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);

it's ugly because, I expect a String value (that can be null) but have to deal with type casting. — thepoosh
– thepoosh, Commented Oct 29, 2012 at 8:40
Would checking for "null", in the first example, be less ugly? — ignis
– ignis, Commented Oct 29, 2012 at 8:45
@thepoosh - you are optimizing prematurely. It is highly unlikely that the difference between == and String.equals will be significant. — Stephen C
– Stephen C, Commented Oct 29, 2012 at 9:15

Sujay · Accepted Answer · 2012-10-29 08:47:25Z

28

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.

edited Oct 29, 2012 at 8:47

answered Oct 29, 2012 at 8:40

Sujay

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2 Comments

EinLama Over a year ago

+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.

Mateus Gondim Over a year ago

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.

jhavatar · Accepted Answer · 2020-03-01 00:57:54Z

3

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")

edited Mar 1, 2020 at 0:57

answered Mar 1, 2020 at 0:40

jhavatar

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Comments

Vlad · Accepted Answer · 2021-01-28 09:14:01Z

0

More simple way in Kotlin

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

answered Jan 28, 2021 at 9:14

Vlad

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Comments

Prilaga · Accepted Answer · 2024-03-01 02:18:11Z

0

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
}

answered Mar 1, 2024 at 2:18

Prilaga

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