How to call a class via a Menu

I have the following class below where the side menu calls are, but what I want to do, if I implement all the menus in this class the project will get too large, and I wanted each menu to call a different class , for example:

public boolean onNavigationItemSelected(MenuItem item) {
// Handle navigation view item clicks here.
int id = item.getItemId();

if (id == R.id.nav_markets) {
    new DownloadJsonAsyncTask().execute("https://bittrex.com/api/v1.1/public/getmarkets");
}

I tried to implement something like this, but the program closes when I click on the menu. I would like an example as I could do.

package br.com.escconsultoria.bittrex.view;

import android.app.AlertDialog;
import android.app.ProgressDialog;
import android.graphics.Color;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.NavigationView;
import android.support.design.widget.Snackbar;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle; 
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.LinearLayout;
import android.widget.ListView;
import android.widget.TableLayout;
import android.widget.TableRow;
import android.widget.TextView;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import br.com.escconsultoria.bittrex.R;

public class BitTrexActivity extends AppCompatActivity implements 
NavigationView.OnNavigationItemSelectedListener {

String data = "";
TableLayout tl;
TableRow tr;
TextView label;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_bit_trex);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);

    //TODO
    new DownloadJsonAsyncTask().execute("https://bittrex.com/api/v1.1/public/getmarkets");
}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.bit_trex, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

/*@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();

    if (id == R.id.nav_markets) {
        new DownloadJsonAsyncTask().execute("https://bittrex.com/api/v1.1/public/getmarkets");
    } /*else if (id == R.id.nav_gallery) {

    } else if (id == R.id.nav_slideshow) {

    } else if (id == R.id.nav_manage) {

    } else if (id == R.id.nav_share) {

    } else if (id == R.id.nav_send) {

    }*/

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

/*@Override
protected void onListItemClick(ListView l, View v, int position, long id) {
    super.onListItemClick(l, v, position, id);

    Trend trend = (Trend) l.getAdapter().getItem(position);

    Intent it = new Intent(Intent.ACTION_VIEW, Uri.parse(trend.url));
    startActivity(it);
}*/

class DownloadJsonAsyncTask extends AsyncTask<String, Void, List<Trend>> {

    ProgressDialog dialog;

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        dialog = ProgressDialog.show(BitTrexActivity.this, "Aguarde","Baixando JSON, Por Favor Aguarde...");
    }

    @Override
    protected List<Trend> doInBackground(String... params) {
        String urlString = params[0];

        HttpClient httpclient = new DefaultHttpClient();
        HttpGet httpget = new HttpGet(urlString);

        try {
            HttpResponse response = httpclient.execute(httpget);

            HttpEntity entity = response.getEntity();

            if (entity != null) {
                InputStream instream = entity.getContent();

                String json = toString(instream);
                instream.close();

                List<Trend> trends = getTrends(json);

                return trends;
            }
        } catch (Exception e) {
            Log.e("DEVMEDIA", "Falha ao acessar Web service", e);
        }
        return null;
    }

    private List<Trend> getTrends(String jsonString) {

        List<Trend> trends = new ArrayList<Trend>();

        try {
            JSONObject trendLists = new JSONObject(jsonString);
            //JSONObject trendList = trendLists.getgetJSONObject(0);
            JSONArray trendsArray = trendLists.getJSONArray("result");

            JSONObject trend;

            for (int i = 0; i < trendsArray.length(); i++) {
                trend = new JSONObject(trendsArray.getString(i));

                Log.i("DEVMEDIA", "nome=" + trend.getString("MarketCurrency"));

                Trend objetoTrend = new Trend();
                objetoTrend.name = trend.getString("MarketCurrency");
                objetoTrend.url = trend.getString("BaseCurrency");

                trends.add(objetoTrend);
            }
        } catch (JSONException e) {
            Log.e("DEVMEDIA", "Erro no parsing do JSON", e);
        }

        return trends;
    }

    @Override
    protected void onPostExecute(List<Trend> result) {
        super.onPostExecute(result);
        dialog.dismiss();

        Log.e("DEVMEDIA", "Passou No onPostExecute");

        //parse json data
        try {
            //JSONArray jArray = new JSONArray(result);
            TableLayout tv = (TableLayout) findViewById(R.id.tableLayout);
            tv.removeAllViewsInLayout();
            int flag = 1;

            Log.e("DEVMEDIA", result.toString());
            for (int i = -1; i < result.size() - 1; i++) {
                TableRow tr = new TableRow(BitTrexActivity.this);
                tr.setLayoutParams(new TableRow.LayoutParams(
                        TableRow.LayoutParams.FILL_PARENT,
                        TableRow.LayoutParams.WRAP_CONTENT));
                if (flag == 1) {
                    TextView b6 = new TextView(BitTrexActivity.this);
                    b6.setText("Id");
                    b6.setTextColor(Color.BLUE);
                    b6.setTextSize(15);
                    tr.addView(b6);
                    TextView b19 = new TextView(BitTrexActivity.this);
                    b19.setPadding(10, 0, 0, 0);
                    b19.setTextSize(15);
                    b19.setText("Name");
                    b19.setTextColor(Color.BLUE);
                    tr.addView(b19);
                    TextView b29 = new TextView(BitTrexActivity.this);
                    b29.setPadding(10, 0, 0, 0);
                    b29.setText("Status");
                    b29.setTextColor(Color.BLUE);
                    b29.setTextSize(15);
                    tr.addView(b29);
                    tv.addView(tr);
                    final View vline = new View(BitTrexActivity.this);
                    vline.setLayoutParams(new TableRow.LayoutParams(TableRow.LayoutParams.FILL_PARENT, 2));
                    vline.setBackgroundColor(Color.BLUE);
                    tv.addView(vline);
                    flag = 0;
                    Log.e("DEVMEDIA", "flag: " + flag);
                } else {
                    Trend trend = (Trend) result.get(i);
                    TextView b = new TextView(BitTrexActivity.this);
                    String stime = trend.name;
                    Log.e("DEVMEDIA", "Passou POR AQUI ");
                    b.setText(stime);
                    b.setTextColor(Color.RED);
                    b.setTextSize(15);
                    tr.addView(b);
                    TextView b1 = new TextView(BitTrexActivity.this);
                    b1.setPadding(10, 0, 0, 0);
                    b1.setTextSize(15);
                    String stime1 = trend.url;
                    b1.setText(stime1);
                    b1.setTextColor(Color.BLACK);
                    tr.addView(b1);
                    TextView b2 = new TextView(BitTrexActivity.this);
                    b2.setPadding(10, 0, 0, 0);
                    String stime2 = trend.name;
                    b2.setText(stime2);
                    b2.setTextColor(Color.BLACK);
                    b2.setTextSize(15);
                    tr.addView(b2);
                    tv.addView(tr);
                    final View vline1 = new View(BitTrexActivity.this);
                    vline1.setLayoutParams(new TableRow.LayoutParams(TableRow.LayoutParams.FILL_PARENT, 1));
                    vline1.setBackgroundColor(Color.WHITE);
                    tv.addView(vline1);
                }
            }
        } catch (Exception e) {
        }
        /*if (result.size() > 0) {
            ArrayAdapter<Trend> adapter = new ArrayAdapter<Trend>(
                    BitTrexActivity.this,
                    android.R.layout.simple_list_item_1, result);
            tableLayout.set.setAdapter(adapter);

        } else {
            AlertDialog.Builder builder = new AlertDialog.Builder(
                    BitTrexActivity.this).setTitle("Atenção")
                    .setMessage("Não foi possivel acessar essas informções...")
                    .setPositiveButton("OK", null);
            builder.create().show();
        }*/
    }

    private String toString(InputStream is) throws IOException {

        byte[] bytes = new byte[1024];
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        int lidos;
        while ((lidos = is.read(bytes)) > 0) {
            baos.write(bytes, 0, lidos);
        }
        return new String(baos.toByteArray());
    }
}*/
}

class Trend {
    String name;
    String url;

    @Override
    public String toString() {
        return name;
    }
}

Error:

FATAL EXCEPTION: main
Process: br.com.escconsultoria.bittrex, PID: 24623
android.content.ActivityNotFoundException: Unable to find explicit activity class {br.com.escconsultoria.bittrex/br.com.escconsultoria.bittrex.Download.DownloadGetMarkers}; have you declared this activity in your AndroidManifest.xml?
   at android.app.Instrumentation.checkStartActivityResult(Instrumentation.java:1723)
   at android.app.Instrumentation.execStartActivity(Instrumentation.java:1511)
   at android.app.Activity.startActivityForResult(Activity.java:3473)
   at android.support.v4.app.BaseFragmentActivityJB.startActivityForResult(BaseFragmentActivityJB.java:54)
   at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:75)
   at android.app.Activity.startActivityForResult(Activity.java:3434)
   at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:708)
   at android.app.Activity.startActivity(Activity.java:3676)
   at android.app.Activity.startActivity(Activity.java:3644)
   at br.com.escconsultoria.bittrex.view.BitTrexActivity.onNavigationItemSelected(BitTrexActivity.java:89)
   at android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:156)
   at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:821)
   at android.support.v7.view.menu.SubMenuBuilder.dispatchMenuItemSelected(SubMenuBuilder.java:88)
   at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:158)
   at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:968)
   at android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:342)
   at android.view.View.performClick(View.java:4560)
   at android.view.View$PerformClick.run(View.java:18636)
   at android.os.Handler.handleCallback(Handler.java:733)
   at android.os.Handler.dispatchMessage(Handler.java:95)
   at android.os.Looper.loop(Looper.java:136)
   at android.app.ActivityThread.main(ActivityThread.java:5021)
   at java.lang.reflect.Method.invokeNative(Native Method)
   at java.lang.reflect.Method.invoke(Method.java:515)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:827)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:643)
   at dalvik.system.NativeStart.main(Native Method)