将ArrayList中的String转换为int时出现Java错误

Question

I am very new at Java. I am trying to write a lottery simulator. I have the following code:

Scanner input = new Scanner(System.in);
System.out.println("Enter six whole numbers between 1 and 49 in a comma-separated list: ");
String userNumbersString = input.nextLine();
String[] userNumbersArray = userNumbersString.replaceAll("\\s+","").split(",");
ArrayList userNumbers = new ArrayList(Arrays.asList(userNumbersArray));
boolean validNumbers = true;
for(int v = 0; v <= 6; v++){
    if((int)userNumbers.get(v) > 49){
        validNumbers = false;
            v = 7;
        }else{
            v++;
        }
    }
if(validNumbers == false){
    String[] str = new String[0];
    main(str);
}

It compiles properly. However, when I input a series of numbers, I get this error:

java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer.

I am very confused about why this string can't be cast to an integer.

Sorry if this is a long-winded question, but I would hugely appreciate any help.

I have just realised that the OutOfBounds exception was due to a stupid maths mistake on my part; v was starting at 0; there were five values in the array, but I gave the for loop the condition <= 6. Sorry about that.

Thank you everybody for all your help. Together, you have resolved my question.

Answer 1

In simple terms

int is a primitive type for representing a 32-bit integer number. String is a reference type wrapped arround an array of char s to represent a sequence of characters.

You cannot just change the type from int to String and make the character sequence in the string magically becomes a number.

Think about how you would implement this conversion. You would look at each character, make sure it is a valid digit etc.

Change:

if((int)userNumbers.get(v) > 49)

To:

if(Integer.parseInt(userNumbers.get(v)) > 49)

Further, when creating Generic type containers like ArrayList make sure you also declare what type of objects they store for type safety reasons.

Change:

ArrayList userNumbers = new ArrayList(Arrays.asList(userNumbersArray));

To:

ArrayList<String> userNumbers = new ArrayList<String>(Arrays.asList(userNumbersArray));

This way you calling userNumber.get() will return a String . If you don't put this it will return an Object so you would actually call Integer.parseInt(Object) which is invalid. You would get a pretty self-explanatory error looking like this:

The method parseInt(String) in the type Integer is not applicable for the arguments (Object)

EDIT:

For your IndexOutOfBounds error, you are trying to access more elements than you have. Basically, you want to have 6 values but your for loop says from 0 until 6 (included) . That is a total of 7 elements: 0, 1, 2, 3, 4, 5, 6

You can fix this by changing the <= 6 to < 6 in your loop but the best way to fix it so it will work no matter how many elements you have is to use the size() property of ArrayList :

Change:

for(int v = 0; v <= 6; v++)

To:

for(int v = 0; v < userNumbers.size(); v++)

Answer 2

First, you should use generics for ArrayList . Please replace

ArrayList userNumbers = new ArrayList(Arrays.asList(userNumbersArray));

with

ArrayList<String> userNumbers = new ArrayList<>(Arrays.asList(userNumbersArray));

However, this is not your problem. The misconception here is that casting will infer what type of conversion you need; it won't . Replace (int)userNumbers.get(v) with Integer.parseInt(userNumbers.get(v)) to parse the int from a String .

Answer 3

Type casting is not meant to parse an Integervalue from an String.

You should use Integer.parseInt for that.

if(Integer.parseInt(userNumbers.get(v)) > 49

should solve your error.

Answer 4

First, you will get an ArrayIndexOutOfBounds exception because of the <= 6 in your for loop.

Secondly, you should use the Integer.parseInt(String intStr) method to turn each number into its int equivalent.

Here is a stripped down version that I think works:

public static void main(String[] args)
{
    String testInputOne = "14,22,18,2,5,11";

    boolean allNumbersAreValid = true;

    String[] userNumbersStrArray = testInputOne.split(",");

    for (String numberStr: userNumbersStrArray)
    {
        Integer number = Integer.parseInt(numberStr.trim());

        if (number > 49)
        {
            allNumbersAreValid = false;
            break;
        }
    }

    System.out.println("Testing " + testInputOne + ": " + (allNumbersAreValid?"All valid!":"At least one invalid"));
}

If you want to further guard against bad input such as someone entering an alpha character, you could wrap the Integer.parseInt call in a try catch block and catch the NumberFormatException and output a suitable error message.

I'm frankly a little scared as to what the recursive call to main is supposed to do? :)