Develop Reference

How to handle the case when field is not present in Object Java?

I have an object something like this in my database and now my requirement is to find the value of particular field such as name and if present return true,
{
"_id" : "123",
"name" : "Team"
}
but in some case the field name itself doesn't exist. Sample can be something like this:
{
"id":1234
}
In this case I need to return false.
How can I validate if name field exist in particular object?
I was trying to use StringUtils method something like this
StringUtils.isBlank(obj.getName); But its throwing It is throwing java.lang.NullPointerException .

You can use Json schema validator. If your json will be in specific format. Please have a look at Jackson library.

JSONObject class has a method named "has". try this way,
if (json.has("name")) {
String status = json.getString("name"));
}
This will work

You can use Gson Java library to serialize and deserialize Java objects to JSON (as given below).
Gson gson = new Gson();
JsonObject jsonObject = gson.fromJson(object, JsonObject.class);
Then, you can use the has method in JsonObject, to check if the key exists in it.
jsonObject.has(key)
Example:
Below is a method to check if given key exists in given json string, and get it's value.
(Instead of String, you can use your object as well. Here, I am considering the jsonStr as a String object, for better understanding.)
private String getValueFromJsonForGivenKey(String key, String jsonStr) {
Gson gson = new Gson();
JsonObject jsonObject = gson.fromJson(jsonStr, JsonObject.class);
if (jsonObject.has(key)) {
// The given JSON string has the given key
String value = jsonObject.get(key).getAsString();
return value;
}
return null;
}
For key id and for jsonStr { "id": "1234" }, we get 1234.
For key name and for jsonStr { "id": "1234" }, we get null.

What you can do is to use JSONObject's opt method
eg.
JSONObject jsonObject = new JSONObject(myJSONString);
String name = jsonObject.optString("name");

Can't parse Java JSON string

From my server is get a JSON response like this:
String json = getJsonFromServer();
The server returns this reply (with the double quotes in the beginning and end):
"{\"error\":\"Not all required fields have been filled out\"}";
I then want to get the error field. I have tried to use the JSONObject class to get the string, but it does not work.
System.out.println("The error is: " + new JSONObject().getJSONObject(response).getString("error");

I have try
String response = "{\"error\":\"Not all required fields have been filled out\"}";
JSONParser parser = new JSONParser();
try {
JSONObject json = (JSONObject) parser.parse(response);
System.out.println(json.get("error").toString());
} catch (ParseException ex) {
}
It have working, you can try it and don't forget add json lib json-simple

You seem to be using the wrong JSONObject constructor; in your example, you are trying to fetch an object from a newlay created object - it will never exist.
Try this:
String response = "{\"error\":\"Not all required fields have been filled out\"}";
JSONObject json = new JSONObject( response );
String error = json.getString( "error" );
System.out.println( error );
Yields
Not all required fields have been filled out
edit
Now that you have completely changed the question...
Why not just first strip the first and last char?
So if
String response = "\"{\"error\":\"Not all required fields have been filled out\"}\"";
response = response.substring(1, response.length() - 1));
// Now convert to JSONObject

Your response object has exactly this string?
{"error":"Not all required fields have been filled out"}
The code below printed the expected output:
Object responseObject = "{\"error\":\"Not all required fields have been filled out\"}";
JSONObject jsonObject = new JSONObject(responseObject.toString());
String errorContent = jsonObject.getString("error");
System.out.println(errorContent);

How to access nested objects of a Json File

the string I have into "jsonString" is the content of this link: http://85.18.173.82/cineca/wp5/json/events.json
Now I want the value "Day" of the second "Event".
JSONObject o = new JSONObject(jsonString);
String day = o.getString("XXXXXXXXXX");
System.out.println(day);
What does I have to put as argument of o.getString?
Many thanks

JSONObject obj = new JSONObject(json);
JSONArray array = obj.getJSONArray("Events");
for(int i = 0 ; i < array.length() ; i++){
System.out.println(array.getJSONObject(i).getJSONObject("Event").getString("Day"));
}
In this way, you can access, thanks.

The way you're constructing your JSONObject is wrong. By using this constructor you're not reading the json from that URL, you're actually using that string as a json representation (which it is not).
If you want to first read the json from your URL you'll have to do an HTTP GET request and then construct a JSONObject out of the response.
For more info, take a look at JSONObject docs

Android: Exception when converting a String into a JSONObject

I get the following Error when I try to convert a JSON String into a JSONObject.
Value 48.466667|9.883333 at location of type java.lang.String
cannot be converted to JSONObject
The String is valid JSON, I tested it with http://jsonlint.com/
Example:
{"name":"An der Decke","location":"48.412583|10.0385","type":"Virtual","size":null,"status":"Available","difficulty":1,"rating":null,"terrain":1}
The code that produces the exception looks like that:
jsonObject = new JSONObject(result);
jsonArray = new JSONArray();
Iterator<String> iter = jsonObject.keys();
while (iter.hasNext()) {
String key = iter.next();
try {
JSONObject value = (JSONObject) jsonObject.get(key); <---- Exception
jsonArray.put(value);
} catch (JSONException e) {
// Something went wrong!
}
}
Is the pipe | symbol not a valid character in Java JSON?
EDIT:
The thing is, it works fine if the JSON String doesn't include the "location":"48.412583|10.0385" part...

You seem to misunderstand how the org.json library works.
As explained on the JSON homepage, a JSON value can be a string, number, object, array, true/false or null. The library maps these value types to String, Number subclasses, JSONArray, JSONObject, Boolean or null.
Not everything in that library is a JSONObject. In fact, a JSONObject is specifically used to represent a name/value pair object. JSONObject.get() can potentially return any of the aforementioned value types, that's why it needs to fall back to the greatest common denominator type: Object (and not JSONObject). Thus, casting everything to a JSONObject won't work.
It's your responsibility to ensure that you're casting to the correct type using your knowledge of the incoming data structure. This seems to be a problem in your case: your JSON string contains strings (for name, location, type and status), integers (for difficulty and terrain) and nulls (for size). What exactly are you trying to do with these?

If your goal is just to get a JSONArray of all your JSON string values, there's a much simpler way to do it.
JSONObject jsonObject = new JSONObject(result);
JSONArray jsonArray = jsonObject.toJSONArray(jsonObject.names());
System.out.println(jsonArray); // prints:
// [1,"Available","48.412583|10.0385","An der Decke",1,null,"Virtual",null]
With that aside, you were wrong to assume that every value encapsulated within JSON would be a JSON object itself. In fact, in your case none of them are. The correct types of all the values in your JSON are
// String
System.out.println(jsonObject.getString("name")); // An der Decke
System.out.println(jsonObject.getString("location")); // 48.412583|10.0385
System.out.println(jsonObject.getString("type")); // Virtual
System.out.println(jsonObject.getString("status")); // Available
// Null
System.out.println(jsonObject.isNull("size")); // true
System.out.println(jsonObject.isNull("rating")); // true
// Integer
System.out.println(jsonObject.getInt("terrain")); // 1
System.out.println(jsonObject.getInt("difficulty")); // 1
On the other hand, if your name was an embedded JSON object consisting of first, middle and last names, your JSON string (ignoring the rest of the keys for brevity) would have looked like
{"name": {"fname" : "An", "mname" : "der", "lname" : "Decke"}}
Now, we can put getJSONObject() to use because we really do have an embedded JSON object.
JSONObject jsonObj = new JSONObject("{\"name\":
{\"fname\" : \"An\", \"mname\" : \"der\", \"lname\" : \"Decke\"}}");
// get embedded "name" JSONObject
JSONObject name = jsonObj.getJSONObject("name");
System.out.println(name.getString("fname") + " "
+ name.getString("mname") + " "
+ name.getString("lname")); // An der Decke

The get() method of JSONObject returns a result of type Object. In this case, it seems it is a String. It's as if you were doing
JSONObject value = (JSONObject) new String("asdasdsa");
which obviously makes no sense as they are incompatible types.
Instead, retrieve the value, create a JSONObject from it and add it to the JSONArray.

Why is this JSONObject not a JSONArray?

i am the following codes in javascript
var people = {
"users" : [{id : this.getJID().toString()},{id : this.getJID().toString()}],
"body" : messageBody
}
on my server end, I have the following:
JSONObject b = new JSONObject(jsonstring);
JSONArray users = b.getJSONArray("users");
I cannot get users because I will have the following exception.
org.json.JSONException: JSONObject["users"] is not a JSONArray.
I tried to cast to JSONObject but also gets error.
JSONObject o = b.getJSONObject("users");
org.json.JSONException: JSONObject["users"] is not a JSONObject.
What is wrong??

As noted above:
The JSON is serialized wrongly. The value of user is not an array but a string: {"users":"[...]","body":"test"} There should be no quotes around [...]. That's why you get the error. It seems like you recursively serialized the contents of people