选择以下代码的输出结果：class A { public String s = "A"; public void setS(String s) { this.s = s; } public String getS() { return this.s; } } public class B extends A { public String s = "B"; public void setS(String s) { this.s = s; } public String getS(){ return this.s; } public static void main(String[] args) { A a = new A(); B b = new B(); a.setS("[AA]"); b.setS("[BB]"); a = b; System.out.print(a.s); System.out.print(b.s); System.out.print(a.getS()); System.out.print(b.getS()); } }