Immutable and pass by value

前端 未结 4 1934
我在风中等你
我在风中等你 2020-12-29 14:01

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         


        
相关标签:
4条回答
  • 2020-12-29 14:35

    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.

    0 讨论(0)
  • 2020-12-29 14:39

    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.

    0 讨论(0)
  • 2020-12-29 14:40

    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");
    

    enter image description here

    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)
    

    enter image description here

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

    changeReference(f);
    

    enter image description here

    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");
    

    enter image description here

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

    enter image description here


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

    enter image description here

    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.

    enter image description here

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

    0 讨论(0)
  • 2020-12-29 14:56

    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.

    0 讨论(0)
提交回复
热议问题