Why can Integer and int be used interchangably?

Question

I am confused as to why Integer and int can be used interchangeably in Java even though one is a primitive type and the other is an object?

For example:

Integer b = 42;
int a  = b;

Or

int d = 12;
Integer c = d;

Answer 1

The first few sentences of the posted article describe it pretty well:

You can’t put an int (or other primitive value) into a collection. Collections can only hold object references, so you have to box primitive values into the appropriate wrapper class (which is Integer in the case of int). When you take the object out of the collection, you get the Integer that you put in; if you need an int, you must unbox the Integer using the intValue method. All of this boxing and unboxing is a pain, and clutters up your code. The autoboxing and unboxing feature automates the process, eliminating the pain and the clutter.

That is basically it in a nutshell. It allows you take advantage of the Collections Framework for primatives without having to do the work yourself.

The primary disadvantage is that it confuses new programmers, and can lead to messy/confusing code if it's not understood and used correctly.