我的扫描仪两次要求输入

Question

I seem to have this problem a lot, I can't quite seem to understand how to work scanners

System.out.println("Please enter a number");
Scanner choice1 = new Scanner(System.in);
int choiceH = choice1.nextInt();

while(!choice1.hasNextInt()){
    System.out.println("Please enter a number");
    choice1.next();
}

What I want the code to do is ask for a number, and check if the input is a number. My problem is that it asks fro the number twice and I don't know why.

Answer 1

If this line of code successfully executes:

int choiceH = choice1.nextInt();

Then the user has entered an int and the parsing succeeded. There's no reason to check hasNextInt() again.

If the user didn't enter an int , then nextInt() will throw an InputMismatchException , and you should simply catch it, and prompt the user again.

boolean succeeded = false;
int choiceH = 0;
Scanner choice1 = new Scanner(System.in);

do {
    try {
        System.out.println("Please enter a number");
        choiceH = choice1.nextInt();
        succeeded = true;
    } 
    catch(InputMismatchException e){
        choice1.next();   // User didn't enter a number; read and discard whatever was entered.
    }
} while(!succeeded);

Answer 2

In the line

Scanner choice1 = new Scanner(System.in);

the buffer will be empty. When you reach the line

int choiceH = choice1.nextInt();

you enter a number, and you press Enter. After this, the number will be stored in the buffer and be consumed (the buffer will be empty again). When you reach the line

while (!choice1.hasNextInt())

the program will check if there is an int in the buffer, but at this moment, it will be empty, so hasNextInt will return false . So, the condition will be true and the program will ask for an int again.

How can you solve it? You can delete the first nextInt :

System.out.println("Please enter a number");
Scanner choice1 = new Scanner(System.in);
int choiceH = -1; // some default value

while (!choice1.hasNextInt()) {
    System.out.println("Please enter a number");
    choice1.nextInt();
}