MainActivity

package info.androidboss.retrofit.activity;

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.design.widget.BottomNavigationView;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.MenuItem;

import info.androidboss.retrofit.R;
import info.androidboss.retrofit.fragment.OneFragment;
import info.androidboss.retrofit.fragment.ThreeFragment;
import info.androidboss.retrofit.fragment.TwoFragment;


public class MainActivity extends AppCompatActivity implements OneFragment.OnFragmentInteractionListener,
        TwoFragment.OnFragmentInteractionListener, ThreeFragment.OnFragmentInteractionListener{

    private Toolbar toolbar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.frame_layout, OneFragment.newInstance());
        fragmentTransaction.commit();

        toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
    }

    //Change fragments by click nav buttons
    private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
            = new BottomNavigationView.OnNavigationItemSelectedListener() {

        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            Fragment fragment = null;
            switch (item.getItemId()) {
                case R.id.navigation_home:
                    fragment = OneFragment.newInstance();
                    break;
                case R.id.navigation_dashboard:
                    fragment = TwoFragment.newInstance();
                    break;
                case R.id.navigation_notifications:
                    fragment = ThreeFragment.newInstance();
                    break;
            }
            if (fragment != null) {
                FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
                fragmentTransaction.replace(R.id.frame_layout, fragment);
                fragmentTransaction.commit();
            }
            return true;
        }

    };


}