Working with nulls in JSON objects

Question

I am making calls to a server which can return key-value mappings in the following format:

{...
 "mykey":null
...}

I am able to create a JSONObject instance from the above (using the JSONObject(String json) constructor) but am not sure how to determine whether this JSONObject instance has a mapping for "mykey" and that the value of the mapping is null? Would the following check do it...

myJSONObject.has("mykey") && !myJSONObject.isNull("mykey")

... or is there a better way? In particular, I am confused as to whether the JSONObject.NULL object means the same thing as null?

In addition: as well as getting mappings of keys whose value can be null, I need to create JSONObject instances that map null to keys. Would...

myJSONObject.put("mykey", JSONObject.NULL);

... do the job, or is there another way I should be doing this?

John O'Connor · Accepted Answer · 2012-01-17 02:52:48Z

If you're using the built-in Android JSON library, then you're actually using the (quite capable) libraries from org.json.

https://github.com/douglascrockford/JSON-java

If that's the case, we can just grep the code to see most of the answers here.

In particular, I am confused as to whether the JSONObject.NULL object means the same thing as null?

This should hopefully help:

/**
 * JSONObject.NULL is equivalent to the value that JavaScript calls null,
 * whilst Java's null is equivalent to the value that JavaScript calls
 * undefined.
 */
 private static final class Null {

    /**
     * There is only intended to be a single instance of the NULL object,
     * so the clone method returns itself.
     * @return     NULL.
     */
    protected final Object clone() {
        return this;
    }

    /**
     * A Null object is equal to the null value and to itself.
     * @param object    An object to test for nullness.
     * @return true if the object parameter is the JSONObject.NULL object
     *  or null.
     */
    public boolean equals(Object object) {
        return object == null || object == this;
    }

    /**
     * Get the "null" string value.
     * @return The string "null".
     */
    public String toString() {
        return "null";
    }
}

/**
 * It is sometimes more convenient and less ambiguous to have a
 * <code>NULL</code> object than to use Java's <code>null</code> value.
 * <code>JSONObject.NULL.equals(null)</code> returns <code>true</code>.
 * <code>JSONObject.NULL.toString()</code> returns <code>"null"</code>.
 */
public static final Object NULL = new Null();

Part 2:

am not sure how to determine whether this JSONObject instance has a mapping for "mykey" and that the value of the mapping is null? Would the following check do it...

myJSONObject.has("mykey") && !myJSONObject.isNull("mykey")

Yep - that seems to be about as good a method as any. In particular, you want to check for null using the isNull method, as JSONObject defines it's own NULL object (see above).

In addition: as well as getting mappings of keys whose value can be null, I need to create JSONObject instances that map null to keys. Would...

myJSONObject.put("mykey", JSONObject.NULL); ... do the job, or is there another way I should be doing this?

That's exactly how you want to do it - in particular, using JSONObject.NULL rather than java's null (see above again).

henrik · Accepted Answer · 2012-01-16 23:29:09Z

1

Have a look at the API docs.

boolean has(String name)
         Returns true if this object has a mapping for name.

boolean isNull(String name)
         Returns true if this object has no mapping for name or if it has a mapping whose value is NULL.

If you are not sure, how the serializer / deserializer behaves, you can always test the output / parsing results with simple code snippets.

answered Jan 16, 2012 at 23:29

henrik

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