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" } ] }, 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; } 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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