I made this program and it prints one extra thing

问题

Program uses a while loop menu in the main to request for the user command:

public static void main(String[] args)throws Exception
{
    Boolean meow = true;

    while(meow)
    {
        System.out.println("\n  1. Show all records.\n"
            + "  2. Delete the current record.\n"
            + "  3. Change the first name in the current record.\n"
            + "  4. Change the last name in the current record.\n"
            + "  5. Add a new record.\n"
            + "  6. Change the phone number in the current record.\n"
            + "  7. Add a deposit to the current balance in the current record.\n"
            + "  8. Make a withdrawal from the current record if sufficient funds are available.\n"
            + "  9. Select a record from the record list to become the current record.\n"
            + " 10. Quit.\n");
        System.out.println("Enter a command from the list above (q to quit): ");
        answer = scan.nextLine();
        cmd.command(answer);
        if(answer.equalsIgnoreCase("10") || answer.equalsIgnoreCase("q"))
        {
            meow = false;
        }

    }
}

If none of the commands you pick are actually commands on the menu then this happens:

else
        {
            System.out.println("Illegal command");
            System.out.println("Enter a command from the list above (q to quit): ");
            answer = scan.nextLine();
            command(answer);
        }

Whenever I add a new person or use any command that requires me to press return to finish entering a value I get the else statement and then the regular command request.

So it looks like:

Enter a command from the list above (q to quit): 
Illegal command
Enter a command from the list above (q to quit):

When this happens.

Not gonna post my full code on here, I'm afraid of it cause it's so much. Have the pastebins of them instead.

My full Main Class: http://pastebin.com/rUuKtpXb
My full not Main Class: http://pastebin.com/UE4H76Cd

Anyone know why this is happening?

回答1:

The problem is that something like Scanner::nextDouble doesn't read the new-line character, so the next Scanner::nextLine returns an empty line.

Replacing all occurrences of Scanner::nextLine with Scanner::next should fix it.

You can also do a Scanner::nextLine after your last non-nextLine next method, but this is a bit messy.

Some other things I'd recommend:

Add scan.useDelimiter("\n"); at the beginning of your program, test with adding spaces to a line and you'll see why this is needed.

Change println to print, so the command can be entered on the same line. i.e.:

Change

System.out.println("Enter a command from the list above (q to quit): ");`

System.out.print("Enter a command from the list above (q to quit): ");

Change this:

else
{
    System.out.println("Illegal command");
    System.out.println("Enter a command from the list above (q to quit): ");
    answer = scan.nextLine();
    command(answer);
}

to:

else System.out.println("Illegal command");

You would print the menu again, but you would avoid unneeded recursion. It would be easy enough to avoid printing the menu again.

It would be better to check for exit before running command (and then you can remove that check in command).

System.out.println("Enter a command from the list above (q to quit): ");
answer = scan.nextLine();
if (answer.equalsIgnoreCase("10") || answer.equalsIgnoreCase("q"))
    meow = false;
else
    cmd.command(answer);