I add items to Arraylist but when I use it it is empty

Question

I cannot add items to my Arraylist

Main class, where the array is instantiated

public class act_principal extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener{

    private List<Medico> medicos = new ArrayList<Medico>();

Method where items should be added to the Array

public void setArrayMedico()
    {
        JsonArrayRequest jsonArrayRequest = new JsonArrayRequest(Request.Method.POST, json_url_getMedicos, (String)null, new Response.Listener<JSONArray>() {
            @Override
            public void onResponse(JSONArray response) {
                int codigo = 0;
                String nome="";

                for(int i=0; i<response.length(); i++)
                {
                    try {
                        JSONObject jsonObject = response.getJSONObject(i);

                        codigo = jsonObject.getInt("Codigo");
                        nome = jsonObject.getString("Nome");

                        medicos.add(new Medico(codigo, nome));

                    } catch (JSONException e) {
                        e.printStackTrace();
                    }
                }
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {

            }
        });

        MySingleton.getInstance(this).addToRequest(jsonArrayRequest);
    }

When debugging, it normally inserts the items into the Array, but when it accesses the Array after this function, it is with size() == 0, i.e., empty. Where is my mistake?

From now on, thank you

Answer 1

Note that it is not the method setArrayMedico() who fills in the list but the method onResponse(), class Jsonarrayrequest.

Put another way: when the method setArrayMedico() is called it returns immediately before the list is filled in.

If the code you’re using is something like this:

setArrayMedico();
boolean isEmpty = medicos.isEmpty();

isEmpty will be true.

A possible solution will be:

• Do not declare Arraylist as an Activity attribute, this will ensure that it is not used unfilled.

• Declare a method that will be called when the list is filled:

  private void onMedicosIsReady(List<Medico> medicos){
  
      //Utilize aqui a lista
  }
  

• In the method onResponse() call this method after the list is filled:

  @Override
  public void onResponse(JSONArray response) {
      int codigo = 0;
      String nome="";
  
      //Declaração do ArrayList
      List<Medico> medicos = new ArrayList<Medico>();
  
      for(int i=0; i<response.length(); i++)
      {
          try {
              JSONObject jsonObject = response.getJSONObject(i);
  
              codigo = jsonObject.getInt("Codigo");
              nome = jsonObject.getString("Nome");
  
              medicos.add(new Medico(codigo, nome));
  
          } catch (JSONException e) {
              e.printStackTrace();
          }
      }
      //Chama o método quando estiver preenchido
      onMedicosIsReady(medicos);
  }