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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