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7FieldSelectionMapReduce (Hadoop 0.20.1 API)
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91org.apache.hadoop.mapred.lib</FONT>
92<BR>
93Class FieldSelectionMapReduce&lt;K,V&gt;</H2>
94<PRE>
95<A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">java.lang.Object</A>
96  <IMG SRC="../../../../../resources/inherit.gif" ALT="extended by "><B>org.apache.hadoop.mapred.lib.FieldSelectionMapReduce&lt;K,V&gt;</B>
97</PRE>
98<DL>
99<DT><B>All Implemented Interfaces:</B> <DD><A HREF="http://java.sun.com/javase/6/docs/api/java/io/Closeable.html?is-external=true" title="class or interface in java.io">Closeable</A>, <A HREF="../../../../../org/apache/hadoop/mapred/JobConfigurable.html" title="interface in org.apache.hadoop.mapred">JobConfigurable</A>, <A HREF="../../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</A>&lt;K,V,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;, <A HREF="../../../../../org/apache/hadoop/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</A>&lt;<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;</DD>
100</DL>
101<HR>
102<DL>
103<DT><PRE>public class <B>FieldSelectionMapReduce&lt;K,V&gt;</B><DT>extends <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</A><DT>implements <A HREF="../../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</A>&lt;K,V,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;, <A HREF="../../../../../org/apache/hadoop/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</A>&lt;<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;</DL>
104</PRE>
105
106<P>
107This class implements a mapper/reducer class that can be used to perform
108 field selections in a manner similar to unix cut. The input data is treated
109 as fields separated by a user specified separator (the default value is
110 "\t"). The user can specify a list of fields that form the map output keys,
111 and a list of fields that form the map output values. If the inputformat is
112 TextInputFormat, the mapper will ignore the key to the map function. and the
113 fields are from the value only. Otherwise, the fields are the union of those
114 from the key and those from the value.
115 
116 The field separator is under attribute "mapred.data.field.separator"
117 
118 The map output field list spec is under attribute "map.output.key.value.fields.spec".
119 The value is expected to be like "keyFieldsSpec:valueFieldsSpec"
120 key/valueFieldsSpec are comma (,) separated field spec: fieldSpec,fieldSpec,fieldSpec ...
121 Each field spec can be a simple number (e.g. 5) specifying a specific field, or a range
122 (like 2-5) to specify a range of fields, or an open range (like 3-) specifying all
123 the fields starting from field 3. The open range field spec applies value fields only.
124 They have no effect on the key fields.
125 
126 Here is an example: "4,3,0,1:6,5,1-3,7-". It specifies to use fields 4,3,0 and 1 for keys,
127 and use fields 6,5,1,2,3,7 and above for values.
128 
129 The reduce output field list spec is under attribute "reduce.output.key.value.fields.spec".
130 
131 The reducer extracts output key/value pairs in a similar manner, except that
132 the key is never ignored.
133<P>
134
135<P>
136<HR>
137
138<P>
139<!-- =========== FIELD SUMMARY =========== -->
140
141<A NAME="field_summary"><!-- --></A>
142<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
143<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
144<TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2">
145<B>Field Summary</B></FONT></TH>
146</TR>
147<TR BGCOLOR="white" CLASS="TableRowColor">
148<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
149<CODE>static&nbsp;org.apache.commons.logging.Log</CODE></FONT></TD>
150<TD><CODE><B><A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html#LOG">LOG</A></B></CODE>
151
152<BR>
153&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</TD>
154</TR>
155</TABLE>
156&nbsp;
157<!-- ======== CONSTRUCTOR SUMMARY ======== -->
158
159<A NAME="constructor_summary"><!-- --></A>
160<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
161<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
162<TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2">
163<B>Constructor Summary</B></FONT></TH>
164</TR>
165<TR BGCOLOR="white" CLASS="TableRowColor">
166<TD><CODE><B><A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html#FieldSelectionMapReduce()">FieldSelectionMapReduce</A></B>()</CODE>
167
168<BR>
169&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</TD>
170</TR>
171</TABLE>
172&nbsp;
173<!-- ========== METHOD SUMMARY =========== -->
174
175<A NAME="method_summary"><!-- --></A>
176<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
177<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
178<TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2">
179<B>Method Summary</B></FONT></TH>
180</TR>
181<TR BGCOLOR="white" CLASS="TableRowColor">
182<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
183<CODE>&nbsp;void</CODE></FONT></TD>
184<TD><CODE><B><A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html#close()">close</A></B>()</CODE>
185
186<BR>
187&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</TD>
188</TR>
189<TR BGCOLOR="white" CLASS="TableRowColor">
190<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
191<CODE>&nbsp;void</CODE></FONT></TD>
192<TD><CODE><B><A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html#configure(org.apache.hadoop.mapred.JobConf)">configure</A></B>(<A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;job)</CODE>
193
194<BR>
195&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Initializes a new instance from a <A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred"><CODE>JobConf</CODE></A>.</TD>
196</TR>
197<TR BGCOLOR="white" CLASS="TableRowColor">
198<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
199<CODE>&nbsp;void</CODE></FONT></TD>
200<TD><CODE><B><A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html#map(K, V, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)">map</A></B>(<A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html" title="type parameter in FieldSelectionMapReduce">K</A>&nbsp;key,
201    <A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html" title="type parameter in FieldSelectionMapReduce">V</A>&nbsp;val,
202    <A HREF="../../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</A>&lt;<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;&nbsp;output,
203    <A HREF="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</A>&nbsp;reporter)</CODE>
204
205<BR>
206&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;The identify function.</TD>
207</TR>
208<TR BGCOLOR="white" CLASS="TableRowColor">
209<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
210<CODE>&nbsp;void</CODE></FONT></TD>
211<TD><CODE><B><A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html#reduce(org.apache.hadoop.io.Text, java.util.Iterator, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)">reduce</A></B>(<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&nbsp;key,
212       <A HREF="http://java.sun.com/javase/6/docs/api/java/util/Iterator.html?is-external=true" title="class or interface in java.util">Iterator</A>&lt;<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;&nbsp;values,
213       <A HREF="../../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</A>&lt;<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;&nbsp;output,
214       <A HREF="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</A>&nbsp;reporter)</CODE>
215
216<BR>
217&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<i>Reduces</i> values for a given key.</TD>
218</TR>
219</TABLE>
220&nbsp;<A NAME="methods_inherited_from_class_java.lang.Object"><!-- --></A>
221<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
222<TR BGCOLOR="#EEEEFF" CLASS="TableSubHeadingColor">
223<TH ALIGN="left"><B>Methods inherited from class java.lang.<A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</A></B></TH>
224</TR>
225<TR BGCOLOR="white" CLASS="TableRowColor">
226<TD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#clone()" title="class or interface in java.lang">clone</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#equals(java.lang.Object)" title="class or interface in java.lang">equals</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#finalize()" title="class or interface in java.lang">finalize</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#getClass()" title="class or interface in java.lang">getClass</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#hashCode()" title="class or interface in java.lang">hashCode</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#notify()" title="class or interface in java.lang">notify</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#notifyAll()" title="class or interface in java.lang">notifyAll</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#toString()" title="class or interface in java.lang">toString</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#wait()" title="class or interface in java.lang">wait</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#wait(long)" title="class or interface in java.lang">wait</A>, <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true#wait(long, int)" title="class or interface in java.lang">wait</A></CODE></TD>
227</TR>
228</TABLE>
229&nbsp;
230<P>
231
232<!-- ============ FIELD DETAIL =========== -->
233
234<A NAME="field_detail"><!-- --></A>
235<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
236<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
237<TH ALIGN="left" COLSPAN="1"><FONT SIZE="+2">
238<B>Field Detail</B></FONT></TH>
239</TR>
240</TABLE>
241
242<A NAME="LOG"><!-- --></A><H3>
243LOG</H3>
244<PRE>
245public static final org.apache.commons.logging.Log <B>LOG</B></PRE>
246<DL>
247<DL>
248</DL>
249</DL>
250
251<!-- ========= CONSTRUCTOR DETAIL ======== -->
252
253<A NAME="constructor_detail"><!-- --></A>
254<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
255<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
256<TH ALIGN="left" COLSPAN="1"><FONT SIZE="+2">
257<B>Constructor Detail</B></FONT></TH>
258</TR>
259</TABLE>
260
261<A NAME="FieldSelectionMapReduce()"><!-- --></A><H3>
262FieldSelectionMapReduce</H3>
263<PRE>
264public <B>FieldSelectionMapReduce</B>()</PRE>
265<DL>
266</DL>
267
268<!-- ============ METHOD DETAIL ========== -->
269
270<A NAME="method_detail"><!-- --></A>
271<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
272<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
273<TH ALIGN="left" COLSPAN="1"><FONT SIZE="+2">
274<B>Method Detail</B></FONT></TH>
275</TR>
276</TABLE>
277
278<A NAME="map(java.lang.Object,java.lang.Object,org.apache.hadoop.mapred.OutputCollector,org.apache.hadoop.mapred.Reporter)"><!-- --></A><A NAME="map(K, V, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)"><!-- --></A><H3>
279map</H3>
280<PRE>
281public void <B>map</B>(<A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html" title="type parameter in FieldSelectionMapReduce">K</A>&nbsp;key,
282                <A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html" title="type parameter in FieldSelectionMapReduce">V</A>&nbsp;val,
283                <A HREF="../../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</A>&lt;<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;&nbsp;output,
284                <A HREF="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</A>&nbsp;reporter)
285         throws <A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></PRE>
286<DL>
287<DD>The identify function. Input key/value pair is written directly to output.
288<P>
289<DD><DL>
290<DT><B>Specified by:</B><DD><CODE><A HREF="../../../../../org/apache/hadoop/mapred/Mapper.html#map(K1, V1, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)">map</A></CODE> in interface <CODE><A HREF="../../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</A>&lt;<A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html" title="type parameter in FieldSelectionMapReduce">K</A>,<A HREF="../../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html" title="type parameter in FieldSelectionMapReduce">V</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;</CODE></DL>
291</DD>
292<DD><DL>
293<DT><B>Parameters:</B><DD><CODE>key</CODE> - the input key.<DD><CODE>val</CODE> - the input value.<DD><CODE>output</CODE> - collects mapped keys and values.<DD><CODE>reporter</CODE> - facility to report progress.
294<DT><B>Throws:</B>
295<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></CODE></DL>
296</DD>
297</DL>
298<HR>
299
300<A NAME="configure(org.apache.hadoop.mapred.JobConf)"><!-- --></A><H3>
301configure</H3>
302<PRE>
303public void <B>configure</B>(<A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;job)</PRE>
304<DL>
305<DD><B>Description copied from interface: <CODE><A HREF="../../../../../org/apache/hadoop/mapred/JobConfigurable.html#configure(org.apache.hadoop.mapred.JobConf)">JobConfigurable</A></CODE></B></DD>
306<DD>Initializes a new instance from a <A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred"><CODE>JobConf</CODE></A>.
307<P>
308<DD><DL>
309<DT><B>Specified by:</B><DD><CODE><A HREF="../../../../../org/apache/hadoop/mapred/JobConfigurable.html#configure(org.apache.hadoop.mapred.JobConf)">configure</A></CODE> in interface <CODE><A HREF="../../../../../org/apache/hadoop/mapred/JobConfigurable.html" title="interface in org.apache.hadoop.mapred">JobConfigurable</A></CODE></DL>
310</DD>
311<DD><DL>
312<DT><B>Parameters:</B><DD><CODE>job</CODE> - the configuration</DL>
313</DD>
314</DL>
315<HR>
316
317<A NAME="close()"><!-- --></A><H3>
318close</H3>
319<PRE>
320public void <B>close</B>()
321           throws <A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></PRE>
322<DL>
323<DD><DL>
324<DT><B>Specified by:</B><DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/Closeable.html?is-external=true#close()" title="class or interface in java.io">close</A></CODE> in interface <CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/Closeable.html?is-external=true" title="class or interface in java.io">Closeable</A></CODE></DL>
325</DD>
326<DD><DL>
327
328<DT><B>Throws:</B>
329<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></CODE></DL>
330</DD>
331</DL>
332<HR>
333
334<A NAME="reduce(org.apache.hadoop.io.Text, java.util.Iterator, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)"><!-- --></A><H3>
335reduce</H3>
336<PRE>
337public void <B>reduce</B>(<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&nbsp;key,
338                   <A HREF="http://java.sun.com/javase/6/docs/api/java/util/Iterator.html?is-external=true" title="class or interface in java.util">Iterator</A>&lt;<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;&nbsp;values,
339                   <A HREF="../../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</A>&lt;<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;&nbsp;output,
340                   <A HREF="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</A>&nbsp;reporter)
341            throws <A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></PRE>
342<DL>
343<DD><B>Description copied from interface: <CODE><A HREF="../../../../../org/apache/hadoop/mapred/Reducer.html#reduce(K2, java.util.Iterator, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)">Reducer</A></CODE></B></DD>
344<DD><i>Reduces</i> values for a given key. 
345 
346 <p>The framework calls this method for each
347 <code>&lt;key, (list of values)></code> pair in the grouped inputs.
348 Output values must be of the same type as input values.  Input keys must
349 not be altered. The framework will <b>reuse</b> the key and value objects
350 that are passed into the reduce, therefore the application should clone
351 the objects they want to keep a copy of. In many cases, all values are
352 combined into zero or one value.
353 </p>
354   
355 <p>Output pairs are collected with calls to 
356 <A HREF="../../../../../org/apache/hadoop/mapred/OutputCollector.html#collect(K, V)"><CODE>OutputCollector.collect(Object,Object)</CODE></A>.</p>
357
358 <p>Applications can use the <A HREF="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred"><CODE>Reporter</CODE></A> provided to report progress
359 or just indicate that they are alive. In scenarios where the application
360 takes an insignificant amount of time to process individual key/value
361 pairs, this is crucial since the framework might assume that the task has
362 timed-out and kill that task. The other way of avoiding this is to set
363 <a href="../../../../../../mapred-default.html#mapred.task.timeout">
364 mapred.task.timeout</a> to a high-enough value (or even zero for no
365 time-outs).</p>
366<P>
367<DD><DL>
368<DT><B>Specified by:</B><DD><CODE><A HREF="../../../../../org/apache/hadoop/mapred/Reducer.html#reduce(K2, java.util.Iterator, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)">reduce</A></CODE> in interface <CODE><A HREF="../../../../../org/apache/hadoop/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</A>&lt;<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>,<A HREF="../../../../../org/apache/hadoop/io/Text.html" title="class in org.apache.hadoop.io">Text</A>&gt;</CODE></DL>
369</DD>
370<DD><DL>
371<DT><B>Parameters:</B><DD><CODE>key</CODE> - the key.<DD><CODE>values</CODE> - the list of values to reduce.<DD><CODE>output</CODE> - to collect keys and combined values.<DD><CODE>reporter</CODE> - facility to report progress.
372<DT><B>Throws:</B>
373<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></CODE></DL>
374</DD>
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