How to Read Json File Without Giving Element Names in Java

Question

I want to read json file as follow;

{
  "M": {
  "row": [
  {
  "col1": "c00"
  },
  {
  "col1": "c10",
  "col2": "c11"
  },
  {
  "col1": "c20",
  "col2": "c21",
  "col3": "c22"
  }
  ]
  }
}

Next to reading, I want to print "c00","c10","c11","c20","c21","c22" but without giving element as "col1","col2","col3"... Thanks for helping.

Answer 1

Use any JSON parsing library such as GSON or Jackson and convert it into Java Object.

Sample code using GSON library

 Type type = new TypeToken<Map<String, Object>>() {}.getType();
 Map<String, Object> data = new Gson().fromJson(jsonString, type);
 System.out.println(new GsonBuilder().setPrettyPrinting().create().toJson(data));
 // get the desired value from map

 Map<String,ArrayList<Map<String,String>>> mMap=(Map<String,ArrayList<Map<String,String>>>)data.get("M");
 ArrayList<Map<String,String>> rowArray=mMap.get("row");
 for(Map<String,String> colMap:rowArray){
     for(String value:colMap.values()){
         System.out.println(value);
     }
 }

---

You can convert JSON string into Java POJO class as well that is replica of the JSON string

class MDetails {
    private MDetail M;
    // getter & setter
}

class MDetail {
    private ArrayList<Map<String, String>> row;
    // getter & setter
}

...
MDetails data = new Gson().fromJson(jsonString, MDetails.class);
for (Map<String, String> colMap : data.getM().getRow()) {
    for (String value : colMap.values()) {
        System.out.println(value);
    }
}

---

You can use different field name using @SerializedName annotation.

 class MDetails {
    @SerializedName("M")
    private MDetail mDetail;
    // getter & setter         
 }

---

As per comments, the keys are dynamic so iterate the map containing another map in it and print all the values whose key starts with col

sample code: (call below method that recursively iterate all keys and values)

public static void printColValues(Object data) {
    if (data instanceof Map) {
        for (Map.Entry<String, Object> entry : ((Map<String, Object>) data).entrySet()) {
            String key = entry.getKey();
            if (key.startsWith("col")) {
                System.out.println(entry.getValue());
            } else {
                printColValues(entry.getValue());
            }
        }
    } else if (data instanceof List) {
        for (Object obj : (List) data) {
            printColValues(obj);
        }
    }
}

output:

c00
c10
c11
c20
c21
c22

---

OR if nothing works then try with regex pattern but keep it as last resort

("col\d+":)("[^"]*")

Here is online demo

Or try with Reluctant Qualifier

("col\d+":)(".*?")

Here is demo

sample code:

String jsonString = "{\"M\":{\"row\":[{\"col1\":\"c00\"},{\"col1\":\"c10\",\"col2\":\"c11\"},{\"col1\":\"c20\",\"col2\":\"c21\",\"col3\":\"c22\"}]}}";
Pattern p = Pattern.compile("(\"col\\d+\":)(\"[^\"]*\")");
Matcher m = p.matcher(jsonString);
while (m.find()) {
    System.out.println(m.group(2));
}

output:

"c00"
"c10"
"c11"
"c20"
"c21"
"c22"

---

Code updated to print all values regardless of keys

public static void printColValues(Object data) {
    if (data instanceof Map) {
        for (Map.Entry<String, Object> entry : ((Map<String, Object>) data).entrySet()) {
            Object value=entry.getValue();
            if (value instanceof String) {
                System.out.println(value);
            } else {
                printColValues(value);
            }
        }
    } else if (data instanceof List) {
        for (Object obj : (List) data) {
            printColValues(obj);
        }
    }
}

Answer 2

You can use org.json library for this. It is here . General idea:

JSONObject obj = new JSONObject(sourceString);
for(String key : obj.keys()){
    String value = obj.getString(key);
    // Process value here
}