int VS Integer

Newbies are often confused by the difference between int and Integer.

To properly understand the difference, you should read an introductory textbook on Java. I will make a small stab at answering here.

Everything I say here applies analogously to char and Character, short and Short, long and Long, float and Float, double and Double.

Definitions

An int is a primitive. It is not an Object. An int is a high performance, streamlined beast for calculating numbers in the range -2,147,483,648 [-2³¹] aka Integer.MIN_VALUE to +2,147,483,647 [2 ³¹-1] aka Integer.MAX_VALUE. An int is a bare bones 32-bit chunk of information. int variables are mutable. Unless you mark them final, you can change their value at any time.

An Integer, is a Object that contains a single int field. An Integer is much bulkier than an int. It is sort like a Fedex box to contain the int. Integers are immutable. If you want to affect the value of a Integer variable, the only way is to create a new Integer object and discard the old one.

Which is better?

That is like asking which is better a volley ball or a volleyball in a Fedex box. It depends what you want to do with it.

ability	unboxed int	boxed Integer
Calculate with it, + - * / % ^ etc.	yes	no
Pass it as a parameter	yes	yes
return it as a value	yes	yes
Use methods on it from java.lang.Integer	no	yes
Store it in a Vector or other Collection	no	yes
Use it as a HashMap key	no	yes
serialize it	no	yes
send it by itself over RMI	no	yes
send it as part of another Object over RMI	yes	yes
pass it as a generic object to a TableCellRenderer. You may pass a variety of different types via the same paramter.	no	yes
Allow for a null value that means there is no value.	no	yes

Converting

Fortunately it is easy to convert back and forth between int and Integer.

// to int i from Integer ii
int i = ii.intValue();

// to Integer ii from int i
Integer ii = new Integer( i );

Starting with Java 1.5, with boxing and unboxing, the compiler will sometimes automatically do the conversions to and from int and Integer for you so you can treat int and Integer as if they were almost the same thing.

Why Both?

Why are there both int and Integer? For speed. ints, without any Object packaging are compact and fast. Would it not have been easier if there were only one of sort of creature that could do everything and have the compiler automatically figure out when the packaging was needed and when not? The Eiffel language designers thought so, and the Java designers are gradually coming around to the same conclusion.