how do i prompt a user to re enter their input value, if the value is invalid?

Question

public int inputNumber() { 
    Scanner input = new Scanner (System.in);
    System.out.print("Enter the number of cookies you'd like to make ");
    int number = input.nextInt();
    if (number <=0) {
       System.out.println(" please enter a valid number")
       int number = input.nextInt(); 
    }
    input.close();
    return number;
}

EDIT:

I should have used a while loop.. throwback to literally my first project.

Answer 1

System.out.print("Enter the number of cookies you'd like to make:");
int number = input.nextInt();

while(number<=0) //As long as number is zero or less, repeat prompting
{
    System.out.println("Please enter a valid number:");
    number = input.nextInt();    
}

This is about data validation. It can be done with a do-while loop or a while loop. You can read up on topics of using loops.

Remarks on your codes: You shouldn't declare number twice. That is doing int number more than once in your above codes (which is within same scope).

Answer 2

This way you have fewer superfluous print statements and you don't need to declare you variable multiple times.

public int inputNumber() {

    Scanner input = new Scanner (System.in);
    int number = 0;

    do {
        System.out.print("Enter the number of cookies you'd like to make ");
        number = input.nextInt();
    } while(number <= 0);

    input.close();
    return number;
}