How to prevent the user to enter numbers for a given data?

Question

How do I make my Java program not accept numbers as given by the user?

I would like, in a field that requires the user name, if a number is inserted, such as a badge, make the program ask again for the name.

I’ve thought of several options like making a "number alphabet", but I don’t know if Java is the best option.

Answer 1

One mode you can use to check whether or not a sequence contains certain characters is through regular expressions, in Java you can use the method String.Matches(), this method returns true if the sequence given as a parameter corresponds to a given regex.

In practice it would be something like this:

public boolean checkLetters(String str) 
{
    return str.matches("[a-zA-Z]+");
}

Example:

public Main()
{
    Scanner sc = new Scanner(System.in);
    System.out.println("Digite o seu nome: ");
    String nome = sc.nextLine().trim();
    if (checkLetters(nome))
    {
        // Fazer alguma coisa aqui.
    }
    else
    {
        System.out.println("Neste campo não é permitido números. Tente Novamente.");
    }
}

Demonstration in Ideone.

Answer 2

You can create a unique method that takes as argument a String to be checked for numbers or not. For example:

public boolean hasNumbers(final String string){
   String numbers = "0123456789";
   for(char a : string.toCharArray())
     for(char b : numbers.toCharArray())
       if(a == b) return true;
   return false;
}

So you can use it in any application, be it Swing, console or web.

---

Console

Scanner input = new Scanner(System.in);
if(hasNumbers(input.nextLine())){
   // tem números, faz algo...
}

Swing

String input = textField.getText(); // pega o valor no jtextfield
if(hasNumbers(input)){
   // tem números, faz algo...
}

---

Test in Ideone.

See also the response from Qmechanic73 using regex.

Answer 3

Assuming you’re using console, you can do something like this:

public static String lerNome(String mensagem, Scanner scanner) {
    while (true) {
        System.out.println(mensagem);
        String lido = scanner.nextLine().trim();
        if (lido.isEmpty()) {
            System.out.println("Desculpe, você não digitou nada. Tente novamente.");
            continue;
        }
        try {
            new BigDecimal(lido);
            System.out.println("Desculpe, mas " + lido + " é um número. Você deveria ter digitado um nome. Tente novamente.");
        } catch (NumberFormatException e) {
            return lido;
        }
    }
}

And then you would use this method so:

Scanner ent = new Scanner(System.in);
String nome = lerNome("Digite o nome.", ent);

This code shows the message Digite o nome. and reads a user-readable line of text. If this line of text is blank or is a number, it gives an error message and does not exit the loop, asking the user to type again. Otherwise (not empty and not number), it accepts and returns the typed text.

Answer 4

Only Put This On Keyreleased Event:

char[] text = txt.getText().toCharArray();

if (txt.getText().length() > 0){

    for (int i = 0; i < text.length; i++){
        if (text[i] >= '0' && text[i] <= '9'){
            txt.setText(txt.getText().replace(String.valueOf(text[i]), ""));
        }
    }
}

Since you use Netbeans it won’t be hard to add it, select the Textbox, go to events in the lower right corner and search for Keyreleased.