Hadoop MapReduce framework Java WordCount program

Mapper Function Basics
Each Mapper function processes a single input key-value pair at a time and produces zero or more output key-value pairs. These outputs become the input for the reducer phase, but the reducer only operates on the keys that it receives.

The mapper generates intermediate key-value pairs, which are shuffled and sorted before reaching the reducer phase. The reducer then aggregates these key-value pairs, for instance, by summing up all counts for each unique word in a word count job.