Added a rudimentary Map/Reduce example

Author: rampatra

src/main/java/com/rampatra/misc/MapReduce.java

@@ -0,0 +1,74 @@
+package com.rampatra.misc;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+/**
+ * A rudimentary example explaining the concept of Map/Reduce paradigm. The question is, provided a list of Person
+ * objects from various countries, compute the total population in each country.
+ *
+ * @author rampatra
+ * @since 2019-02-26
+ */
+public class MapReduce {
+
+    private static class Person {
+        String name;
+        int age;
+        String country;
+
+        Person(String country) {
+            this.country = country;
+        }
+    }
+
+    /**
+     * The mapper function will take all the data and output only the information which we need, and in this
+     * case, it is just the country name to which a person belongs.
+     *
+     * @param personList a list of all persons
+     * @return a list of country names
+     */
+    private static List<String> mapper(List<Person> personList) {
+        return personList.stream().map(person -> person.country).collect(Collectors.toList());
+    }
+
+    /**
+     * The reducer function will take all the useful information from the mapper function and then compute the result
+     * we need. In this case, it is to count the number of persons in each country.
+     *
+     * @param countryNamesOfAllPersons a list of country names taken out of all {@code Person} objects
+     * @return a map containing country names as the keys and their resp. population as values
+     */
+    private static Map<String, Integer> reducer(List<String> countryNamesOfAllPersons) {
+        Map<String, Integer> countryToPopulationMap = new HashMap<>();
+
+        countryNamesOfAllPersons.forEach(countryName -> {
+            countryToPopulationMap.computeIfPresent(countryName, (country, population) -> population + 1);
+            countryToPopulationMap.computeIfAbsent(countryName, country -> countryToPopulationMap.put(country, 1));
+        });
+
+        return countryToPopulationMap;
+    }
+
+    /**
+     * Just to print the output.
+     *
+     * @param countryToPopulationMap a map containing country names as the keys and their resp. population as values
+     */
+    private static void printPopulation(Map<String, Integer> countryToPopulationMap) {
+        countryToPopulationMap.forEach((k, v) -> System.out.println(k + ": " + v));
+    }
+
+    public static void main(String[] args) {
+
+        Person[] persons = new Person[]{new Person("India"), new Person("Ireland"),
+                new Person("Sweden"), new Person("United States"), new Person("India"),
+                new Person("Ireland"), new Person("India")};
+
+        printPopulation(reducer(mapper(Arrays.asList(persons))));
+    }
+}