Determine if the student has passing grades
Reduce code repetition / redundant checks
Your if statements are redundant. If grade = 90, it already satisfies the grade &gt;= 80, so just use that. No need to check if it equals 90 exactly because that's already covered.
Instead of having four different lines where you show a message dialog, have a string that you assign the message to and print that at the end? Something like this:
String message;
if (grades &gt;= 80) {
    message = grades + &quot;: Passed!&quot;;
} else if (grades &gt;= 75) {
    message = grades + &quot;: Academic Warning!&quot;;
} else {
    message = grades + &quot;: Failed!&quot;;
}
JOptionPage.showMessageDialog(null, message);

Reduce unused code
You created a Scanner in all of your functions, but never use them. I'd remove them for code clarity.