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There is nothing new in Listing 1 so that code shouldn't require further explanation. Note however, that the name "Tom" will become significant in alater discussion.

Instantiate and populate a JSONArray object

The code in Listing 2 instantiates and populates a JSONArray object.

Listing 2 . Instantiate and populate a JSONArray object. JSONArray arrayListA = new JSONArray(); arrayListA.add("2-club");arrayListA.add("3-heart"); arrayListA.add("4-diamond");arrayListA.add("5-spade");

Previous pages in this book have constructed JSON strings using a subclass of the Java HashMap class -- (the JSONObject class) . It is worth noting that JSON strings constructed in that manner are unordered.However, JSON arrays constructed using the JSONArray class, which is a subclass of the ArrayList class, are ordered lists.

The program that I will explain in this page is intended to represent the beginning state of a two-person card game where each player receives four cards.The code in Listing 2 constructs a list of the cards that will be dealt to one of the players.

Populate hashMapA with a key/value pair

The code in Listing 3 populates the JSONObject object referred to as hashMapA with a key/value pair where the key is "cards" and the value is an object of the JSONArray class containing a list of strings naming specific playing cards.

Listing 3 . Populate hashMapA. hashMapA.put("cards",arrayListA);

Note that hashMapA already contained a key/value pair identifying one of the players in the game named "Tom" (see Listing 1 ). Thus the code in Listing 1 through Listing 3 can be though of as "dealing" the cards identified in Listing 2 to the player named "Tom".

Create and populate another similar JSON object

Listing 4 creates and populates a second list of playing cards and deals them to the second player in the game whose name is "Joe".

Listing 4 . Create and populate another similar JSON object. JSONObject hashMapB = new JSONObject(); hashMapB.put("name","Joe");JSONArray arrayListB = new JSONArray();arrayListB.add("4-heart"); arrayListB.add("5-heart");arrayListB.add("6-club"); arrayListB.add("7-diamond");hashMapB.put("cards",arrayListB);

Put the players in the game

Now that the players have been created and have received their cards, it is time to put them in the game.

Listing 5 begins by adding the two players and their card arrays to a new object of type JSONArray . This results in nested arrays.

Listing 5 . Put the players in the game. JSONArray arrayListC = new JSONArray(); arrayListC.add(hashMapA);arrayListC.add(hashMapB); JSONObject hashMapC = new JSONObject();hashMapC.put("game",arrayListC);

Then Listing 5 creates a new JSONObject object to represent the game and populatesit with a key/value pair where the key is "game" and the value is the array containing the two players and their card arrays.

Write the JSON string to an output file

Listing 6 calls the writeJSONString method on the JSONObject object to encode the object into a JSON string and write it to an output filenamed junk.json .

Listing 6 . Write the JSON string to an output file. try{ PrintWriter out =new PrintWriter(new File("junk.json")); hashMapC.writeJSONString(out);out.flush(); }catch(IOException ex){ex.printStackTrace(); }//end catch}//end main }//end class Code

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Source:  OpenStax, Object-oriented programming (oop) with java. OpenStax CNX. Jun 29, 2016 Download for free at https://legacy.cnx.org/content/col11441/1.201
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