
android:text="Button" doesn’t work.
I should make a special string for it in the values/strings.xml and change the android:text="Button" to android:text="@string/button" for button having the value "Button".
What’s the problem it used to work before?
Those are not error only warning, there is tool in every ide called lint which helps programmer to write code in standard format of that language. In android standard practice is to use string.xml for storing all of your string value that is used in xml, so lint shows a warning that you should not directly set label hard-coded rather add them in string resource then set the resource id.
And there is some advantage of using string values like
etc
You can find more information here —
String Resources