Static class

package com.thieumao.learnfirst;

import android.app.DatePickerDialog;
import android.app.Dialog;
import android.app.DialogFragment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import java.util.Calendar;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        DatePickerFragment m = new DatePickerFragment();
        DatePickerFragment m2 = new DatePickerFragment();{}

        Mao.thuMao();
        Mao.a = 2;
    }

    public static class Mao {
        public static int a = 2;
        public static void thuMao() {}

    }

    public static class DatePickerFragment extends DialogFragment {

        public int a = 0;

        @Override
        public Dialog onCreateDialog(Bundle savedInstanceState) {
            final Calendar c = Calendar.getInstance();
            int year = c.get(Calendar.YEAR);
            int month = c.get(Calendar.MONTH);
            int day = c.get(Calendar.DAY_OF_MONTH);

            return new DatePickerDialog(getActivity(),
                    (DatePickerDialog.OnDateSetListener)
                            getActivity(), year, month, day);
        }

    }
}