JAVA: How to convert String ArrayList to Integer Arraylist?

Question

My question is - how to convert a String ArrayList to an Integer ArrayList?

I have numbers with ° behind them EX: 352°. If I put those into an Integer ArrayList, it won't recognize the numbers. To solve this, I put them into a String ArrayList and then they are recognized.

I want to convert that String Arraylist back to an Integer Arraylist. So how would I achieve that?

This is my code I have so far. I want to convert ArrayString to an Int Arraylist.

        // Read text in txt file.
    Scanner ReadFile = new Scanner(new File("F:\\test.txt"));

    // Creates an arraylist named ArrayString
    ArrayList<String> ArrayString = new ArrayList<String>();

    // This will add the text of the txt file to the arraylist.
    while (ReadFile.hasNextLine()) {

        ArrayString.add(ReadFile.nextLine());
    }

    ReadFile.close();


    // Displays the arraystring.
    System.out.println(ArrayString);

Thanks in advance

Diego

PS: Sorry if I am not completely clear, but English isn't my main language. Also I am pretty new to Java.

Answer 1

You can replace any character you want to ignore (in this case °) using String.replaceAll:

 "somestring°".replaceAll("°",""); // gives "sometring"

Or you could remove the last character using String.substring:

"somestring°".substring(0, "somestring".length() - 1); // gives "somestring"

One of those should work for your case.

Now all that's left is to parse the input on-the-fly using Integer.parseInt:

ArrayList<Integer> arrayInts = new ArrayList<Integer>();

while (ReadFile.hasNextLine()) {
    String input = ReadFile.nextLine();
    try {
        // try and parse a number from the input. Removes trailing `°`
        arrayInts.add(Integer.parseInt(input.replaceAll("°","")));
    } catch (NumberFormatException nfe){
        System.err.println("'" + input + "' is not a number!");
    }
}

You can add your own handling to the case where the input is not an actual number.

Answer 2

For a more lenient parsing process, you might consider using a regular expression.

Note: The following code is using Java 7 features (try-with-resources and diamond operator) to simplify the code while illustrating good coding practices (closing the Scanner). It also uses common naming convention of variables starting with lower-case, but you may of course use any convention you want).

This code is using an inline string instead of a file for two reasons: It shows that data being processed, and it can run as-is for testing.

public static void main(String[] args) {
    String testdata = "55°\r\n" +
                      "bad line with no number\r\n" +
                      "Two numbers: 123   $78\r\n";
    ArrayList<Integer> arrayInt = new ArrayList<>();
    try (Scanner readFile = new Scanner(testdata)) {
        Pattern digitsPattern = Pattern.compile("(\\d+)");
        while (readFile.hasNextLine()) {
            Matcher m = digitsPattern.matcher(readFile.nextLine());
            while (m.find())
                arrayInt.add(Integer.valueOf(m.group(1)));
        }
    }
    System.out.println(arrayInt);
}

This will print:

[55, 123, 78]

Answer 3

You would have to create a new instance of an ArrayList typed with the Integer wrapper class and give it the same size buffer as the String list:

List<Integer> myList = new ArrayList<>(ArrayString.size());

And then iterate through Arraystring assigning the values over from one to the other by using a parsing method in the wrapper class

for (int i = 0; i < ArrayString.size(); i++) {
    myList.add(Integer.parseInt(ArrayString.get(i)));

}