Variable vs Object vs Reference


Variables, Objects and References, we frequently listen to these terms while development but usually gets confused between them.

Let's try to understand a very basic definition along with an example for all three. The concept is similar in any object-oriented programming language but for reference, I'll be using Java examples. Feel free to comment below if you need examples in any specific language.

Variables

Variables are named storage of any primitive data type.

Name storage means we (developers) define the name of the variable.
Primitive data types are something which predefined in any language like int, float, boolean etc.
Keep in mind Int with the capital I or Float with the capital F are not primitive data types, those are non-primitive data types.
The size of the variable depends on the type of it.
In Java, a char takes 2 bytes, int takes 4 bytes, float takes 8 bytes and so on.
  
int age = 5;
float pi = 3.14;

Objects

Objects are variables of non-primitive data types or user-defined classes.
An object can't exist without its class.
It is created by the new keyword which calls the constructor of that class which ultimately assigns some memory to that object.
Its size depends on the properties defined in that class.
In java, an object is created on the heap.
  
Employee emp = new Employee();
Here an object of Employee will be created by calling the new keyword. Keep in mind that technically emp is not an object, its just a reference pointing to the object which is created on heap.
Read Refrence below to understand it better.

Reference

A reference is nothing but a pointer pointing to the object. In other words, it is holding the memory address of the object created by new keyword.
  
Employee emp = new Employee();
In the same above example, emp is a reference to the object which is created using the new keyword.

Basic differences between them

  • You can't create a variable using the new keyword.
  • Variables can't hold null values as primitive data types doesn't allow that.
  • If you just write new Employee(); it will call the constructor and will even assign the memory in heap to that object but there is no reference so you can't access it.
  • Chaining is possible in references using the dot(.) is most of the languages. you can call a function using object reference followed by a dot(.)
  • If you assign once reference to another like Employee emp2 = emp; it just copies the reference and but internally both the references are pointing to the same object.