Force user to enter a valid number using the Java scanner

Question

I have a simple code that asks for the age of a user. Everything is working as it should, except, if I type nothing and simply press enter I enter into something - maybe infinite loop? I am not sure but I cannot get out of it.

I have seen countless questions and responses for this topic when dealing with strings but barely any with byte (or int).

This is my code:

    //App #2: Age

    System.out.print("Enter your Age: ");

    byte age = scanner.nextByte();

    while (!scanner.hasNext()) {
        System.out.println("Please enter a valid number.");
        byte test = scanner.nextByte();
    }

    if (age > 16) {
        applicant.age = age;
        System.out.println(age);
    }
    else
        System.out.println("You must be at least 17 years old to obtain a microloan.");

What can I do to force a user to enter a valid number?

Mureinik · Accepted Answer · 2021-04-22 08:45:38Z

3

Your loop needs to continue as long as additional input is available (i.e., scanner.hasNext()) but it isn't a valid byte (i.e., !scanner.hashNextByte()). You should also remember to consume (next()) any invalid string so the next inputed string can be evaluated:

while (scanner.hasNext() && !scanner.hasNextByte()) {
    System.out.println("Please enter a valid number.");
    scanner.next(); // Discard the invalid string
}

// Now we have a valid byte, read it:
byte age = scanner.nextByte();

answered Apr 22, 2021 at 8:45

Mureinik

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1 Comment

Frank Jimenez Over a year ago

Got ya. That makes sense. Thanks!

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