How to find specified name and its value in JSON-string from Java?

Let's assume we have the next JSON
string:

{ 

            "name" : "John",
 "age" : "20",
 "address" : "some
            address",
 "someobject" : {
 "field" : "value" 

            }
}

What
is the easiest (but still correct, i.e. regular expressions are not acceptable) way to
find field age and its value (or determine that there's no
field with given name)?

p.s. any open-source
libs are ok.

p.s.2: please, don't post links to
the libraries - it's not a useful answer. 'Show me the code'(c).