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Kooin-Shin
Kooin-Shin

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How to find value using path array on structural document in Java

Sometimes we need to find a value on structural document like JSON, YAML. Also there is sometime to need to find a value by path of document.

Structural document can be easily converted to Map-List object with parser libraries. Because of it, we can get a value of the document using key and index array.

To do that,

First, we need to get a parser library that can parse JSON or YAML to Java Map-List structure. That library can be found easily in internet library repository like Maven Central.

For this post, we assume that we target JSON document.
It's down below

{
  "id": "0001",
  "type": "donut",
  "name": "Cake",
  "ppu": 0.55,
  "batters": {
    "aaa": {
      "bbb": {
        "ccc": {
          "value": 123.0
        }
      }
    },
    "batter": [
      {
        "id": "1001",
        "type": "Regular"
      },
      {
        "id": "1002",
        "type": "Chocolate"
      },
      {
        "id": "1003",
        "type": "Blueberry"
      },
      {
        "id": "1004",
        "type": "Devil\u0027s Food"
      }
    ]
  },
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We use recursive algorithm to find a value and if path can't find value or path isn't exist, It will return null.

Completed code is down below

    /**
     * To find value with path array on structural data
     * 
     * @param obj
     * @param keys
     * @return
     */
    public static Object findValue(Object obj, Object[] keys) {
        if (obj instanceof List) {
            List list = (List) obj;
            if (keys.length == 1) {
                if (keys[0] instanceof Integer && list.size() > (int) keys[0]) {
                    return list.get((int) keys[0]);
                }
            } else if (keys.length > 1 && list.size() > 0) {
                int idx = (int)keys[0];
                if(idx < list.size()) {
                    return findValue(list.get((int) keys[0]), Arrays.copyOfRange(keys, 1, keys.length));
                }
            }
        } else if (obj instanceof Map) {
            Map map = (Map) obj;
            if (keys.length == 1 && map.containsKey(keys[0])) {
                return map.get(keys[0]);
            } else if (keys.length > 1 && map.containsKey(keys[0])) {
                if(keys.length > 0) {
                    return findValue(map.get(keys[0]), Arrays.copyOfRange(keys, 1, keys.length));
                }
            }
        }
        return null;
    }
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Well, let's start test now

Above JSON document save to file named 'sample.json'
Below code is read 'sample.json' and result is image more below.

Gson gson = new Gson();
Map<String, Object> map = gson.fromJson(new FileReader("simple.json"), Map.class);
Object[] keys = new Object[] {"batters", "batter", 3, "type"};

Object obj = findValue(map, keys);
System.out.println(obj);
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