public class LongSumReducer<K> extends MapReduceBase implements Reducer<K,LongWritable,K,LongWritable>
Reducer that sums long values.| 构造器和说明 |
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LongSumReducer() |
| 限定符和类型 | 方法和说明 |
|---|---|
void |
reduce(K key,
Iterator<LongWritable> values,
OutputCollector<K,LongWritable> output,
Reporter reporter)
Reduces values for a given key.
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close, configureclone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, waitconfigurepublic void reduce(K key, Iterator<LongWritable> values, OutputCollector<K,LongWritable> output, Reporter reporter) throws IOException
ReducerThe framework calls this method for each
<key, (list of values)> pair in the grouped inputs.
Output values must be of the same type as input values. Input keys must
not be altered. The framework will reuse the key and value objects
that are passed into the reduce, therefore the application should clone
the objects they want to keep a copy of. In many cases, all values are
combined into zero or one value.
Output pairs are collected with calls to
OutputCollector.collect(Object,Object).
Applications can use the Reporter provided to report progress
or just indicate that they are alive. In scenarios where the application
takes an insignificant amount of time to process individual key/value
pairs, this is crucial since the framework might assume that the task has
timed-out and kill that task. The other way of avoiding this is to set
mapred.task.timeout to a high-enough value (or even zero for no
time-outs).
reduce 在接口中 Reducer<K,LongWritable,K,LongWritable>key - the key.values - the list of values to reduce.output - to collect keys and combined values.reporter - facility to report progress.IOExceptionCopyright © 2009 The Apache Software Foundation