Object类与上下转型

package prepare1;
class Book{
	
}
public class Test {

	public static void main(String[] args) {
		Object obj = new Object();
		Object obj1 = new Book();//向上转型
		Object obj2 = "字符串";//向上转型
		
		Book bk = (Book)obj;//向下转型
		String str = (String)obj2;//向下转型

	}

}