android.support.v7.widget.AppCompatTextView cannot be cast to android.widget.RelativeLayout

All you need to do is Assign Id to Relative Layout it should be like

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"android:id="@+id/rl_Container"android:layout_width="match_parent"android:layout_height="match_parent"><TextView    android:id="@+id/content"    android:layout_width="match_parent"    android:layout_height="match_parent"></TextView></RelativeLayout>

then just make a slight modification to your activity

public class DisplayMessageActivity extends AppCompatActivity {@Overrideprotected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_display_message);    Intent intent = getIntent();    String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);    TextView textView = new TextView(this);    textView.setTextSize(40);    textView.setText(message);    RelativeLayout layout = (RelativeLayout) findViewById(R.id.rl_Container);    layout.addView(textView);}}

RelativeLayout layout = (RelativeLayout) findViewById(R.id.content);

causing the problem.Because You are defining RelativeLayout with id of TextView.

Assign

android:id="@+id/content"

to RelativeLayout. Not to TextView in XML

Change your layout to this, the id content should go to the RelativeLayout and not the TextView.

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"xmlns:tools="http://schemas.android.com/tools"android:id="@+id/content"android:layout_width="match_parent"android:layout_height="match_parent"tools:context="com.example.flover.hellloworld.DisplayMessageActivity">   <TextView   android:layout_width="wrap_content"   android:layout_height="wrap_content"/></RelativeLayout>