Object vs Reference in Java

Question

In Java, if I declare,

MyClass obj;

Is obj called a "reference" or an "object". I am not instantiating class here.

Answer 1

obj is a Reference to an instance of MyClass.

Currently, that Reference is NULL because you haven't assigned it to refer to any instance.

Technically MyClass must be a subclass of Object, so it is possible to say that obj is a Reference to an instance of Object as well.

Answer 2

Reference: A variable that points to some object in memory. It is stored in stack they can be contained in other objects (then they are not really variables, but fields), which puts them on the heap also.

Object: An instance of class that is created dynamically. It is stored in heap

Example:

MyClassI aObj,aObj1;

aObj=new MyClass2();

At first line aObj and aObj1 are references

At second line aObj referencing to object of MyClass2(New operator creates an object of Myclass2 and its address is assigned to aObj).

To understand even better consider a class Car which has driverName as a member.

Car c1,c2;

c1.driverName="Andrew"
c2.driverName="Gabriel"

System.out.println(c1.driverName);//gives Andrew
System.out.println(c2.driverName);//gives Gabriel

c1=c2;

c2=null;

// gives gabriel because the address of c2 is copied to reference c1.
// the object is not nullified because c2 is just a reference when 
// assigning null the address that is stored on c2 in nullified not 
// the object it points..

system.out.println(c1.driverName);