MainActivity

package antonmatveyenka.testapp;

import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
        TestFragment fragment = new TestFragment();
        fragment.setMainActivity(this);
        ft.replace(R.id.container, fragment);
        ft.commit();
    }

    public void replaceFragment() {
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
        ft.replace(R.id.container, new TestFragmentSecond());
        ft.commit();
    }
}