Using int vs Integer

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) 

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.

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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

If you can use int do so. If the value can be null or is used as an Object e.g. Generics, use Integer

Usually it doesn't matter which one you use but often int performs slightly better.

This approach is not good in practice, use int whenever possible. Usage of Integer indicates that this particular variable can be null (or it was inside a collection, damn generics...) - which is not the case.

Also using Integer introduces an extra overhead of boxing and unboxing.