.. highlight:: java


************
Java Library
************

Overview
========

The Java library for WMR is based on two user classes, and using an iterator to
manage the list of values passed to the reducer. The two classes that users must
provide are::

   public class Mapper
   {
     public void map(String, String);
   }

::

   public class Reducer
   {
     public void reduce(String, WmrIterator);
   }

API
===

Class Wmr
---------

::

   public class Wmr
   {
     public static void emit(String, String);
   }

The **Wmr** class contains a single method:

public static void emit(String, String)
    Prints the two provided ``String`` objects to
    standard output separated by a tab character.

Class WmrIterator
-----------------

::

   public class WmrIterator implements Iterator<String>, Iterable<String>
   {
     public WmrIterator iterator();
     public boolean hasNext();
     public String next() throws NoSuchElementException;
     public void remove() throws UnsupportedOperationException;
   }

The **WmrIterator** class provides the list of values to the reducer, and is
compatible with Java-style iterators.

public WmrIterator iterator()
    Returns this WmrIterator when called.

public boolean hasNext()
    Returns ``true`` when there is at least one additional value contained in
    the list (standard input) managed by the iterator. Otherwise returns
    ``false``. If **hasNext()** returns ``true``, it is safe to call the
    **next()** method.

public String next()
    Returns the next value in the list, if available. If there is no next value
    (as indicated by **hasNext()**\ ), a **NoSuchElementException** is thrown.

public void remove()
    This method is not implemented, and will throw an
    **UnsupportedOperationException** if called.

Example Usage
=============

::

   public class Mapper
   {
     public void map(String key, String value)
     {
       String words[] = key.split(" ");
       for (int i = 0; i < words.length; i++)
       {
         Wmr.emit(words[i], "1");
       }
     }
   }

::

   public class Reducer
   {
     public void reduce(String key, WmrIterator values)
     {
       int sum = 0;
       for (String value : values)
       {
         sum += Integer.parseInt(value);
       }
       Wmr.emit(key, Integer.valueOf(sum).toString());
     }
   }