JSON.getString doesn't return null

Question

I have a response coming back from a server and I am expecting a String value, so I wrote this for parsing it

public String getMessageFromServer(JSONObject response) {
    String msg = response.getString("message");
    return msg;
}

then, when I use this in my code and get a null value from the server, the function doesn't return null, it returns "null" instead.

I have seen this bug report, but I don't see a solution.

EDIT:

I have a small hack to solve this but it's ugly and I'm looking for a better solution:

public String getMessageFromServer(JSONObject response) {
    Object msg = response.get("message");
    if(msg == null) {
         return null;
    }
    return (String) msg;
}    

EDIT #2:

after years, going back to this question, I see that I was not entirely wrong here and that JSONObject has a built in method for this.

The way to get an optional value from a JSONObject is with using this method JSONObject.optString("message", DEF_VALUE);

Paternal

it is ok, but it's an ugly hack and I dislike it – Paternal

Why is that ugly or a hack? You are just doing it well – Embed

it's ugly because, I expect a String value (that can be null) but have to deal with type casting. – Paternal

Would checking for "null", in the first example, be less ugly? – Herminiahermione

it'll be time consuming, testing for the null value takes almost no time, testing for value of String takes a long time. – Paternal

@Paternal - you are optimizing prematurely. It is highly unlikely that the difference between == and String.equals will be significant. – Bloodmobile

@Paternal Please note that JSONObject.optString() does coerce JSONObject.NULL to string with value "null" too. In other words "null".equals(new JSONObject().put("test", JSONObject.NULL).optString("test", null)) gives true. – Bracer

The way to get an optional value from a JSONObject is with using this method JSONObject.optString("message", DEF_VALUE); This worked for me! Thank you – Pluvious

Answer 1

The hack looks okay for your situation.

The other option would be to use the method boolean isNull(String key) and then based on the returned boolean value proceed with your option. Something like:

public String getMessageFromServer(JSONObject response) {
    return ((response.has("message") && !response.isNull("message"))) ? response.getString("message") : null;
} 

But then, I don't think there's much of a difference between the your current implementation and this.

Hindrance

+1 for pointing out the correct method to check for a null value. But I don't agree when you say there is not much difference. The isNUll-call is cleaner and should be preferred towards relying on Object and casting. – Lachrymose

can't you remove the response.has("message)" check? Whenever that key doesn't exist, isNull will return true, so getString won't be called and no exception will be thrown. – Madonna

Answer 2

This is easy to solve when using Kotlin class extensions:

fun JSONObject.optNullableString(name: String, fallback: String? = null) : String? {
    return if (this.has(name) && !this.isNull(name)) {
        this.getString(name)
    } else {
        fallback
    }
}

Then e.g. name will be null in:

val name : String? = JSONObject("""{"id": "foo", "name":null}""").optNullableString("name")

Salpingitis

Answer 3

More simple way in Kotlin

fun JSONObject.getNullableString(name: String) : String? {
    if (has(name) && !isNull(name)) {
        return getString(name)
    }
    return null
}

Lattermost

Answer 4

In android sdk default value in optString method is annotated as @NonNull. Just skip it and simplify to get value or null:

fun JSONObject.optStringOrNull(key: String, fallback: String? = null): String? {
    val obj: Any? = opt(key)
    if (obj is String) {
        return obj
    } else if (obj != null) {
        return obj.toString()
    }
    return fallback
}

Vellicate