confusion regarding mitidimentional array please help

[shashank pratap]

class Dims{
public static void main(String[] args){
int [][] a={{1,2},{3,4}};
int[] b=(int[])a[1];
Object o1=a;
int [][] a2=(int[][])o1;
System.out.println(b[1]);
}}

the answer to this q is given is " an exception is thrown at runtime" page number 278 Kathy & Bert (310-065)
but when i compile this program on java5 compiler this showing me the output
please help m,e

[M K Rayapudi]

in the text book, the following line of code is also there

int[] b2 = (int[]) o1;

check once again the text book