Immutable and pass by value

Question

I have the following code which has
a mutable Person class, String and a method to modify the instances of String and Person

    class Person{

int a = 8         


        

Answer 1

Sometimes people get confused when passing by reference. It's possible to change the object that the reference refers to (giving the impression of pass-by-reference), but it is not possible to modify reference itself. So it still remains pass-by-value.

Answer 2

In modifyObject, When you assign to str, you're not mutating str, you're setting it so that it points to a different object. Since it's passed by value, the str pointer local to your modifyObject method is a copy of the s pointer in main, so when you change the former, it does not affect le later.

On the other hand, when it comes to p, the one in modifyObject is still a copy of the one in main, but both pointers refer to the very same object in memory, hence if you call a method on it from modifyObject, you're actually mutating the thing pointed to by p.

Answer 3

Java always passes arguments by value NOT by reference.

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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 change 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".

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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 :)

Answer 4

In this function call "modifyObject(s, p);" you are sending the value of variable s to modifyObject method's local variable str. So a new variable is created and its value is changed but the original one remains unchanged.