Hash join: Difference between revisions

=={{header|Java}}==

{{trans|PHP}}

{{works with|Java|8}}

<lang java>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[]> value = map.get(record[idx1]);

if (value == null)

value = new ArrayList<>();

value.add(record);

map.put(record[idx1], value);

}

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;

}

}</lang>

<pre>[[27, Jonah], [Jonah, Whales]]

[[27, Jonah], [Jonah, Spiders]]

[[18, Alan], [Alan, Ghosts]]

[[28, Alan], [Alan, Ghosts]]

[[18, Alan], [Alan, Zombies]]

[[28, Alan], [Alan, Zombies]]

[[28, Glory], [Glory, Buffy]]</pre>