我的do-while循环似乎没有执行我的if语句

Question

I'm new to java, so sorry if my programming skills are poor. My program needs to use a queue to simulate an I/O buffer. It continually accepts lines of user input. If the user inserts an O(Oh) it returns the first line entered. If an X is entered it needs to end.

This is my program:

public static void main(String[] args) {

   Queue<String> queue = new LinkedList<String>();

    Scanner input = new Scanner(System.in);
    String check = input.nextLine();

do {

  queue.add(check);

  if(check.equals("O")) {

     if(queue.peek() != null){
        String remove = queue.remove();
        System.out.println("Data: " + remove);
        }

     else if(queue.peek() == null) {
        System.out.println("Buffer empty");         
        }
  } 

 } while(!"X".equals(check)) ;

The expected input-output is:

line1
line2
O
Data: line1
line3
line4
O
Data: line2
O
Data: line3
line5
O
Data: line4
O
Data: line5
O
Buffer empty 
X   

My program doesn't seem to execute my if statement, as it never stops asking for input and doesn't exit once and X is given by the user.

I'm sorry if I'm missing something obvious.

Answer 1

The issue is you need to read the input again at the end, after the if condition, you need to add the line:

check = input.nextLine();

this way you read the next inputline, otherwise check will always be the same thing which is the first thing entered.

Answer 2

First of all,

String check = input.nextLine();

is called only once whereas it should be called again at the end of the loop. So it is not updated.

Answer 3

Your input.nextLine() is outside of the loop so it's only called once, when adding input to your queue is called on every loop iteration, that's why it's happening. Change it this way:

String check;
do {
  check = input.nextLine();
  queue.add(check);