Question

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What does Map<?, ?> mean in Java?
Java Generics – What is this syntax for?

I was wondering what does the notation < > does in Java, for example.

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

I am not a java expert in any fashion. I hope you can provide some clear answer for the reasons behind using the < > notation.

Answer 1

It denotes generics. Mapper is a generic and you’re inheriting from Mapper<LongWritable, Text, Text, IntWritable>, which is that generic specialized for those types.

It’s like Vector – also a generic – you can have Vector<Object> and Vector<SomeOtherClass>.