reference type in Java [duplicate]

Question

Possible Duplicate:
Is Java pass-by-reference?
Java pass by reference

For the following Java program, my understanding is that a is a reference type to an Integer, like pointer type in C/C++. So any changes done in method f to its value will be reflected after the method returns. But println still prints its original value 0 instead of 3.

Integer and int does not make a difference. Was my previous understanding wrong? Please help. Thank you!

  public static void f(Integer b){
            b=3;
        }
        public static void main(String[] args){
            Integer a=0;
            f(a);
            System.out.println(a);
      }

Answer 1

Java always passes arguments by value NOT by reference.

---

Let me explain this through an example:

public class Main
{
     public static void main(String[] args)
     {
          Foo f = new Foo("f");
          changeReference(f); // It won't change the reference!
          modifyReference(f); // It will modify the object that the reference variable "f" refers to!
     }
     public static void changeReference(Foo a)
     {
          Foo b = new Foo("b");
          a = b;
     }
     public static void modifyReference(Foo c)
     {
          c.setAttribute("c");
     }
}

I will explain this in steps:

1. Declaring a reference named f of type Foo and assign it to a new object of type Foo with an attribute "f".

   Foo f = new Foo("f");
   

   

2. From the method side, a reference of type Foo with a name a is declared and it's initially assigned to null.

   public static void changeReference(Foo a)
   

   

3. As you call the method changeReference, the reference a will be assigned to the object which is passed as an argument.

   changeReference(f);
   

   

4. Declaring a reference named b of type Foo and assign it to a new object of type Foo with an attribute "b".

   Foo b = new Foo("b");
   

   

5. a = b is re-assigning the reference a NOT f to the object whose its attribute is "b".

   

   ---

6. As you call modifyReference(Foo c) method, a reference c is created and assigned to the object with attribute "f".

   

7. c.setAttribute("c"); will change the attribute of the object that reference c points to it, and it's same object that reference f points to it.

   

I hope you understand now how passing objects as arguments works in Java :)