Java 8：将要映射的Group Object数组作为JSON返回

Question

I have an Object 2D array ( Object [][] ) that is returned from querying a database. I now want to map it to Objects which can be returned in API call after grouping.

Here is my 2D Object array.

Object [][] dbResult = 
{
  {1, "a", 9, "Content1", "format1", false, true},
  {1, "a", 9, "Content1", "format2", true, false},
  {2, "b", 8, "Content2", "format3", true, false},
  {2, "b", 8, "Content2", "format4", false, false},
  {3, "c", 7, "Content3", "format5", true, true},
  {4, "d", 8, "Content2", "format6", false, true},
  {4, "d", 6, "Content3", "format7", true, true},
  {4, "d", 5, "Content4", "format8", false, false},
  {5, "e", 4, "Content5", "format9", false, true},
  {6, "f", 3, "Content6", "format10", true, false}
};

Here is the legend/key for the index.
[ID, Name, AnotherID, AnotherName, Format, Boolean, Boolean]

I want to return

List<IdName> idsNames;

Where each of the classes should be mapped like this.

class IdName {
    String id;
    String name;
    List<Another> anotherNameList;
}

class Another {
    String anotherId;
    String anotherName;
    List<Format> formatList;
}

class Format {
    String format;
    Boolean isEnabled;
    Boolean isManaged;
}

I tried using Java 8's groupingBy but I couldn't get to the state that I want.

Sample expected result:

[
      {
      "id": 1,
      "name": "a",
      "another": [
        {
          "anotherId": 9,
          "anotherName": "Content1",
          "format": [
            {
              "format": "format1",
              "isEnabled": true,
              "isManaged": false
            },
            {
              "format": "format2",
              "isEnabled": true,
              "isManaged": false
            }
          ]
        }
      ]
    }
]

Answer 1

Looks like you should use Collectors.collectingAndThen .

First create the extractors (Assuming your classes have constructors & getters):

// The cast types are just an example. You can Cast/convert the array values to any type you want

IdName extractIdName(Object[] row) {
    return new IdName((String) row[0], (String) row[1], null);
}

Another extractAnother(Object[] row) {
    return new Another((String) row[2], (String) row[3], null);
}

Format extractFormat(Object[] row) {
    return new Format((String) row[4], (boolean) row[5], (boolean) row[6]);
}

Then you will need the merge functions:

List<Another> setFormats(Map<Another, List<Format>> map) {
    return map.entrySet()
              .stream()
              .map(e -> {
                  e.getKey().setFormatList(e.getValue());
                  return e.getKey();
              })
              .collect(toList());
}

List<IdName> setAnothers(Map<IdName, List<Another>> map) {
    return map.entrySet()
              .stream()
              .map(entry -> {
                  entry.getKey().setAnotherNameList(entry.getValue());
                  return entry.getKey();
              })
              .collect(toList());
}

Finally this will do the trick:

// Converting Object[][] to List<IdName>
List<IdName> list = 
      Arrays.stream(dbResult)
            .collect(
                collectingAndThen(
                    groupingBy(this::extractIdName,
                        collectingAndThen(
                            groupingBy(this::extractAnother,
                                mapping(this::extractFormat, toList())),
                            this::setFormats
                        )),                                                             
                    this::setAnothers));

Answer 2

It can be done in several steps. Let all values be String for simplicity. Also you are supposed to have constructors and equals / hashcode methods implemented.

Map<IdName, Map<Another, List<String[]>>> map = Arrays.stream(dbResult)
    .collect(
        groupingBy(s -> new IdName(s[0], s[1], null),
            groupingBy(s -> new Another(s[2], s[3], null))));

Then we can create Format objects and put everything together.

for (Map.Entry<IdName, Map<Another, List<String[]>>> entry : map.entrySet()) {
    IdName idName = entry.getKey();        // main object
    Set<Another> anothers = entry.getValue().keySet();
    for (Another another : anothers) {        // create list<Format> for each Another
        List<Format> formatList = entry.getValue().get(another).stream()
            .map(format -> new Format(format[4], format[5], format[6]))
            .collect(Collectors.toList());

        another.setFormatList(formatList);
    }

    idName.setAnotherNameList(anothers);
}

Now we can get all assembled objects

Set<IdName> idNames = map.keySet();