What does this Java Syntax mean?

Question

In the code below, what does Iterator<V> and OutputCollector<K, V> mean? Is it a special data type?

public void reduce(K key, 
  Iterator<V> values, 
  OutputCollector<K, V> output, 
  Reporter reporter) throws IOException {

Answer 1

Sort of. Those are generic types:

http://en.wikipedia.org/wiki/Generics_in_Java

Answer 2

K stands for Key and V for Value, like in a HashMap. There isn't a Key class or a Value class that you have to instantiate or subclass, it's a semantic thing for generics. Those letters are only placeholders for whatever classes you decide should fill the Key and Value roles.

Answer 3

Those are Generic types. Check out the tutorial, it is really helpful.

http://download.oracle.com/javase/tutorial/extra/generics/index.html