How do you set a value to null with org.json.JSONObject in java?

Try to set JSONObject.NULL instead of null:

A sentinel value used to explicitly define a name with no value. Unlike null, names with this value:

• show up in the names() array
• show up in the keys() iterator
• return true for has(String)
• do not throw on get(String)
• are included in the encoded JSON string.

This value violates the general contract of equals(Object) by returning true when compared to null. Its toString() method returns "null".