzmir";
now the arraylist third element at position 2 is Denizli Why? I don't understand this point. String is pass by value of references but this is not ok
Thanks.

[Burkhard Hassel]

Hi ranchers,

ilter wrote:

ArrayList a = new ArrayList();
a.add("istabul");
a.add("Ankara");
a.add("Denizli");
String string = (String)a.get(2); --this is "Denizli"
string = "İzmir";

now the arraylist third element at position 2 is Denizli Why? I don't understand this point. String is pass by value of references but this is not ok

You create a String object with the content "Denizli" at the line where you add it to the ArrayList. The ArrayList variable a refers to it (and to the other turkish towns).
In the line --this is "Denizli" a String variable named string now also refers to the "Denizli" - String-object within the ArrayList.
In the last line, the variable string now refers to (~ points to) a new object ("İzmir"). But this will not change the ArrayList. It's like this:

last but one linevariable       object(s)
                    a       -->  İstanbul, Ankara, Denizli
                  string    -->                    --/
 
last line
                     a      -->  İstanbul, Ankara, Denizli
                   string   -->  İzmir
 

Denizli is still at its place in the ArrayList.
The method get(int i) does not remove an alement, it only returns a reference to it. Using the get-method does not change a list.

Yours,
Bu.

[ahmet oguz]

hi, thanks but my mind went blank.

  
class Deneme{
public int x;
public String s;
public Deneme(){ x = 5; s = "istanbul";}


}
public class Reference {
 
/**
 * @param args
 */
public static void change(Deneme s){
s.x = 10;
s.s = "ankara";
}
public static void change(String s){
s = "ankara";
}
public static void main(String[] args) {
// TODO Auto-generated method stub
String k = "denizli";
change(k);
System.out.println(k);
Deneme d = new Deneme();
change(d);
System.out.println(d.x + "-" + d.s);
}
 
}
 
  

when i run this java code the result is:
denizli
10-ankara

why public static void change(String s) function does not change the value
but
public static void change(Deneme s) fucntion changes the value of string.

I know that Objects pass by value of their references but the change function that parameter type is string does not change the string.
Please help me
thanks

[Ilja Preuss]

Strings never get changed - they are immutable, their value is fixed.

s.s = "ankara";

causes the String reference in the object referenced by s to refer to the new String "ankara". That change to the *reference* is seen by everyone using that object.

s = "ankara";

causes the *local* reference s to refer to the new String "ankara". As the reference is local, that change is not seen from code outside the method.

Does that help?

[Arnav Velimala]

In Java, it is pass-by-value.

String k = "denizli";
change(k);

When you invoke change(k) method, a *copy* of 'k' is created and is passed to the method. Not the original 'k' is passed(since it is pass-by-value). So any changes made in the method will be reflected in the copy-variable and not the original value. That's why change(String s) method does not change the value.

References are also passed-by-value - a copy of reference will be created(the created copy will now be an alias to the object the original reference is refering to).

Deneme d = new Deneme();
change(d);

public static void change(Deneme s){
s.x = 10;
s.s = "ankara";
}

d & s are aliases to the Deneme object. With 's' reference you can change the *state* of the object as 'd' and 's' denotes the same object.

Since Java passes by-value,

Deneme d = new Deneme();
change(d);

public static void change(Deneme s){
s=null
}

will not nullify the "d" reference. The statement System.out.println(d.x + "-" + d.s); will give NullPointerException IF it has been nullified.