Java always passes arguments by value NOT by reference.

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Let me explain this through an

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".
   
   

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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 :)