Object reference in java

Question

consider this simple servlet sample:

protected void doGet(HttpServletRequest request, HttpServletResponse response){
    Cookie cookie = request.getCookie();
    // do weird stuff with cookie object
}

I always wonder.. if you modify the object cookie, is it by object or by reference?

Answer 1

Java methods get passed an object reference by value. So if you change the reference itself, e.g.

cookie = new MySpecialCookie();

it will not be seen by the method caller. However when you operate on the reference to change the data the object contains:

cookie.setValue("foo");

then those changes will be visible to the caller.

Answer 2

Java objects are always passed by value. So if you change value of coolie (for example cookie = null) you will not change original cookie.

Answer 3

if you modify the object cookie, is it by object or by reference?

Depends on what you mean by "modify" here. If you change the value of the reference, i.e. cookie = someOtherObject, then the original object itself isn't modified; it's just that you lost your reference to it. However, if you change the state of the object, e.g. by calling cookie.setSomeProperty(otherValue), then you are of course modifying the object itself.

Take a look at these previous related questions for more information:

• Java, pass-by-value, reference variables
• Is Java pass by reference?

Answer 4

In the following line of code

Cookie cookie = request.getCookie(); /* (1) */

the request.getCookie() method is passing a refrence to a Cookie object.

If you later on change cookie by doing something like

cookie = foo_bar(); /* (2) */

Then you are changing the internal refrence. It in no way affects your original cookie object in (1)

If however you change cookie by doing something like

cookie.setFoo( bar ); /* assuming setFoo changes an instance variable of cookie */

Then you are changing the original object recieved in (1)

Answer 5

object reference is different from object. for eg: class Shape { int x1=200; } class Shape1 { public static void main(String arg[]) { Shape s=new Shape();//creating object by using new operator System.out.println(s.x); Shape s1;//creating object reference s1=s;//assigning object to reference System.out.println(s1.x); } }