blocks|key|228404|text|尝试使用json.isNull(+"field-name"+)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|228405|参考：http://developer.android.com/reference/org/json/JSONObject.html#isNull%2528java.lang.String%2529|228406|entityMap|0|LINK|mutability|MUTABLE|url|http://developer.android.com/reference/org/json/JSONObject.html#isNull%2528java.lang.String%2529^0|4|R|0|3|2K|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@]|D|@$9|T|A|U|1|V]]|E|$]]|$1|H|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

Try with <code>json.isNull( "field-name" )</code>.

Reference: <a href="http://developer.android.com/reference/org/json/JSONObject.html#isNull%28java.lang.String%29">http://developer.android.com/reference/org/json/JSONObject.html#isNull%28java.lang.String%29</a>

blocks|key|230096|text|首先检查isNull()....if+cant，然后尝试下面的内容|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|230097|您还可以让JSONObject.NULL检查空值...|230098|+if+((resultObject.has("username")
++++&&+null+!=+resultObject.getString("username")
++++&&+resultObject.getString("username").trim().length()+!=+0)
++++++{
+++++++++++++++//not+null
++++++++}|code-block|syntax|javascript|230099|在你的例子中，还要检查resultObject.getString("username").trim().eqauls("null")|230100|entityMap^0|4|8|0|5|F|0|0|B|1K|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|T|8|@$9|U|A|V|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|P|$]]

first check with <code>isNull()</code>....if cant work then try belows

and also you have <code>JSONObject.NULL</code> to check null value...

<pre><code> if ((resultObject.has("username")
 &amp;&amp; null != resultObject.getString("username")
 &amp;&amp; resultObject.getString("username").trim().length() != 0)
 {
 //not null
 }
</code></pre>

and in your case also check <code>resultObject.getString("username").trim().eqauls("null")</code>

blocks|key|3097453|text|因为如果给定的键存在，JSONObject#getString就会返回值，所以根据定义它不是null。这就是JSONObject.NULL存在的原因:表示一个null+JSON值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3097454|json.getString("hello").equals(JSONObject.NULL);+//+should+be+false
json.getString("bye").equals(JSONObject.NULL);+//+should+be+true|code-block|syntax|javascript|3097455|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Because JSONObject#getString returns a value if the given key exists, it is not null by definition. This is the reason JSONObject.NULL exists: to represent a null JSON value.

<pre><code>json.getString("hello").equals(JSONObject.NULL); // should be false
json.getString("bye").equals(JSONObject.NULL); // should be true
</code></pre>

blocks|key|228477|text|如果您必须先解析json，然后再处理对象，那么让我们试试这个|type|unstyled|depth|inlineStyleRanges|entityRanges|data|228478|解析器|228479|Object+data+=+json.get("username");|code-block|syntax|javascript|228480|处理程序|228481|if+(data+instanceof+Integer+%7C%7C+data+instanceof+Double+%7C%7C+data+instanceof+Long)+{
+++++//+handle+number+;
}+else+if+(data+instanceof+String)+{
+++++//+hanle+string;+++++++++++++++
}+else+if+(data+==+JSONObject.NULL)+{
+++++//+hanle+null;+++++++++++++++++
}|228482|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

If you must parse json first and handle object later, let try this 

Parser 

<pre><code>Object data = json.get("username");
</code></pre>

Handler

<pre><code>if (data instanceof Integer || data instanceof Double || data instanceof Long) {
 // handle number ;
} else if (data instanceof String) {
 // hanle string; 
} else if (data == JSONObject.NULL) {
 // hanle null; 
}
</code></pre>

blocks|key|2883501|text|对于android，如果不存在这样的映射，它将引发一个JSONException。所以你不能直接调用这个方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2883502|json.getString("bye")|code-block|syntax|javascript|2883503|如果您的数据可以为空(可能不存在密钥)，请尝试|2883504|json.optString("bye","callback+string");|2883505|或|2883506|json.optString("bye");|2883507|而不是。|2883508|在您的演示代码中，|2883509|JSONObject+json+=+new+JSONObject("{\"hello\":null}");
json.getString("hello");|2883510|您得到的是字符串"+null“not+null。|2883511|你应该使用|2883512|if(json.isNull("hello"))+{
++++helloStr+=+null;
}+else+{
++++helloStr+=+json.getString("hello");
}|2883513|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|14|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|16|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|18|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|19|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|1A|8|@]|9|@]|A|$E|F]]|$1|U|3|V|5|6|7|1B|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1C|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|D|7|1D|8|@]|9|@]|A|$E|F]]|$1|10|3|-4|5|6|7|1E|8|@]|9|@]|A|$]]]|11|$]]

For android it will raise an JSONException if no such mapping exists. So you can't call this method directly. 

<pre><code>json.getString("bye")
</code></pre>

if you data can be empty(may not exist the key), try 

<pre><code>json.optString("bye","callback string");
</code></pre>

or

<pre><code>json.optString("bye");
</code></pre>

instead.

In your demo code, the 

<pre><code>JSONObject json = new JSONObject("{\"hello\":null}");
json.getString("hello");
</code></pre>

this you get is String "null" not null.

your shoud use 

<pre><code>if(json.isNull("hello")) {
 helloStr = null;
} else {
 helloStr = json.getString("hello");
}
</code></pre>

blocks|key|2883537|text|下面是我使用的一个帮助器方法，这样我只需一行代码就可以获得JSON字符串：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2883538|public+String+getJsonString(JSONObject+jso,+String+field)+{
++++if(jso.isNull(field))
++++++++return+null;
++++else
++++++++try+{
++++++++++++return+jso.getString(field);
++++++++}
++++++++catch(Exception+ex)+{
++++++++++++LogHelper.e("model",+"Error+parsing+value");
++++++++++++return+null;
++++++++}
}|code-block|syntax|javascript|2883539|然后就像这样：|2883540|String+mFirstName+=+getJsonString(jsonObject,+"first_name");|2883541|将给出字符串值或安全地将字符串变量设置为null。我尽可能地使用Gson来避免像这样的陷阱。在我看来，它处理空值要好得多。|2883542|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Here's a helper method I use so that I can get JSON strings with only one line of code:

<pre><code>public String getJsonString(JSONObject jso, String field) {
 if(jso.isNull(field))
 return null;
 else
 try {
 return jso.getString(field);
 }
 catch(Exception ex) {
 LogHelper.e("model", "Error parsing value");
 return null;
 }
}
</code></pre>

and then something like this:

<pre><code>String mFirstName = getJsonString(jsonObject, "first_name");
</code></pre>

would give you your string value or safely set your string variable to null. I use Gson whenever I can to avoid pitfalls like these. It handles null values much better in my opinion.

How can I detect when a json value is null?
for example: [{"username":null},{"username":"null"}]

The first case represents an unexisting username and the second a user named "null". But if you try to retrieve them both values result in the string "null"

<pre><code>JSONObject json = new JSONObject("{\"hello\":null}");
json.put("bye", JSONObject.NULL);
Log.e("LOG", json.toString());
Log.e("LOG", "hello="+json.getString("hello") + " is null? "
 + (json.getString("hello") == null));
Log.e("LOG", "bye="+json.getString("bye") + " is null? "
 + (json.getString("bye") == null));
</code></pre>

The log output is

<pre><code>{"hello":"null","bye":null}
hello=null is null? false
bye=null is null? false
</code></pre>

Android Json and null values

JSON

如何检测json值何时为空？例如：{“用户名”：空}，{“用户名”：“空”}第一种情况表示不存在的用户名，第二种情况表示名为"null“的用户。但是，如果您尝试检索这两个值，则会得到字符串"null“JSONObject json = new JSONObject("{\"hello\":null}");json.pu...

问Android Json和空值
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