Difference between creating a “new object” and “Class objectname”

Question

Say for example I have a class called Phone.

What is the difference between:

Phone p;

and

Phone p = new Phone(200)          


        

Answer 1

The first snippet only declares a variable p with a given type Phone. You have not created any object, only a variable.

The third instantiates a new Phone object (which is not assigned to a variable).

The second combines the two, the declaration and instantiation, into a single line of code.

Phone p; // declaration of variable 
p = new Phone(200); // instantiation of object, assigned to variable 

Phone p = new Phone(200); // declaration and instantiation in a single line

For p to be used, you need to instantiate it (or otherwise initialize it). The declaration itself is not useful. In a local (a variable declared inside the scope of a method), it is illegal to use it without first initializing it. If it's a class-level member, then it will simply be a null pointer when you try to access any of its member functions.