How to "fix" an exception?

Question

We can treat an exception thrown by a line of code in order not to let the program break, but how we can fix it?

Scanner sc = new Scanner(System.in);

try {
    String[] vect = new String[1];
    vect[0] = "Maria";

    int posicao = sc.nextInt();
    System.out.println(vect[posicao]);
}
catch(IndexOutOfBoundsException e) {
    System.out.println("Posição inacessível !");
}

If any value is entered in the variable posicao other than 0 will cast the exception of catch, but how can we give the option to type this variable again?

Answer 1

Instead of trying to "fix" the exception after it happens, you could simply prevent it from happening.

In this case, it would be sufficient to test the position by checking whether the value is valid or not. And if it is not, you ask that this be re-typed, until it is valid. For this we can use a while (true):

int posicao;
System.out.println("Digite o valor da posição:");
while (true) {
    posicao = sc.nextInt();
    if (posicao >= 0 && posicao < vect.length) {
        break; // valor da posição válido, pode sair do while
    } else {
        System.out.println("Valor inválido, digite outro valor:");
    }
}

System.out.println(vect[posicao]);

The while(true) is a way to make a loop infinity. Then the value of posicao is read, and if it is greater than or equal to zero and smaller than the size of the array (vect.length) - that is, it has a value that will not launch IndexOutOfBoundsException - I use the break, to get out of while.

If the value is invalid (negative or larger than the size of the array - that is, any value you throw IndexOutOfBoundsException), the while continues running, and another value is requested, until a valid value is entered.

---

Like the if has already ensured that the value of posicao is valid, you can even withdraw the try/catch(IndexOutOfBoundsException).

However, if you want to increment the code, you can capture java.util.InputMismatchException, which is the exception launched by nextInt() if something other than an integer is entered:

int posicao;
System.out.println("Digite o valor da posição:");
while (true) {
    try {
        posicao = sc.nextInt();
        if (posicao >= 0 && posicao < vect.length) {
            break; // valor da posição válido, pode sair do while
        } else {
            System.out.println("Valor inválido, digite outro valor:");
        }
    } catch (InputMismatchException e) {
        sc.nextLine(); // limpar o buffer
        System.out.println("Valor digitado não é um número inteiro, digite outro valor:");
    }
}

System.out.println(vect[posicao]);

Within the try I keep doing the same checks on the value of posicao. If something other than an integer is typed (such as abc xyz or even 1.234), the method nextInt() spear one InputMismatchException.

Within the catch I call nextLine() for "clear" the buffer of Scanner, and then inform the user what happened and ask him to type the value again (and the while continues running until a valid value is entered).

---

You are tempted use the exception to control the program flow, but in this case it is not necessary. It is possible to do the necessary checks and logics without needing it, using only the basic language structures (if, while, etc.).

Except, of course, in the case of InputMismatchException, since this exception is the way the method nextInt() has to indicate that it could not read an integer. But in the case of IndexOutOfBoundsException, doesn’t need the try/catch to do what you need.

Answer 2

To solve your problem just use an Infinite Loop and allow your stop with the Return only when the actions you wish to happen, if there is an exception it will give a Continue and return to the beginning of the loop

while(true) {
    System.out.println("Digite a posição");
    Scanner sc = new Scanner(System.in);

    try {
        String[] vect = new String[1];
        vect[0] = "Maria";

        int posicao = sc.nextInt();
        System.out.println(vect[posicao]);
        return;
    } catch (IndexOutOfBoundsException e) {
        System.out.println("Posição inacessível !");
        continue;
    }
}