Error String Index Out Of Range

I write a code but it show a error message

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String ind
ex out of range: 10
at java.lang.String.charAt(String.java:658)
at StringChar.main(StringChar.java:11)

Code :

class StringChar{
 public static void main(String ss[]){
String str="HelloWorld";
int a;
System.out.println("String is = " + str);
a=str.length();
System.out.println("String is After Reverse");
for(int i=a;i>=0;i--)
System.out.print(str.charAt(i));
}
}