res/values/strings.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">StudyDemo</string>
    <string name="hello_world">Hello world!</string>
    <string name="action_settings">Settings</string>

</resources>

myText.setText(getResources().getString(R.string.hello_world));

android:text="@string/hello_world"