TeacherActivity.java

package com.darshanbshah.odsystem;

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.Toast;

import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.List;


public class TeacherActivity extends AppCompatActivity {

    class ODTable {
        String flag, reason, from, to, fullday;

        public ODTable(String flag, String reason, String from, String to, String fullday) {
            this.flag = flag;
            this.reason = reason;
            this.from = from;
            this.to = to;
            this.fullday = fullday;
        }

    }
    ODTable table;
    int i = 0;
    AlertDialog.Builder builder;
    DataProvider itemValue;
    FirebaseAuth mAuth;
    FirebaseDatabase database = FirebaseDatabase.getInstance();

    DatabaseReference root;
    DatabaseReference adv;
    DatabaseReference student;
    DatabaseReference od, od_flag, od_student;
    String message = "";
    List<String> uid_list = new ArrayList<String>();
    HashMap<String, String> uid_map = new HashMap<String, String>();
    HashMap<String, String> student_map = new HashMap<String, String>();
    Hashtable<String, ODTable> odtable = new Hashtable<String, ODTable>();
    ListView listView;
    CustomListAdapter adapter;
    String adv_name, key, value, uid = "";
    List<String> roll = new ArrayList<String>();
    List<String> random = new ArrayList<String>();
    List<DatabaseReference> od_student_list = new ArrayList<DatabaseReference>();

    String flag = "", reason = "", from = "", to = "", fullday = "";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_teacher);
        mAuth = FirebaseAuth.getInstance();