Mapper maps input key/value pairs into intermediate key/value pairs.
E.g.
Input: (docID, doc)
Output: (term, 1)
Mapper Class Prototype:
Mapper<Object, Text, Text, IntWritable>
// Object:: INPUT_KEY
// Text:: INPUT_VALUE
// Text:: OUTPUT_KEY
// IntWritable:: OUTPUT_VALUE
Special Data Type for Mapper
IntWritable
A serializable and comparable object for integer.
Example:
private final static IntWritable one = new IntWritable(1);
Text
A serializable, deserializable and comparable object for string at byte level. It stores text in UTF-8 encoding.
Example:
private Text word = new Text();
Hadoop defines its own classes for general data types.
-- All "values" must have Writable interface;
-- All "keys" must have WritableComparable interface;
Map Method for Mapper
Method header
public void map(Object key, Text value, Context context
) throws IOException, InterruptedException
// Object key:: Declare data type of input key;
// Text value:: Declare data type of input value;
// Context context:: Declare data type of output. Context is often used for output data collection.
Tokenization
// Use Java built-in StringTokenizer to split input value (document) into words:
StringTokenizer itr = new StringTokenizer(value.toString());
Building (key, value) pairs
// Loop over all words:
while (itr.hasMoreTokens()) {
// convert built-in String back to Text:
word.set(itr.nextToken());
// build (key, value) pairs into Context and emit:
context.write(word, one);
}
Map Method Summary
Mapper class produces Mapper.Context object, which comprise a series of (key, value) pairs
public void map(Object key, Text value, Context context
) throws IOException, InterruptedException {
StringTokenizer itr = new StringTokenizer(value.toString());
while (itr.hasMoreTokens()) {
word.set(itr.nextToken());
context.write(word, one);
}
}
Overview of Mapper Class
public static class TokenizerMapper
extends Mapper<Object, Text, Text, IntWritable>{
private final static IntWritable one = new IntWritable(1);
private Text word = new Text();
public void map(Object key, Text value, Context context
) throws IOException, InterruptedException {
StringTokenizer itr = new StringTokenizer(value.toString());
while (itr.hasMoreTokens()) {
word.set(itr.nextToken());
context.write(word, one);
}
}
}