Joins in Java 8 Collection API

Question

I have two List<Map<String, Object>> object.

[{Month=August-2013, Sales=282200}, {Month=July-2013, Sales=310400}, 
 {Month=June-2013, Sales=309600}, {Month=May-2013, Sales=318200}, 
 {Month=September-2013, Sales=257800}]

and

[{Month=April-2013, NoOfTranx=8600}, {Month=August-2013, NoOfTranx=6700}, 
 {Month=July-2013, NoOfTranx=14400}, {Month=June-2013, NoOfTranx=8500}, 
 {Month=May-2013, NoOfTranx=14400}]

I want to join(Merge) these two List on Month key. How can I perform several join operation (i.e. FULL OUTER JOIN, RIGHT OUTER JOIN etc.) on these List using new Collection API.

Answer 1

One way to perform a full outer join is to first construct a Map<String, Map<String, Object>> that links the month value to the maps themselves and get the values in a list:

//first concatenate the two lists
Map<String, Map<String, Object>> result = Stream.concat(list1.stream(),
                                                        list2.stream())
                //then collect in a map where the key is the value of the month
                .collect(toMap(m -> (String) m.get("Month"),
                //the value is the map itself
                               m -> m,
                //merging maps (i.e. adding the "Sales" and "NoOfTranx" infos)
                               (m1, m2) -> {m1.putAll(m2); return m1; }));

//finally put that in a list
List<Map<String, Object>> merge = new ArrayList<>(result.values());

Notes:

• the original maps are modified - if this is undesirable you can create a new HashMap<>() in the merging part.
• for left/right outer join, you can stream one list only and add the relevant entries in the stream
• for inner join you can start from one of the list, filter elements present in the other list and continue as above

Full example below which prints:

list1 = [{Month=August-13, Sales=282200}, {Month=July-13, Sales=310400}]
list2 = [{Month=August-13, NoOfTranx=6700}, {Month=July-13, NoOfTranx=14400}]
merge = [{Month=August-13, Sales=282200, NoOfTranx=6700}, {Month=July-13, Sales=310400, NoOfTranx=14400}]

Code:

public static void main(String[] args) {
    List<Map<String, Object>> list1 = new ArrayList<>();
    list1.add(map("Month", "August-13", "Sales", 282200));
    list1.add(map("Month", "July-13", "Sales", 310400));
    System.out.println("list1 = " + list1);

    List<Map<String, Object>> list2 = new ArrayList<>();
    list2.add(map("Month", "August-13", "NoOfTranx", 6700));
    list2.add(map("Month", "July-13", "NoOfTranx", 14400));
    System.out.println("list2 = " + list2);

    Map<String, Map<String, Object>> result = Stream.concat(list1.stream(),
                                                            list2.stream())
                .collect(toMap(m -> (String) m.get("Month"),
                               m -> m,
                               (m1, m2) -> {m1.putAll(m2); return m1; }));

    List<Map<String, Object>> merge = new ArrayList<>(result.values());
    System.out.println("merge = " + merge);
}

private static Map<String, Object> map(Object... kvs) {
    Map<String, Object> map = new HashMap<>();
    for (int i = 0; i < kvs.length; i += 2) {
        map.put((String) kvs[i], kvs[i+1]);
    }
    return map;
}