使用try-catch块的while循环在循环一次后不会等待try块中的输入

Question

So I've got a method getInput, that is meant to, well, ask user for input, use a try-catch to make sure it's a correct data type(int in this case) and validate whether it's in a predefined range. Problem is, when I enter, say, a string, it repeats the message about incorrect input, prints the message I have asking for input in try {} and goes straight to catch without waiting for input. Here's the code:

public static int getInput(String message, int lowerLimit, int upperLimit) {
    while (true) {
        int input;
        try {
            System.out.println(message);
            input = scanner.nextInt();
            if (lowerLimit > input && input > upperLimit) {
                return input;
            } else {
                System.out.println("Incorrect input value, try again");
            }
        } catch (Exception e) {
            System.out.println("Incorrect input, try again");
        }
    }
}

I know there are other, working ways of accomplishing this, but I'd like to know why this one doesn't work - I think it should. Any ideas?

Answer 1

if (lowerLimit > input && input > upperLimit)

This is ALWAYS false . You meant

if (input >= lowerlimit && input <= upperLimit)
    return input;

Answer 2

The behaviour you're observing stems from the fact that scanner (assuming you're working with java.util.Scanner ) only advances if nextInt() is successful. That means that you have to skip over the faulty input (ie by using next() ) before proceding.

Answer 3

Assuming you are using Scanner class, you could check, whether token has int or not

 if(scanner.hasNextInt()){
  int input = scanner.nextInt();
     if (lowerLimit > input && input < upperLimit) {
     return input;
      } 
   }
  else 
   {
      System.out.println("Incorrect input value, try again");
    }