Why can not a button object be created as Button bt = "abc"

Question

Hi.......

Ques: I have  create a String object as below  String str = "abc" & String str = new String("abc")  Why can not a button object be created as  Button bt = "abc"? Why is it compulsory to create a button object as Button bt = new Button("abc")? Why this is not compulsory in String's case? please explain...