Java pass by value example

January 17, 2016

Java is always pass-by-value. Unfortunately, they decided to call pointers references, thus confusing newbies. Because those references are passed by value.

It goes like this:

public static void main( String[] args ){
    Dog aDog = new Dog("Max");
    foo(aDog);

    if (aDog.getName().equals("Max")) { //true
        System.out.println( "Java passes by value." );

    } else if (aDog.getName().equals("Fifi")) {
        System.out.println( "Java passes by reference." );
    }
}

public static void foo(Dog d) {
    d.getName().equals("Max"); // true

    d = new Dog("Fifi");
    d.getName().equals("Fifi"); // true
}

In this example aDog.getName() will still return "Max". The value aDog within main is not overwritten in the function foo with the Dog "Fifi" as the object reference is passed by value. If it were passed by reference, then the aDog.getName() in main would return "Fifi" after the call to foo.

Likewise:

Dog aDog = new Dog("Max");
foo(aDog);
aDog.getName().equals("Fifi"); // true

public void foo(Dog d) {
    d.getName().equals("Max"); // true
    d.setName("Fifi");
}


Java always passes arguments by value NOT by reference.


Let me explain 
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:


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");




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)




As you call the method changeReference, the reference a will be assigned to the object which is passed as an argument.
changeReference(f);




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");




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





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




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.