Java – setText with onClick button and onCreate crashes the application

How do I change

my text field when the app starts, and how do I change it again when I click the button?

MainActivity.java

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

TextView myText=(TextView)findViewById(R.id.myActivityTextView);
    myText.setText("Launch Text Change");        
}

public void myButtonclicked(View v){
    TextView myText=(TextView)findViewById(R.id.myActivityTextView);
    myText.setText("Button Change Text");
}

activity_main.xml

<TextView android:text="@string/hello_world" android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/myText" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Change Text"
    android:id="@+id/myButton"
    android:layout_below="@+id/myText"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:onClick="myButtonclicked"/>

The ID of your TextView in xml is myText. Change your

lookup for

TextView myText=(TextView)findViewById(R.id.myActivityTextView); 

to

TextView myText=(TextView)findViewById(R.id.myText);

It should work.