about context object in map-reduce

Question

Can anyone explain why we are writing arguments in angle brackets in below statement and why we are defining output key/value pairs in arguments.

public static class Map extends Mapper <LongWritable, Text, Text, IntWritable> 

What is context object and why we are using in the below statement.

public void map(LongWritable key, Text value, Context context ) throws IOException, InterruptedException

Answer 1

<> is used to indicate generics in Java.

Mapper <LongWritable, Text, Text, IntWritable> takes only <LongWritable,Text> as keys and <Text,IntWritable> as values. If you try to provide any other writable types to your mapper, this will throw an error.

Context context object is used to write output Key-Values as well as get configuration, counters, cacheFiles etc in the Mapper.

Answer 2

To add to what @Vasu answered..

Context stores references to RecordReader and RecordWriter. Whenever context.getCurrentKey() and context.getCurrentValue() are used to retrieve key and value pair, the request is assigned to RecordReader. And when context.write() is called, it is assigned to RecordWriter.

Here RecordReader and RecordWriter are actually abstract classes.