How to split objects based on an element from a nested object present in an ArrayList in Java?

Question

I have the following list of objects (showing only one object in an array for example). How to do I split the users list into different objects with same original object using Java. I have given the result I am getting from the database and the result am expecting after processing in Java.

As you can see in the result I am expecting in the given expected JSON , the objects all remain the same except the users array list in each object. We have to split the array in terms of "parent_user_id" in all the objects in users array, so common objects with same "parent_user_id" will be in one object as a whole.

Result am getting from DB:

[
    {
        "_id" : "63a8808652f40e1d48a3d1d7",
        "name" : "A",
        "description" : null,
        "users" : [
            {
                "id" : "63a8808c52f40e1d48a3d1da",
                "owner" : "John Doe",
                "purchase_date" : "2022-12-25",
                "status" : "active",
                "parent_user_id" : "63a8808c52f40e1d48a3d1da"
            },
            {
                "id" : "63a880a552f40e1d48a3d1dc",
                "owner" : "John Doe 1",
                "purchase_date" : "2022-12-25",
                "parent_user_id" : "63a8808c52f40e1d48a3d1da"
            },
            {
                "id" : "63a880f752f40e1d48assddd",
                "owner" : "John Doe 2",
                "purchase_date" : "2022-12-25",
                "parent_user_id" : "63a8808c52f40e1d48a3d1da"
            },
            {
                "id" : "63a880f752f40e1d48a3d207",
                "owner" : "John Doe 11",
                "dt" : "2022-12-25",
                "status" : "inactive",
                "parent_user_id" : "63a880f752f40e1d48a3d207"
            },
            {
                "id" : "63a880f752f40e1d48agfmmb",
                "owner" : "John Doe 112",
                "dt" : "2022-12-25",
                "status" : "active",
                "parent_user_id" : "63a880f752f40e1d48agfmmb"
            },
            {
                "id" : "63a880f752f40e1d48agggg",
                "owner" : "John SS",
                "dt" : "2022-12-25",
                "status" : "inactive",
                "parent_user_id" : "63a880f752f40e1d48agggg"
            },
            {
                "id" : "63a880f752f40e1d487777",
                "owner" : "John SS",
                "dt" : "2022-12-25",
                "parent_user_id" : "63a880f752f40e1d48agggg"
            }
        ]
    }
]

Result I need after processing in Java:

[
  {
    "_id": "63a8808652f40e1d48a3d1d7",
    "name": "A",
    "description": null,
    "users": [
      {
        "id": "63a8808c52f40e1d48a3d1da",
        "owner": "John Doe",
        "purchase_date": "2022-12-25",
        "status": "active",
        "parent_user_id": "63a8808c52f40e1d48a3d1da"
      },
      {
        "id": "63a880a552f40e1d48a3d1dc",
        "owner": "John Doe 1",
        "purchase_date": "2022-12-25",
        "parent_user_id": "63a8808c52f40e1d48a3d1da"
      },
      {
        "id": "63a880f752f40e1d48assddd",
        "owner": "John Doe 2",
        "purchase_date": "2022-12-25",
        "parent_user_id": "63a8808c52f40e1d48a3d1da"
      }
    ]
  },
  {
    "_id": "63a8808652f40e1d48a3d1d7",
    "name": "A",
    "description": null,
    "users": [
      {
        "id": "63a880f752f40e1d48a3d207",
        "owner": "John Doe 11",
        "dt": "2022-12-25",
        "status": "inactive",
        "parent_user_id": "63a880f752f40e1d48a3d207"
      }
    ]
  },
  {
    "_id": "63a8808652f40e1d48a3d1d7",
    "name": "A",
    "description": null,
    "users": [
      {
        "id": "63a880f752f40e1d48agfmmb",
        "owner": "John Doe 112",
        "dt": "2022-12-25",
        "status": "active",
        "parent_user_id": "63a880f752f40e1d48agfmmb"
      }
    ]
  },
  {
    "_id": "63a8808652f40e1d48a3d1d7",
    "name": "A",
    "description": null,
    "users": [
      {
        "id": "63a880f752f40e1d48agggg",
        "owner": "John SS",
        "dt": "2022-12-25",
        "status": "inactive",
        "parent_user_id": "63a880f752f40e1d48agggg"
      },
      {
        "id": "63a880f752f40e1d487777",
        "owner": "John SS",
        "dt": "2022-12-25",
        "parent_user_id": "63a880f752f40e1d48agggg"
      }
    ]
  }
]

Answer 1

You can use JSON library such as Josson to do the transformation.

https://github.com/octomix/josson

Deserialization

Josson josson = Josson.fromJsonString(
    "[" +
    "    {" +
    "        \"_id\": \"63a8808652f40e1d48a3d1d7\"," +
    "        \"name\": \"A\"," +
    "        \"description\": null," +
    "        \"users\": [" +
    "            {" +
    "                \"id\": \"63a8808c52f40e1d48a3d1da\"," +
    "                \"owner\": \"John Doe\"," +
    "                \"purchase_date\": \"2022-12-25\"," +
    "                \"status\": \"active\"," +
    "                \"parent_user_id\": \"63a8808c52f40e1d48a3d1da\"" +
    "            }," +
    "            {" +
    "                \"id\": \"63a880a552f40e1d48a3d1dc\"," +
    "                \"owner\": \"John Doe 1\"," +
    "                \"purchase_date\": \"2022-12-25\"," +
    "                \"parent_user_id\": \"63a8808c52f40e1d48a3d1da\"" +
    "            }," +
    "            {" +
    "                \"id\": \"63a880f752f40e1d48assddd\"," +
    "                \"owner\": \"John Doe 2\"," +
    "                \"purchase_date\": \"2022-12-25\"," +
    "                \"parent_user_id\": \"63a8808c52f40e1d48a3d1da\"" +
    "            }," +
    "            {" +
    "                \"id\": \"63a880f752f40e1d48a3d207\"," +
    "                \"owner\": \"John Doe 11\"," +
    "                \"dt\": \"2022-12-25\"," +
    "                \"status\": \"inactive\"," +
    "                \"parent_user_id\": \"63a880f752f40e1d48a3d207\"" +
    "            }," +
    "            {" +
    "                \"id\": \"63a880f752f40e1d48agfmmb\"," +
    "                \"owner\": \"John Doe 112\"," +
    "                \"dt\": \"2022-12-25\"," +
    "                \"status\": \"active\"," +
    "                \"parent_user_id\": \"63a880f752f40e1d48agfmmb\"" +
    "            }," +
    "            {" +
    "                \"id\": \"63a880f752f40e1d48agggg\"," +
    "                \"owner\": \"John SS\"," +
    "                \"dt\": \"2022-12-25\"," +
    "                \"status\": \"inactive\"," +
    "                \"parent_user_id\": \"63a880f752f40e1d48agggg\"" +
    "            }," +
    "            {" +
    "                \"id\": \"63a880f752f40e1d487777\"," +
    "                \"owner\": \"John SS\"," +
    "                \"dt\": \"2022-12-25\"," +
    "                \"parent_user_id\": \"63a880f752f40e1d48agggg\"" +
    "            }" +
    "        ]" +
    "    }," +
    "    {" +
    "        \"_id\": \"111111111111111111111111\"," +
    "        \"name\": \"B\"," +
    "        \"users\": [" +
    "            {" +
    "                \"id\": \"22222222222222222222222\"," +
    "                \"owner\": \"John XX\"," +
    "                \"purchase_date\": \"2022-12-25\"," +
    "                \"status\": \"active\"," +
    "                \"parent_user_id\": \"22222222222222222222222\"" +
    "            }" +
    "        ]" +
    "    }" +
    "]");
    

Transformation

JsonNode node = josson.getNode(
    "[]@" + // 1
    ".users" + // 2
    ".group(parent_user_id)" + // 3
    ".map(..._id, ...name, ...description, users:elements)" + // 4
    ".@flatten()"); // 5
System.out.println(node.toPrettyString());

1. Divert each element to separate branches
2. Proceed to node users
3. Group by parent_user_id
4. Map a new object, get _id/name/description from 2 steps back, and rename elements to users
5. Merge all branch results into a single array and then flatten the array

Output

[ {
  "_id" : "63a8808652f40e1d48a3d1d7",
  "name" : "A",
  "description" : null,
  "users" : [ {
    "id" : "63a8808c52f40e1d48a3d1da",
    "owner" : "John Doe",
    "purchase_date" : "2022-12-25",
    "status" : "active",
    "parent_user_id" : "63a8808c52f40e1d48a3d1da"
  }, {
    "id" : "63a880a552f40e1d48a3d1dc",
    "owner" : "John Doe 1",
    "purchase_date" : "2022-12-25",
    "parent_user_id" : "63a8808c52f40e1d48a3d1da"
  }, {
    "id" : "63a880f752f40e1d48assddd",
    "owner" : "John Doe 2",
    "purchase_date" : "2022-12-25",
    "parent_user_id" : "63a8808c52f40e1d48a3d1da"
  } ]
}, {
  "_id" : "63a8808652f40e1d48a3d1d7",
  "name" : "A",
  "description" : null,
  "users" : [ {
    "id" : "63a880f752f40e1d48a3d207",
    "owner" : "John Doe 11",
    "dt" : "2022-12-25",
    "status" : "inactive",
    "parent_user_id" : "63a880f752f40e1d48a3d207"
  } ]
}, {
  "_id" : "63a8808652f40e1d48a3d1d7",
  "name" : "A",
  "description" : null,
  "users" : [ {
    "id" : "63a880f752f40e1d48agfmmb",
    "owner" : "John Doe 112",
    "dt" : "2022-12-25",
    "status" : "active",
    "parent_user_id" : "63a880f752f40e1d48agfmmb"
  } ]
}, {
  "_id" : "63a8808652f40e1d48a3d1d7",
  "name" : "A",
  "description" : null,
  "users" : [ {
    "id" : "63a880f752f40e1d48agggg",
    "owner" : "John SS",
    "dt" : "2022-12-25",
    "status" : "inactive",
    "parent_user_id" : "63a880f752f40e1d48agggg"
  }, {
    "id" : "63a880f752f40e1d487777",
    "owner" : "John SS",
    "dt" : "2022-12-25",
    "parent_user_id" : "63a880f752f40e1d48agggg"
  } ]
}, {
  "_id" : "111111111111111111111111",
  "name" : "B",
  "users" : [ {
    "id" : "22222222222222222222222",
    "owner" : "John XX",
    "purchase_date" : "2022-12-25",
    "status" : "active",
    "parent_user_id" : "22222222222222222222222"
  } ]
} ]