2016 Update

Task/Hash-join/Java/hash-join.java

@@ -0,0 +1,40 @@
+import java.util.*;
+
+public class HashJoin {
+
+    public static void main(String[] args) {
+        String[][] table1 = {{"27", "Jonah"}, {"18", "Alan"}, {"28", "Glory"},
+        {"18", "Popeye"}, {"28", "Alan"}};
+
+        String[][] table2 = {{"Jonah", "Whales"}, {"Jonah", "Spiders"},
+        {"Alan", "Ghosts"}, {"Alan", "Zombies"}, {"Glory", "Buffy"},
+        {"Bob", "foo"}};
+
+        hashJoin(table1, 1, table2, 0).stream()
+                .forEach(r -> System.out.println(Arrays.deepToString(r)));
+    }
+
+    static List<String[][]> hashJoin(String[][] records1, int idx1,
+            String[][] records2, int idx2) {
+
+        List<String[][]> result = new ArrayList<>();
+        Map<String, List<String[]>> map = new HashMap<>();
+
+        for (String[] record : records1) {
+            List<String[]> v = map.getOrDefault(record[idx1], new ArrayList<>());
+            v.add(record);
+            map.put(record[idx1], v);
+        }
+
+        for (String[] record : records2) {
+            List<String[]> lst = map.get(record[idx2]);
+            if (lst != null) {
+                lst.stream().forEach(r -> {
+                    result.add(new String[][]{r, record});
+                });
+            }
+        }
+
+        return result;
+    }
+}