OP solved it, closing

In operators:
if (a[i] != b[j]) {
difference.add(a[i]);
You output element a[i] as many times as it does not match the elements b. And it must be equal to 1 time, and then only if it does not coincide with any of the elements b.

When your result looks like the expected one, but still fails, it is helpful to print your string between delimiters and escape whitespace characters.

For your situation:

String s = sb.toString();
System.out.print('"');
for (int i = 0; i < s.length(); i++) {
    char ch = s.charAt(i);
    System.out.print(ch == '\n' ? "\\n" : Character.toString(ch));
}
System.out.print('"');

for n = 1, this prints:

"*\n \n"

while the expected answer is:

"*\n"