为什么出者句话Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException

我照书抄的程序如下：
public class aa{
public static void main (String arg[]){
int i;
int a[]=new int[5];
for(i=0;i<=5;i++)
a[i]=i;
for(i=a.length-1;i>=0;i--)
System.out.println("a["+i+"]="+a[i]);
}
}
然后编译没报错运行却出现了：Exception in thread "main"java.lang.ArrayIndexOutOfBoundsException at ArrDemo.main(ArrDemo.java:7)
编译与运行如下：
C:\j2sdk1.4.0>javac ArrDemo.java

C:\j2sdk1.4.0>java ArrDemo
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException
at ArrDemo.main(ArrDemo.java:7)
为什么啊？
附：我运行//My first Application program in Java
public class jj{
public static void main(String args[]){
System.out.println("hahaha");
}
}这个程序没报错