Use MapReduce to find prime numbers

Just want to write a small example of MapReduce of Hadoop for finding prime numbers. The first question is: how could I generate numbers from 1 to 1000000 by my own application instead of reading from file of HDFS? The answer is: inherit the InputSplit, RecordReader, and InputFormat by yourself, just like teragen program
Then comes the second question: could I just use mapper without reducer stage? The answer is yes, simply use job.setNumReduceTasks(0) to disable reducer stage.

The complete code is here (I know the algorithm for checking a number for prime is naive, but it works):

Copy the code to file, compile and run it:

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