Notes

private void getData(String url){
final ProgressDialog progress = new ProgressDialog(MainActivity.this);
progress.setMessage("Loading..."); // Setting Message
//progress.setTitle("ProgressDialog"); // Setting Title
progress.setProgressStyle(ProgressDialog.STYLE_SPINNER); // Progress Dialog Style Spinner
progress.show(); // Display Progress Dialog
progress.setCancelable(false);

StringRequest stringRequest = new StringRequest(Request.Method.POST, url,
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
// Extrak JSON
try {
progress.dismiss();

categories.clear();

JSONObject jsonRootObject = new JSONObject(response);
JSONArray jsonArray = jsonRootObject.optJSONArray("result");

for(int i=0; i<jsonArray.length(); i++){
JSONObject jsonObject = jsonArray.getJSONObject(i);
String vexana = jsonObject.optString("model").trim();

categories.add(vexana);

}
}catch (Exception e){
Toast.makeText(MainActivity.this, e.getMessage(), Toast.LENGTH_SHORT).show();
}
}
},
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Toast.makeText(MainActivity.this, "The server unreachable", Toast.LENGTH_SHORT).show();
// close prosgress bar
progress.dismiss();
}
})
{
@Override
protected Map<String, String> getParams() throws AuthFailureError {
Map<String, String> params = new HashMap<>();
//Adding parameters to request
params.clear();
//returning parameter
return params;
}
};

//Adding the string request to the queue
Volley.newRequestQueue(MainActivity.this).add(stringRequest);
}