Java – How to generalize all key/value pairs in Hadoop

I’m new to hadoop and I’m trying to do some map/reduce tasks in Java. I would like to know how we can do a reduction operation on all key/value pairs.

For example, let’s say we have the highest temperature of the day for each day of the month. We use date as key and temperature as value, I want to get the key/value of the highest temperature for the whole month.

I hope my question is clear!

Thanks for your help.

Yes, it is possible. Simply > by Configure your work to use a single reducer, which will iterate through all key/value pairs. In the reduce() method, you simply search for the maximum value, while in the cleanup() method, you output the final result. (k, v) = (year, temperature) Example:

public class MaxTemperatureReducer extends Reducer<IntWritable, DoubleWritable, IntWritable, DoubleWritable> {
    private static int year = 0;
    private static double maxTemp = 0.0;

@Override
    public void reduce(IntWritable key, Iterable<DoubleWritable> values, Context context) throws IOException, InterruptedException {
        for (DoubleWritable value : values) {
            if (value.get() > maxTemp) {
                year = key.get();
                maxTemp = value.get();
            }
        }
    }

@Override
    public void cleanup(Context context) throws IOException, InterruptedException {
        context.write(new IntWritable(year), new DoubleWritable(maxTemp));
    }
}