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
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.
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
.
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 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"
.
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 :)
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.