Java: Why does my code automatically and infinitely loop when an exception is entered instead of wai-CodePudding

I am trying to make a loop that prompts the user for a positive integer so it can be saved into int x.

If a negative or non-integer is entered it will prompt the user again.

It seems to work when a negative value is entered, but not when an exception is entered.

When an exception is entered it just endlessly prints "Please enter a positive integer." without allowing input.

What am I doing wrong? Please help guide me.

Thanks in advance.

int x = 0;

boolean firstLoop = false;
        
while (firstLoop == false)
{
    firstLoop = true;

    try
    {
        x = in.nextInt();
    }
            
    catch (InputMismatchException e)
    {
        System.out.println("Please enter a positive integer.");
        firstLoop = false;
    }
            
    if (x < 0)
    {
        System.out.println("Please enter a positive integer.");
        firstLoop = false;
    }    
}

CodePudding user response：

If you type in an illegal character that can't be parsed to an int, you will get an InputMismatchException. Since firstloop is set to false, the loop continues. But the illegal character is still in the buffer so it continues to repeat. So do something like in.nextLine(); in the first catch block to start over and clear the bad character including the newline.

CodePudding user response：

You miss in.hasNextInt()/in.hasNextLine()/in.nextLine().

If you can do without console in the IDE, better use java.io.Console instead of Scanner on System.in.

You then need to convert yourself: int Integer.parseInt(String) but it is much cleaner, and allows better prompts.

Console con = System.console();
int x = 0;
for (;;) {
    String input = con.readLine("Please enter a positive integer <= %d: ",
            Integer.MAX_VALUE);
    if (input == null) { // User ended console.
        break;
    }
    if (input.matches("\\d ")) { // 1 or more digits.
        x = Integer.parseInt(input);
        break;
    } else {
        con.printf("This '%s' is not a positive integer.%n", input);
    }
}