Question

I came across a class using Integer variables to capture size to be used in a for loop. Is this good practice or should we use the int primitive data type?

Integer size = something.getFields().size();
for (Integer j = 0; j < size - 1; ++j) 

Answer 1

the Integer class is provided so that values can be boxed/unboxed in a pure OO manner. use int where appropriate unless you specifically need to use it in an OO way; in which case Integer is appropriate.

Java Int vs Integer

However, very different things are going on under the covers here. An int is a number; an > Integer is a pointer that can reference an object that contains a number.

…

An int is not an object and cannot passed to any method that requires
objects. A common case is in using the provided collection classes (
List , Map , Set ) – though it is possible to write versions of these
classes that provide similar capabilities to the object versions. The
wrapper classes ( Integer , Double , etc) are frequently required
whenever introspection is used (such as in the reflection API).

A better description of when to use one vs. the other:

Choosing between int and Integer

I’ll start with how these types should be used before going into
detail on why.

• Prefer int for performance reasons
• Methods that take objects (including generic types like List<T>)
  will implicitly require the use of Integer
• Use of Integer is relatively cheap for low values (-128 to

127. because of interning – use Integer.valueOf(int) and not new
     Integer(int)

• Do not use == or != with Integer types
• Consider using Integer when you need to represent the
  absence of a value (null)
• Beware unboxing Integer values to int with null values