int vs Integer : Java Glossary

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 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?

Converting

// 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.

Learning More