This is really out of scope for JSON libraries, since JSON format itself has no support or notion of variable substitution. Your best bet may be to use a JSON library (like Jackson) to get a tree representation (for Jackson that would be JsonNode), then traverse it, and use another library for handling textual substitution. There are many that can do that, from stringtemplate to others (perhaps MessageFormat that other answer refers to).

It may also be possible to revert the other, if your substitutions will never funny "funny characters" (quotes, linefeeds); if so, you could use string templating lib first, JSON parser next feeding processed text. But it is bit riskier, as usually there is eventually one case where you do end up trying to add a quote, say, and then parsing fails.

If you are struggling to arrange the "'s the correct syntax would be

String jsonRequestString = "{\"access_code\" : \""+code+"\" , ";

Instead of formatting Json string manually, which takes alot of effort, consider using a library or util.

For ex (going to use Jackson library) :

Request re = new Request();
re.setCode(code);
...
ObjectMapper mapper = new ObjectMapper();
String jsonStr = mapper.writeValueAsString(re);

You cannot replace a value as such. Therefore, you need to remove it from the json and add it back.

JSONObject js = new JSONObject(); // Your jsonobject here, this is just a sample
js.remove("id"); // Since you want to replace the value associated with id, remove it
js.put("id", "HELLO"); // add the new value for id

String.format

String str = String.format("Action %s occured on object %s.",
   objectA.getAction(), objectB);

Or

String str = String.format("Action %s occured on object %s with outcome %s.",
   new Object[]{objectA.getAction(), objectB, outcome});

You can also use numeric positions, for example to switch the parameters around:

String str = String.format("Action %2$s occured on object %1$s.",
   objectA.getAction(), objectB);

You can always use String.format(....). i.e.,

String string = String.format("A String %s %2d", aStringVar, anIntVar);

I'm not sure if that is attractive enough for you, but it can be quite handy. The syntax is the same as for printf and java.util.Formatter. I've used it much especially if I want to show tabular numeric data.

That link you provided is an excellent source since matching using patterns is the way to go. The basic idea here is first get the tokens using a matcher. After this you will have Operators and Operands

Then, do the replacement individually on each Operand.

Finally, put them back together using the Operators.

So I'm assuming that you have a json object called jsonObject:

var url = "http://example.com/";
jsonObject.playlist[0] = url;

On the other hand, if you're talking about construction the json object, then you just put the variable into the right position:

var url = "http://example.com/";
var jsonObject = {playlist: [ 
    url, 
    { 
        // our song 
        url: 'another URL goes here'
    }
  ]  
}

Note that there's no problem with url being used as a variable in our list, while also being used as a key in the object that comes right after it.