How do I check if my input is number or text?

Question

I wonder how I can check the kind of value that goes into my Scanner, to return to the user that that entry only receives a number and that he has typed a text or character.

public class WhileChallenge {
    
    public static void main(String[] args) 
    {
        double grades = 0.0;
        double grades_total = 0.0;
        int grades_amount = 0;
        
        Scanner input_value = new Scanner(System.in);
        
        while(grades != -1)
        {
            System.out.println("Insert a grade:");
            grades = Double.parseDouble(input_value.next().replace(",", "."));
            
            if(grades < 0.0 || grades > 10.0)
            {
                System.out.println("Out of range value");
            }else if(grades >= 0.0 && grades <= 10.0)
            {
                grades_total += grades;
                grades_amount++;
            }
            //Se a nota for diferente de um número; Pede pro usuário digitar um número
        }
        
        System.out.println("Average: " + ((grades_total)/grades_amount));
        
        input_value.close();    
    }
    
}

Answer 1

According to the documentation, Double.parseDouble makes an exception (a NumberFormatException) if the string does not have a valid number. Then just capture this exception to see if it went wrong:

Scanner input = new Scanner(System.in);
double gradesTotal = 0;
double gradesAmount = 0;
while (true) {
    double grade;
    System.out.println("Insert a grade:");
    while (true) { // enquanto não digitar um número válido, continua no while
        try {
            grade = Double.parseDouble(input.nextLine().replace(",", "."));
            break; // se chegou aqui é porque o número é válido, então posso sair do while
        } catch (NumberFormatException e) {
            // número inválido
            System.out.println("Digite um número válido");
        }
    }

    // se for -1, sai do while mais externo
    if (grade == -1)
        break;

    if (grade < 0.0 || grade > 10.0) {
        System.out.println("Out of range value");
    } else {
        gradesTotal += grade;
        gradesAmount++;
    }
}

Notice I made one while(true) external (to keep repeating everything, until finding a -1, which is the output condition you have placed), and another more internal one just to read the number (and it keeps asking you to type again, until the number is valid).

See also that in the else do not need to test all conditions again. If the number is between 0 and 10 it will not enter the if, then it came to else is because it is certainly not less than zero and not more than 10, so testing it again is redundant and unnecessary.

I also changed the names of the variables to match the java conventions, who prefers nomesAssim instead of nomes_assim.