Call by Value

[David Houghton]

The question relates to java basics, im currently studyiung towards my SCJP cert. This is about a passage ive read in a book that doesnt make sense. Its not about the actual cert so ive put it here in the begginer section. The passage states:


If the argument passed into the method is a primitve type, it is impossible in Java for the method to alter the value of the original primitive (SCJP Study Guide, Richard F Raposa, page37, passing primitives vs passing references).

The section is on Call by Value.

My question is, doesnt the code i have given below make this passage incorrect?

  

public class MC
{
   public int x = 0;
   /** Creates a new instance of MC */
   public MC()
   {
   }
   
   public static void main(String[] args)
   {
      MC m = new MC();
      System.out.println(m.x);
      m.setX(5);
      System.out.println(m.x);
   }
   
   public void setX(int anInt)
   {
      x = anInt;
   }
   
   
}
  

[Gavin Tranter]

In what way do you think that your example makes the statement about how Java passes primitive types incorrect?

[David Houghton]

If you comile and run the code, the following print statment show that the x value has changed from 0 to 5 due toi being changed by the method

       System.out.println(m.x);   
      m.setX(5);   
      System.out.println(m.x);    

[David Houghton]

This is the netbeans output

  init:
deps-jar:
compile-single:
run-single:
0
5
BUILD SUCCESSFUL (total time: 0 seconds)  

[Fred Hamilton]

look at it this way. if you had int n=5; in your main method, and you passed n instead of 5, e.g. m.setX(n);

then no matter what happens in the method setX, n would still be equal to 5 in your main method. That is what is meant by "the method can't change the the value of the original primitive"

[fred rosenberger]

you are not changing the value of the passed parameter. you change the value of a class variable. Try changing your code to something like this:

 public static void main(String[] args)  
    {  
        int temp = 5;
        System.out.println("main, before calling set(), temp is " + temp);
        set(temp);
        System.out.println("main, after calling set(), temp is " + temp);
    }  
      
    public void set(int temp)  
    {  
      System.out.println("at the beginning of set() method, temp is " + temp);
       temp = 7;  
      System.out.println("at the end of set() method, temp is " + temp);
    }  

Here, I pass temp into a method, and change it's value. But back in main, themp says as 5 (assuming i wrote this correctly - I didn't test or verify it even compiles).