每当我输入值时，java scanner总是跳过一行

这是我第一次在这里发帖。我对扫描仪的方法有问题。这是我正在使用的代码

public void loanBook() {
        Scanner input = new Scanner(System.in);
        boolean successful = false;
        do {


            System.out.println(
                    "\nPlease enter the book ID of the book that you wish to take out (Press 9 to exit to the main menu)");

            if (input.nextInt() == 9) {
                successful = true;
                break;
            }
            int bookID = input.nextInt();
            for (int i = 0; i < Books.size(); i++) {
                if (Books.get(i).getBookID() == bookID) {
                    do {
                        System.out.println("\nHow long would you like to loan the book for (20 Days maximum):");
                        int durationOnLoan = input.nextInt();
                        if (durationOnLoan <= 20 && 1 <= durationOnLoan) {
                            Books.get(i).setDurationOnLoan(durationOnLoan);
                            successful = true;
                        } else {
                            System.out.println("The number of days you have entered is invalid");
                        }
                    } while (successful == false);

                    System.out.println("\nThe book " + Books.get(i).getTitle() + " is now on loan");
                    Books.get(i).setOnLoan(true);
                    Books.get(i).setNumOfLoan(Books.get(i).getNumOfLoans() + 1);
                    successful = true;
                }

            }

public void loanBook（）{
扫描仪输入=新扫描仪（System.in）；
布尔成功=假；
做{
System.out.println(
“\n请输入要取出的图书的图书ID（按9键退出主菜单）”；
if（input.nextInt（）==9）{
成功=正确；
打破
}
int bookID=input.nextInt（）；
对于（int i=0；i
if (input.nextInt() == 9) {

你读取了数字，如果不是9，你可以继续。但你也放弃了它。这就是为什么你必须再次输入它。一个简单的解决方案是使用一个变量，这是你已经做过的

int bookID = input.nextInt();
if (bookID == 9) // check the save value so you don't have to enter it twice.
    break;

如果你想知道，我怎么能自己解决这个问题呢？答案是使用你的调试器，因为这应该正好是每行代码正在做的事情