Student results with custom validation exceptions

import java.util.*;

public class Main {
    private static final Scanner IN = new Scanner(System.in).useLocale(Locale.ROOT);

java.util.* imports utility type names such as Scanner, Locale and collections. Main is the public class matching Main.java. The private static final IN field holds one shared Scanner for System.in; final prevents replacing that reference. The scanner uses Locale.ROOT.

    static String text() {
        if (!IN.hasNextLine()) throw new IllegalArgumentException("Incomplete input.");
        String value = IN.nextLine().trim();
        if (value.isEmpty() || value.length() > 200) throw new IllegalArgumentException("Text must have 1–200 characters.");
        return value;
    }

text checks whether a line exists, reads it and removes leading/trailing whitespace. Reject empty text and lengths above 200 by throwing IllegalArgumentException. Otherwise return the string. One field is read per line, so these helpers do not mix nextInt with leftover newlines.

    static int integer(int low, int high) {
        int value = Integer.parseInt(text());
        if (value < low || value > high) throw new IllegalArgumentException("Integer out of range.");
        return value;
    }

Integer.parseInt converts the returned string to int or throws NumberFormatException, a subtype of IllegalArgumentException. Reject values outside the caller’s inclusive low/high limits, then return the accepted integer.

    static double number(double low, double high) {
        double value = Double.parseDouble(text());
        if (!Double.isFinite(value) || value < low || value > high) throw new IllegalArgumentException("Number out of range.");
        return value;
    }

Double.parseDouble reads the string as a double. Double.isFinite rejects NaN and infinities as well as the explicit low/high range check. Return only a finite accepted value; exceptions transfer control to a matching catch block.

    static void money(String label, double value) { System.out.printf(Locale.ROOT, "%s: %.2f%n", label, value); }

money is a formatting helper: %s prints the label, %.2f prints two decimal places, and %n ends the line. Locale.ROOT fixes locale-sensitive formatting. The helper can also display non-currency quantities; it does not change the stored value.

static class InvalidMarksException extends Exception {
        private static final long serialVersionUID=1L;
        InvalidMarksException(){super("Marks must be finite values from 0 to 100.");}
    }
    static class DivideByZeroException extends Exception {
        private static final long serialVersionUID=1L;
        DivideByZeroException(){super("Subject count cannot be zero.");}
    }

Both custom classes extend Exception and provide specific messages through super. serialVersionUID is a serialization-version identifier. These checked exceptions must be declared or caught; they are distinct from input parsing errors.

    static double[] calculate(int subjects) throws InvalidMarksException,DivideByZeroException {
        if(subjects==0)throw new DivideByZeroException();
        double total=0;

calculate declares both checked exceptions with throws. Rejecting subjects == 0 before computing an average prevents division by zero. total starts at zero.

        for(int i=0;i<subjects;i++){
            double mark=Double.parseDouble(text());
            if(!Double.isFinite(mark)||mark<0||mark>100)throw new InvalidMarksException();total+=mark;
        }

Read the mark as text and parse a double. Reject NaN, infinities and values outside 0–100 with InvalidMarksException. Only accepted values contribute to total.

        return new double[]{total,total/subjects};
    }

The new double array has total at index 0 and total/subjects at index 1. A zero subject count cannot reach this return because it threw earlier.

    public static void main(String[] args){
        try{
            int subjects=integer(0,10);double[] result=calculate(subjects);double average=result[1];
            String grade=average>=90?"A":average>=80?"B":average>=70?"C":average>=60?"D":average>=40?"E":"F";
            money("Total",result[0]);money("Average",average);System.out.println("Grade: "+grade);

main accepts 0–10 subjects so zero can demonstrate the custom exception. After calculate succeeds, read the average, test grade thresholds downward and display the two numeric results.

        }catch(InvalidMarksException|DivideByZeroException e){System.out.println(e.getMessage());}
        catch(IllegalArgumentException e){System.out.println("Invalid input: "+e.getMessage());}
    }
}

The multi-catch prints either custom exception’s message. A separate IllegalArgumentException catch handles bad numeric strings and helper range failures. The final braces close main and Main.