Friday, June 29, 2018

parameter passing - Java pass by reference




What is the difference between this 2 codes:



Code A:



Foo myFoo;
myFoo = createfoo();


where




public Foo createFoo()
{
Foo foo = new Foo();
return foo;
}


Vs. Code B:



Foo myFoo;

createFoo(myFoo);

public void createFoo(Foo foo)
{
Foo f = new Foo();
foo = f;
}


Are there any differences between these 2 pieces of codes?



Answer



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");


    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 :)


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