阅读下面代码： import java.awt.*; public class Exam11_1 { private Frame. f; private Button b1,b2,b3,b4; public static void main(String args[] { Exam11_1 that = new Exam11 1 (); that.go(); } public void go() { ______; f.setLayout(new FlowLayout()) ;; b1 = new Button ('Button 1'); b2 = new Button ('Button 2'); b3 = new Button ('Button 3'); b4 = new Button ('Button 4'); f.add (b1); f.add (b2); f.add (b3); f.add (b4); f.pack (); f.setVisible (true); } } 请在画线处填写正确的语句【 】，以便编译运行程序后得到正确的结果。