Converting a datetime string type to datetime type in apex

Question

This question is based on what I encountered on another question which goes like this: How do I convert text to datetime?

In our legacy system feed we are getting the following text value. "2016-11-17T10:04:31.000+08:00" I would like to know whether it is possible to convert this text value to a datetime data type. 2016-11-17T18:04:31.000Z

Normally, I would solve it like this:

String oldD = '2016-11-17T10:04:31.000+08:00';
Datetime newD = Datetime.newInstance(
    Integer.valueOf(oldD.subString(0,4)), 
    Integer.valueOf(oldD.subString(5,7)),
    Integer.valueOf(oldD.subString(8,10)),  
    Integer.valueOf(oldD.subString(11,13)),
    Integer.valueOf(oldD.subString(14,16)),
    Integer.valueOf(oldD.subString(17,19))
); //this will be constructed in user timezone

system.debug('newD = '+ newD);

However, one fine gentleman solved it with one line of code! and it was:

DateTime t = (DateTime)json.deserialize('"2016-11-17T10:04:31.000+08:00"', datetime.class);

Of course, this one line rocks!!

Q1. Can anyone explain me how and why this actually worked? The string is not even a JSON string!

Q2. Is this mentioned in the salesforce dev docs?

Q3. If no, then how can I learn such awesome "tricks"?

Thanks in advance.

Answer 1

The answer to the question of whether a single value is valid JSON is probably "it depends on which standard you consider" - see What is the minimum valid JSON?. Apex (at least in the JSON class) considers that it is which is arguably a helpful thing. The documentation even includes an example of a single value.

Internally, the JSON class is probably using a set of type conversion methods to allow it to map string values to specific type fields when a .class argument is supplied. For your example the DateTime converter is getting used.