Get Arraylist file json using java

Question

I have problems with my code, I need to extract the "materias" markup in the arraylist. Example data:

[{
    "name": "A114",
    "grupo": "DAW2",
    "tutor": 15,
    "materias": ["DWES", "DWEC", "IW", "DAW", "IE"]
}]

I tried this, work but i need the content of the arraylist:

try { // this read the JSON file
    line = new String(Files.readAllBytes(Paths.get("fileAulas")));
} catch (Exception ex) {
    ex.printStackTrace();
}
JSONArray recs = new JSONArray(line);
for (Object rec : recs) {

    Aula datos = new Aula();

    JSONObject obj = ((JSONObject) rec);
    String name = obj.getString("name");    
    //datos.setNombre(name);

    String grupo = obj.getString("grupo");
    //datos.setGrupo(grupo);

    int tutor = obj.getInt("tutor");
    //datos.setTutor(tutor);
}

My objetive is to read the arraylist and then datos.setArraylist to my another class. All works fine except read the arraylist

PD: I use java downloaded from Maven, to execute i use "javac -cp ./*: program.java"

What is rec and where does it come from?

TobiSH
– TobiSH

2019-02-04 13:35:31 +00:00
Commented Feb 4, 2019 at 13:35 — TobiSH
– TobiSH, Commented Feb 4, 2019 at 13:35
I update the code

user11012672
– user11012672

2019-02-04 13:37:31 +00:00
Commented Feb 4, 2019 at 13:37 — user11012672
– user11012672, Commented Feb 4, 2019 at 13:37

João Dias Amaro · Accepted Answer · 2019-02-04 13:51:21Z

1

materias is also an array, so you need something like:

JSONArray materias = obj.getJsonArray("materias");
List<String> materiasList = materias.getValuesAs(String.class);

This is not type-safe so you need to make sure the array will only have String values.

Also, if you are using Java 8, you can iterate over the values and store them manually (JsonArray is also a Collection<JsonValue>)

edited Feb 4, 2019 at 13:51

answered Feb 4, 2019 at 13:41

João Dias Amaro

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2 Comments

user11012672 Over a year ago

sorry, but how i can read (or see) the content of that array? ¡thanks for advance!

João Dias Amaro Over a year ago

Hi @UsuarioConMiga just edited, you can iterate directly over a JsonArray. It is a Java Collection :)

lhaidacher · Accepted Answer · 2019-02-04 13:54:32Z

Quickly created short test implementation of your needs:

    String json = "[{ \"name\":\"A114\",\"grupo\": \"DAW2\",\"tutor\":15,\"materias\": [\"DWES\",\"DWEC\",\"IW\",\"DAW\",\"IE\"]}]";

    JSONArray objectArray = new JSONArray(json);

    for (int x = 0; x < objectArray.length(); x++) {
        JSONObject obj = objectArray.getJSONObject(x);

        System.out.println("Name: " + obj.get("name"));
        System.out.println("Grupo: " + obj.get("grupo"));
        System.out.println("Tutor: " + obj.get("tutor"));

        JSONArray materias = obj.getJSONArray("materias");
        for (int y = 0; y < materias.length(); y++) {
            System.out.println("Materia " + y + ": " + materias.get(y));
        }
    }

Output

Name: A114 Grupo: DAW2 Tutor: 15 Materia 0: DWES Materia 1: DWEC Materia 2: IW Materia 3: DAW Materia 4: IE

Bakar · Accepted Answer · 2019-02-04 13:50:49Z

First gotta make your JSON Valid.

[{
        "name": "A114",
        "grupo": "DAW2",
        "tutor": 15,
        "materias": ["DWES", "DWEC", "IW", "DAW", "IE"]
    }]

Secondly, you can try the following :

for (JsonElement rec : recs) {

    Aula datos = new Aula();

    JSONObject obj = rec.getAsJsonObject();
    String name = obj.get("name").getAsString();    
    //datos.setNombre(name);

    String grupo = obj.get("grupo").getAsString();
    //datos.setGrupo(grupo);

    int tutor = obj.get("tutor").getAsInt();

     JsonObject obj = rec.getAsJsonObject();
      JsonArray materias = obj.getAsJsonArray("materias");

          List<String> materiasList = new ArrayList<>();

for (JsonElement item:
                 materias) {
                materiasList.add(item.getAsString());
            }

}

ladyhawk · Accepted Answer · 2019-02-04 14:17:12Z

I believe that the syntax for an array in json is different that what you have in your file. For example - a simple complete json that contains an array of simple elements would be:

    {"contacts":[
      { "firstName":"John", "lastName":"Doe" },
      { "firstName":"Jen", "lastName":"Smith" },
      { "firstName":"David", "lastName":"Jones" }
    ]}

In your text, you specify an array, without the opening and closing JSON '{' and '}'. Also, You would need to name the array element ("contacts" in the example above). I think that what you want would be (I used "arrayName" for the array element name):

    {"arrayName":[{ "name":"A114","grupo": "DAW2","tutor":15,"materias":["DWES","DWEC","IW","DAW","IE"] }, 
      { "name":"121","grupo": "DAW1","tutor":8,"materias":["PROGRA","BD","SIS","LM","FOL"] }, 
      { "name":"A112","grupo": "AUTO1","tutor":5 }, 
      { "name":"A127","grupo": "ASIR1","tutor":2 }, 
      { "name":"A114","grupo": "SMR2","tutor":11 }, 
      { "name":"A128","grupo": "ASIR2","tutor":9,"materias":["IAW","ABD","ASIS","SI","IE"] }, 
      { "name":"A125","grupo": "ADM2","tutor":12 } ] }

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