Java Review 12 Exception 问题？

package cn.com.ch12;public class ClassCastExceptionTest {public static void main(String[] args) {Book book=new NovelBook();Object o=book;String s=(String)o;System.out.println(s);}}class Book{private String title;private String content;private String author;}class NovelBook extends Book{private  String type="友谊";public String toString(){return "主题："+type;}}

Console

Exception in thread "main" java.lang.ClassCastException: cn.com.ch09.NovelBook cannot be cast to java.lang.String
at cn.com.ch09.ClassCastExceptionTest.main(ClassCastExceptionTest.java:7)