面试题与答案

� Lighter and faster than XML as on-the-wire data format
� JSON objects are typed while XML data is typeless
> JSON types: string, number, array, boolean,
> XML data are all string
� Native data form for JavaScript code
> Data is readily accessible as JSON objects in your JavaScript
code vs. XML data needed to be parsed and assigned to variables through tedious DOM APIs
> Retrieving values is as easy as reading from an object property in your JavaScript code

� A collection of name/value pairs
> In various languages, this is realized as an object, record, struct, dictionary, hash table, keyed list, or associative array
� An ordered list of values
> In most languages, this is realized as an array, vector, list, or sequence
� These are universal data structures supported
� A JSON object is an unordered set of name/value pairs
� A JSON object begins with { (left brace) and ends with } (right brace)
� Each name is followed by : (colon) and the name/value pairs are separated by , (comma)

� JSON is a subset of the object literal notation of JavaScript
> JSON can be used in the JavaScript language with no muss or fuss
Example: JSON Object
var myJSONObject = {"bindings": [ {"ircEvent": "PRIVMSG", "method": "newURI", "regex": "^http://.*"},
{"ircEvent": "PRIVMSG", "method": "deleteURI", "regex": "^delete.*"},
{"ircEvent": "PRIVMSG", "method": "randomURI", "regex": "^random.*"}
]
};
� In this example, a JSON JavaScript object is created
containing a single member "bindings", which contains
an array containing three objects, each containing
"ircEvent", "method", and "regex" members
� Members can be retrieved using dot or subscript
operators myJSONObject.bindings[0].method // "newURI"
Text to Object Conversion in
JavaScript code
var myObject = eval('(' + myJSONtext + ')');
� To convert a JSON text into an JSON object, use
the eval() function > eval() invokes the JavaScript compiler
> Since JSON is a proper subset of JavaScript, the compiler will
correctly parse the text and produce an object structure

� Create JSON Java object
� Add name and value pairs using put method
� Convert it to String type using toString method and send it to the client with content-type as "text/xml" or "text/plain"
myString = new JSONObject().put("JSON", "Hello, World!").toString();
// myString is {"JSON": "Hello, World"}

� JSON data is received as a string
� Calling eval() will generate JSON object in JavaScript code
var JSONdata = eval(req.responseText);
� Once you have JSON object, you can use . notation to access its properties
var name = JSONdata.name;
var address = JSONdata.addresses[3];
var streetname = JSONdata.addresses[3].street;

� Create JSON JavaScript object
� Use "POST" HTTP method in the open method of the XMLHttpRequest object
� Pass JSON JavaScript object in the send method of XMLHttpRequest object
var carAsJSON = JSON.stringify(car);
var url = "JSONExample?timeStamp=" + new Date().getTime(); createXMLHttpRequest();
xmlHttp.open("POST", url, true);
xmlHttp.onreadystatechange = handleStateChange;
xmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
xmlHttp.send(carAsJSON);

� Read the JSON data as a String type
� Create JSONObject Java object from the string String json = readJSONStringFromRequestBody(request);
//Use the JSON-Java binding library to create a JSON object in Java JSONObject jsonObject = null;
try {
jsonObject = new JSONObject(json);
}
catch(ParseException pe) {
}