当输入为数字时，如何检查Scanner（System.in）？

Question

[我注意到扫描仪中的next()方法“从该扫描仪中查找并返回下一个完整令牌。”然后我使用它，如下所示：

    public static void main(String[] args) {
        System.out.println(menu);
        Scanner scanner = new Scanner(System.in);


        String command = scanner.next();
        System.out.printf("command is \"%s\".", command);
        System.out.println(command == "5");
        while (command != "5") {

            switch (command) {
                case "1":       // update the database

                    System.out.println("Enter first name: ");
                    String firstName = scanner.next();
                    System.out.println("Enter last name: ");
                    String lastName = scanner.next();
                    System.out.println("Enter email address: ");
                    String emailAddress = scanner.next();
                    System.out.println("Is paying member (T or F): ");
                    String isPayingMember = scanner.next();
                    System.out.println("Is staff member (T or F): ");
                    String isStaff = scanner.next();
                    System.out.println("Is problem contributor (T or F): ");
                    String isProblemContributor = scanner.next();
                    System.out.println("Enter subscription start date (as MMDDYY, e.g., 120219):");
                    String startDate = scanner.next();
                    if (isFormatCorrect(firstName, lastName, emailAddress, isPayingMember, isStaff,         
                                        isProblemContributor, startDate)) {
                        update();
                    }
                    break;

                default:
                    System.out.println(warning);
                    System.out.println(menu);
                    command = scanner.next();
                    break;


            }

        }
        scanner.close();
        System.out.println("Exiting...");
        System.out.println("Done");

并且我得到了如下所示的输出：

1) Enter member information
2) List the member(s) who have paid more than $400
3) Add a problem into a specified problem pool
4) List the problems and problem pools that each member has
5) Quit
Enter 1-4 or 5 to quit:
5
command is "5".false
Warning: Invalid input, please try again!
1) Enter member information
2) List the member(s) who have paid more than $400
3) Add a problem into a specified problem pool
4) List the problems and problem pools that each member has
5) Quit
Enter 1-4 or 5 to quit:

它说字符串command不等于“ 5”，这真让我感到困惑。但是，在使用nextInt()的Scanner方法后，它可以正常工作。是什么原因呢？

我正在MacOS 10.15.1和Java（TM）SE Runtime Environment（内部版本1.8.0_221-b11）下工作

Answer 1

如果要比较Java中的字符串，则必须使用equals方法。

while (command.equals("5")) {
  ...
}

Answer 2

您的输入是字符串，而不是整数。然后，您不能使用==运算符比较2个字符串，必须使用equals方法。

[command.equals("5")而不是command == "5"

有关Java中字符串比较的更多信息，请参见this post。>>

基本上，==运算符比较2个字符串的引用，等于比较值。您要比较这些值。