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Is Java pass-by-reference or pass-by-value?

asked May 15, 2015 in Core java by rajesh
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Java is always pass-by-value. The difficult thing to understand is that Java passes objects as references and those references are passed by value.

It goes like this:

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

    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.


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

public void foo(Dog d) {
    d.getName().equals("Max"); // true
answered May 15, 2015 by rajesh
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I just noticed you referenced my article ;)

The Java Spec says that everything in Java is pass-by-value. There is no such thing as "pass-by-reference" in Java.

The key to understanding this is that something like

Dog myDog;
is not a Dog; it's actually a pointer to a Dog.

What that means, is when you have

Dog myDog = new Dog("Rover");
you're essentially passing the address of the created Dog object to the foo method.

(I say essentially because Java pointers aren't direct addresses, but it's easiest to think of them that way)

Suppose the Dog object resides at memory address 42. This means we pass 42 to the method.

if the Method were defined as

public void foo(Dog someDog) {
    someDog.setName("Max");     // AAA
    someDog = new Dog("Fifi");  // BBB
    someDog.setName("Rowlf");   // CCC
let's look at what's happening.

the parameter someDog is set to the value 42
at line "AAA"
someDog is followed to the Dog it points to (the Dog object at address 42)
that Dog (the one at address 42) is asked to change his name to Max
at line "BBB"
a new Dog is created. Let's say he's at address 74
we assign the parameter someDog to 74
at line "CCC"
someDog is followed to the Dog it points to (the Dog object at address 74)
that Dog (the one at address 74) is asked to change his name to Rowlf
then, we return
Now let's think about what happens outside the method:

Did myDog change?

There's the key.

Keeping in mind that myDog is a pointer, and not an actual Dog, the answer is NO. myDog still has the value 42; it's still pointing to the original Dog (but note that because of line "AAA", its name is now "Max" - still the same Dog; myDog's value has not changed.)

It's perfectly valid to follow an address and change what's at the end of it; that does not change the variable, however.

Java works exactly like C. You can assign a pointer, pass the pointer to a method, follow the pointer in the method and change the data that was pointed to. However, you cannot change where that pointer points.

In C++, Ada, Pascal and other languages that support pass-by-reference, you can actually change the variable that was passed.

If Java had pass-by-reference semantics, the foo method we defined above would have changed where myDog was pointing when it assigned someDog on line BBB.

Think of reference parameters as being aliases for the variable passed in. When that alias is assigned, so is the variable that was passed in.

Does that help? (I'll have to add this as an addendum to my article...)
answered May 15, 2015 by rajesh
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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){

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.


    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 :)

answered May 15, 2015 by rajesh