Java Pass by value

Question

Possible Duplicate:
Java, pass-by-value, reference variables

Consider the following simple java program

class main{
          public static void main(String args[]){
                int x = 5;
                change(x);
                System.out.println(x);
          }
          static void change(int x){
             x = 4;
          }
 }

Since java uses pass by value the value of x will not change in the main..To overcome this problem we have the concept of pass by reference in c..But i do not find any such concept in java..How can i really change the value of x..? If there is no way of changing x then is this not a disadvantage of java?

Answer 1

C also passes parameter by value. But if you pass a pointer (also by value of a pointer), you can change the value of a variable that the pointer points to.

You can check that C pass pointer by value by changing the value of a pointer in a function. When the function returns, the pointer still points to the same location (not the one that it points in the function).

Pass by a value is not a disadvantage. I feel safer if I'm sure that a function or a method cannot change the value of an argument.

If you want to change the value of x, use the following code:

x = change(x);

and change void change(...) to int change(...).

Answer 2

You can't change the value of x in the calling code within change. (It would have been clearer if you'd used two different variable names.) I don't find it makes my life harder in Java - you can use pass by reference in C#, but it's rarely a good idea.

Pass by value is usually easier to understand, and leads to simpler code. Usually I either want a method to simply compute something - in which case that should be the return value - or I want to mutate an object; preferably the object I call the method on, but potentially an object which I indicate by passing a reference by value for one of the parameters. I very, very rarely wish I could change the value of a variable in the calling code other than by using simple assignment.

(Oh, and C doesn't have pass-by-reference either, by the way. It allows you to pass pointers by value, but that's not the same thing.)

Answer 3

Basically primitives are passed by value, objects are passed by reference and there are no pointers. So make a wrapper class or return the new value.

More information can be found here: http://javadude.com/articles/passbyvalue.htm

Answer 4

if you need to change a value using a method then don't use primitive data types...alternatively since you are using an int you could do this:

 class main{
          public static void main(String args[]){
                int x = 5;
                x = change(x);
                System.out.println(x);
          }
          static int change(int x){
             x = 4;
             return x;
          }
 }

Answer 5

One way to do this is to give x class scope by moving the declaration outside the main function, that way it's scope is available to all members of the class, including the function change. You could also make x public, protected, or a member of a public class.

class main{
      int x = 5;
      public static void main(String args[]){
            change(x);
            System.out.println(x);
      }
      static void change(int x){
         x = 4;
      }
}

So the default private scope of all objects is not a disadvantage, quite the opposite. It minimizes side effect of unintended changes, unless the programmer explicitly allows it.