Question1_1 in java,the book's code is different from the code on github

[SmileEye]

int the book,the code is:
if (str.length() > 26) {
return false;
}
it's contains only the lowercase a to z characters.

but on github,the code is:

if (str.length() > 128) {
return false;
}

it's contains all ASCII.

public static boolean isUniqueChars(String str) {
if (str.length() > 128) {
return false;
}
int checker = 0;
for (int i = 0; i < str.length(); i++) {
int val = str.charAt(i) - 'a';
System.out.println(val);
if ((checker & (1 << val)) > 0) return false;
checker |= (1 << val);
}
return true;
}