如果用户输入了无效字符，我如何使我的程序向用户输出消息？

Question

If the user enters the wrong character such as "E" or "A" in my program how can I output a message to the user? The line which I'm speaking about in my program is when the user has to choose to input one out of three characters P, M or D. However, if the user inputs another character which isn't listed I want the program to display a message.

As you can see if the user inputs "E" the program skips to the next step. However, I want the user to be notified and asked again to enter the correct value which is "P, M or D" .

In order to do what I'm trying to achieve I believe I need to do an if statement so here what I have tried.

if((i = P,M,D) && (i < ?)) {
   System.out.println("ERROR, please input only P,M or D.");
}

Screenshot of the program

JAVA CODE

String grade;
int yr2cred[] = new int[18];
char yr2grade[] = new char[18];
for (int i = 1; i <= 18; i++) {
    System.out.println("Please enter your grade for unit " + i +" "+(username) + ".");
    grade = userinput.next();
    yr2grade[i - 1] = grade.charAt(0);
    if (yr2grade[i - 1] == 'P' || yr2grade[i - 1] == 'p') {
        yr2cred[i - 1] = yr2cred[i - 1] + P;
    }
    if (yr2grade[i - 1] == 'M' || yr2grade[i - 1] == 'm') {
        yr2cred[i - 1] = yr2cred[i - 1] + M;
    }
    if (yr2grade[i - 1] == 'D' || yr2grade[i - 1] == 'd') {
        yr2cred[i - 1] = yr2cred[i - 1] + D;
    }
}

Answer 1

Possible code is here. We iterate on units, for each unit we ask to enter the grade value until valid one will be entered. At the end the list of grades is printed.

import java.util.Arrays;
import java.util.Scanner;

public class CheckInput {
    public static void main(String[] args) {

        int unitsCount = 18;
        String[] unitGrades = new String[unitsCount];

        String grade;

        Scanner userInput = new Scanner(System.in);

        for (int i = 0; i < unitsCount; i++) {
            grade = "";
            while ( ! (grade.equalsIgnoreCase("P") || grade.equalsIgnoreCase("M") || grade.equalsIgnoreCase("D") ) ) {
                if (!"".equals(grade)){
                    System.out.println("ERROR, please input only P,M or D.");
                }
                System.out.print("Please enter your grade for unit " + i + ": ");
                grade = userInput.next();
            }
            unitGrades[i] = grade;
        }
        System.out.println("Unit grades: "+Arrays.toString(unitGrades));
    }
}

Answer 2

You can do this using a while loop within the for loop:

for (int i = 1; i <= 18; i++) {
    while (grade.equals("")) {
        System.out.println("Please enter your grade for unit " + i + " " + username + ".");
        grade = userinput.next().toLowerCase();
        yr2grade[i - 1] = grade.charAt(0);
        if (yr2grade[i - 1] == 'p') {
            yr2cred[i - 1] = yr2cred[i - 1] + P;
        } else if (yr2grade[i - 1] == 'm') {
            yr2cred[i - 1] = yr2cred[i - 1] + M;
        } else if (yr2grade[i - 1] == 'd') {
            yr2cred[i - 1] = yr2cred[i - 1] + D;
        } else {
            System.err.println("Invalid Grade Supplied! Try Again.\n");
            grade = "";
        }
    }
    grade = "";
}