如果突破内部if，为什么我的消息打印两次？

Question

I am having a little problem with my code. Compiling and running works well, however, when I attempt to break out of the inner loop,

System.out.println("Type which category you want to add to.");
System.out.println("Homework, Classwork, Labs, Test, Quizzes, Midterm, Final");

The code above is printing twice to the terminal when I only want it to print once.

I have a feeling that is a simple mistake with the way my brackets are aligned but I am having difficulty with figuring out how to do it. Any help would be greatly appreciated!

import java.util.Scanner;

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

        Scanner input = new Scanner(System.in);

        final int MAX = 15;

        int[] homework = new int[MAX];
        int[] classwork = new int[MAX];
        int[] lab = new int[MAX];
        int[] test = new int[MAX];
        int[] quizzes = new int[MAX];
        int[] midterm = new int[MAX];
        int[] fin = new int[MAX];
        int hwCount, clCount, labCount, testCount, quizCount, midCount, finCount;
        double hwTotal, clTotal, labTotal, testTotal, quizTotal, midTotal, finTotal;
        double grade = 0;

        String selection = "";

        System.out.println();
        System.out.println("Welcome to GetGrade!");
        System.out.println();

        while (true) {

            System.out.println("Type which category you want to add to.");
            System.out.println("Homework, Classwork, Labs, Test, Quizzes, Midterm, Final");
            selection = input.nextLine();

            if (selection.equals("homework")) {

                System.out.print("What percentange of your grade is homework? > ");
                double hwPercent = input.nextDouble();
                System.out.println("Now begin typing your grades. When you are finished, type -1.");

                for (int i = 0; i < homework.length; i++) {
                    homework[i] = input.nextInt();
                    hwTotal = homework[i] * hwPercent;
                    grade += hwTotal;
                    if (homework[i] == -1) break;
                }
            }
        }
    }
}

Answer 1

It's just as trivial as it seems:
The call to input.nextInt(); in your inner loop does not include the newline. So you are breaking of the innerloop, receiving the next line which only contains the newline - character in input.nextLine(); which is the remaining input of your "-1\\n" line and proceed with the main loop again as it does not match "homework".

Answer 2

Try setting the conditional variable in your while loop to an actual boolean rather than true.

Also, when you invoke "break", you are only breaking out of the for loop. If you reassign a boolean variable to false at this point, you would exit the while loop completely.

Answer 3

Just before while loop ends, add a "Do you want to continue? (Y/N)" functionality.

If user enters "N" or anything else, execute another break. And that break will make you get out of the while loop.

Answer 4

The simple way to get your code working is to change

selection = input.nextLine();

to

selection = input.next();

next() only reads in a string value (which is what you are actually doing in your code) instead of the newline character as Peter has suggested.

So the an extra iteration of the while does not take place when you read the newline character.

Answer 5

When you use a scanner to read a line from the keyboard, it reads everything up to and including the newline character the user types to submit their input. So for example:

Type which category you want to add to.
Homework, Classwork, Labs, Test, Quizzes, Midterm, Final
>

If you type "homework" and then ENTER, the actual input becomes "homework\\n". input.nextLine() will scan the input until it encounters the first newline character, '\\n', which it will consume and then it returns everything up to that point (ie "homework").

Your problem here is that input.nextInt() does NOT consume a newline character, and so there is still a newline character in the input buffer by the time your while loop starts another round.

Now begin typing your grades. When you are finished, type -1.
> ...
> -1
      => User input is "-1\n"
-------------------------------

// Meanwhile, back in the code...
for (int i=0;i<homework.length;i++) {
   homework[i] = input.nextInt();      // <--- This call consumes "-1" but leaves '\n'
   hwTotal = homework[i] * hwPercent;
   grade += hwTotal;
   if (homework[i] == -1) break; 
}

That newline is consumed by the next call to input.nextLine() , leaving the input buffer empty.

while (true) {
    System.out.println("Type which category you want to add to.");
    System.out.println("Homework, Classwork, Labs, Test, Quizzes, Midterm, Final");
    selection = input.nextLine();    // <--- This call consumes the leftover '\n' and returns the empty string
    ...

And because "" is not equal to "homework", the while loop goes around one more time, but this time the input buffer is empty, and so the call to input.nextLine() behaves as you would expect.

// selection is empty, so this condition fails and the program loops back around
if (selection.equals("homework")) {
    ...

There are two easy solutions to this problem. You can

• Use Integer.parseInt(input.nextLine()) instead of input.nextInt()
• Add an extra call to input.nextLine() at the end of your while loop to consume the final newline character

The first option is probably the most robust, and you get the added benefit of a run-time error being thrown if they do not give you a valid integer as input.