How to use MapReduce default partition Hash Partitioner

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1 the default partitioning method for projection MapReduce

The source code of the default partition

Public int getPartition (K2 key, V2 value,int numReduceTasks) {return (key.hashCode () & Integer.MAX_VALUE)% numReduceTasks;}

Key.hashCode (), which takes the hashCode value of the key output from map

& is the and operator in java, that is, in binary terms, it is equal to 1 and 1, otherwise it is 0.

Demo:

Public class test {public static void main (String [] args) {int a = 129 int b = 128 System.out.println (aqb); / / 128}}

/ * parsing: "a" is 129. the conversion to binary is: 10000001 "b" is 128. the conversion to binary is: 10000000 according to the characteristics of & and: 1 and 1 are 11 and 0 are 0so, and the result is 10000000, that is, 128 /.

Therefore, the purpose of key.hashCode () & Integer.MAX_VALUE is to ensure that the partition determined by any key and numReduceTashs output on the map side is a positive integer.

2, hashCode value of common data

1Numeric value of type 1 minint

Public void test () {for (int I =-3; I 98. X-> 120y-> 121z-> 122cm /

Thus it can be seen that the hashCode value of English a-> z or A-> Z is its corresponding integer value.

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