How to merge two ArrayList> to one?

Question

ArrayList<Map<String, String>> result1

result1 is like

(1, a)
(2, a)
(3, b)
(4, e)
(5, e)

ArrayList<Map<String, String>> result2

result2 is like

(1,android)
(2,ios)
(3,android)
(4,android)
(5,ios)

I want to merge the two maps to build a map like this one

(1, ( a, android))
(2, ( a, ios))
(3, ( b, android))
(4, (e, android))
(5, (e, ios))

How to make this happen?

1) Create your own class with the two fields (e.g. for `a` and `ios`) 2) Create a new list `result3`, 3) Iterate over `result1` and `result2` at once, 4) Create objects of your class with the values from the lists your iterating and add them to your new list `result3` — Lino, Feb 05 '19 at 12:08
I'm using arraylist just because I only can retrieve the data from database in the form of Arraylist — DavidW, Feb 05 '19 at 12:15
I see this as a `ArrayList>`, why would you complicate with maps? — Mark, Feb 05 '19 at 12:22
@DavidW "_I only can retrieve the data from database in the form of Arraylist_" what kind of library/framework are you using?.. :S — Kevin Cruijssen, Feb 05 '19 at 12:23

Prasad Karunagoda · Answer 1 · 2019-02-05T12:42:05.340

For the requirement you have specified here, you can do it like this.

I'm iterating over the keys of the first map. And collecting values for each key from all the maps and putting them in a list. Then put the list to the resulting map.

import java.util.*;

public class MergeMaps
{
  public static void main(String[] args)
  {
    Map<String, String> map1 = new HashMap<>();
    map1.put("1", "a");
    map1.put("2", "a");
    map1.put("3", "b");
    map1.put("4", "e");
    map1.put("5", "e");

    Map<String, String> map2 = new HashMap<>();
    map2.put("1", "android");
    map2.put("2", "ios");
    map2.put("3", "android");
    map2.put("4", "android");
    map2.put("5", "ios");

    Set<String> keys = new HashSet<>();
    keys.addAll(map1.keySet());
    keys.addAll(map2.keySet());

    Map<String, List<String>> mergedMap = new HashMap<>();
    for (String key : keys)
    {
      List<String> list = new ArrayList<>();
      list.add(map1.get(key));
      list.add(map2.get(key));
      mergedMap.put(key, list);
    }

    System.out.println(mergedMap);
  }
}

Output will be:

{1=[a, android], 2=[a, ios], 3=[b, android], 4=[e, android], 5=[e, ios]}

This isn't a complete answer. What if there's an entry `map2.put(6, "windows")` that isn't in map1? — Ben R., Feb 05 '19 at 12:36
@BenR., thanks for pointing it out. I edited the answer and fixed it. — Prasad Karunagoda, Feb 05 '19 at 12:43

score 0 · Accepted Answer · answered Feb 05 '19 at 12:27

0

You can merge two streams with Stream.concat() and group them with Collectors.groupingBy() and Collectors.mapping():

Map<String, String> first = Map.of("1", "a", "2", "a");
Map<String, String> second = Map.of("1", "android", "2", "ios");
Map<String, List<String>> result = Stream.concat(first.entrySet().stream(), second.entrySet().stream())
            .collect(groupingBy(Entry::getKey, mapping(Entry::getValue, toList())));
System.out.println(result);

will output:

{1=[a, android], 2=[a, ios]}

answered Feb 05 '19 at 12:27

Karol Dowbecki

43,645
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`first` and `second` (or `result1` and `result2`) are supposed to be an `ArrayList>` – Andrew Tobilko Feb 05 '19 at 12:34
@AndrewTobilko true, however OP also states "I want to merge the two maps to build a map like this one" which contradicts the `ArrayList` part. – Karol Dowbecki Feb 05 '19 at 12:35

Saeed · Answer 3 · 2019-02-05T12:56:32.010

You can try this approach as well:

Map<String, String> result1 = new HashMap<>();
// initialize result1 ...

Map<String, String> result2 = new HashMap<>();
// initialize result2 ...

Map<String, Map<String, String>> mergedResult = new HashMap<>();

Up to Java 8

result1.forEach((k1, v1) ->
            mergedResult.put(k1, new HashMap<String, String>() {{
                put(v1, result2.get(k1));
            }}));

Java 9 or later

result1.forEach((k1, v1) -> mergedResult.put(k1, 
                                 Map.of(v1, result2.get(k1))));

prasad_ · Answer 4 · 2019-02-05T14:11:39.703

This is one way arriving at the result:

Input Data:

// The first list of data
List<Map<String, String>> list1 = new ArrayList<>();
list1.add(getMapData("1", "a"));
list1.add(getMapData("2", "a"));
list1.add(getMapData("3", "b"));
list1.add(getMapData("4", "e"));
list1.add(getMapData("5", "e"));
list1.add(getMapData("999", "x"));
System.out.println(list1);

Data 1: [{1=a}, {2=a}, {3=b}, {4=e}, {5=e}, {999=x}]

// The second list of data
List<Map<String, String>> list2 = new ArrayList<>();
list2.add(getMapData("1", "android"));
list2.add(getMapData("2", "ios"));
list2.add(getMapData("3", "android"));
list2.add(getMapData("4", "android"));
list2.add(getMapData("5", "ios"));
list2.add(getMapData("888", "zzzzz"));
System.out.println(list2);

Data 2: [{1=android}, {2=ios}, {3=android}, {4=android}, {5=ios}, {888=zzzzz}]

// utility method for creating test data
private static Map<String, String> getMapData(String k, String v) {
    Map<String, String> m = new HashMap<>();
    m.put(k, v);
    return m;
}

The Result Process:

The output is stored to a Map<String, List<String>>:

Map<String, List<String>> result = new HashMap<>();

// process the first list
for (Map<String, String> m : list1) {
    for (Map.Entry<String, String> entry : m.entrySet()) {
        List<String> valueList = new ArrayList<>();
        valueList.add(entry.getValue());
        result.put(entry.getKey(), valueList);
    }
}

// process the second list; merge with the first
for (Map<String, String> m : list2) {
    for (Map.Entry<String, String> entry : m.entrySet()) {
        String k = entry.getKey();
        List<String> valueList = result.get(k);
        if (valueList == null) {
            valueList = new ArrayList<>();
        }
        valueList.add(entry.getValue());
        result.put(k, valueList);
    }
}

System.out.println(result);

The Result:

{1=[a, android], 2=[a, ios], 3=[b, android], 4=[e, android], 5=[e, ios], 888=[zzzzz], 999=[x]}

How to merge two ArrayList> to one?

4 Answers4