Why do I need to convert from Integer[] to int[]?

Question

I have the following code

public static int[] readCSV() {
    ArrayList<Integer> entries = new ArrayList<>();
    try {
        File file = new File("someDataFile.csv");
        FileReader fr = new FileReader(file);
        BufferedReader br = new BufferedReader(fr);
        String line = "";
        String[] row;
        while((line = br.readLine()) != null) {
            row = line.split(",");
            for(String value : row) {
                int entry = Integer.parseInt(value);
                entries.add(entry);
            }
        }
        br.close();
    } catch(IOException ioe) {
        ioe.printStackTrace();
    }
    int[] IDs = entries.toArray();
    return IDs;
}

Every entry of the csv is an integer stored as a string. I get the following error: "Type mismatch: cannot convert from Object[] to int[]". As far as I understand, "entries" is not an Object[] here, it's an ArrayList<Integer>.

I was using an example given on geeksforgeeks. That didn't work and I'm not sure why.

I also checked the previous answers to the same question, and the top answer works for me. That said, I still don't have an int[], I only have Integer[]. Then I have to do this to convert from Integer[] to int[]. My question is why do I have to do all that instead of int[] IDs = entries.toArray();?

If I do

int[] IDs = new int[entries.size()];
for (int i=0; i<entries.size(); i++) {
    IDs[i] = entries.get(i);
}

it works fine. Why is that different from int[] IDs = entries.toArray()?

Is there a better way to get the contents of the csv file in an int[]?

My question is why do I have to do all that instead of int[] IDs = entries.toArray();? : Simple answer: Integer and int are not same. See this - in particular the statement : "The Integer class wraps a value of the primitive type int in an object. An object of type Integer contains a single field whose type is int. " — SomeDude
– SomeDude, Commented Jun 18, 2022 at 18:00

Elliott Frisch · Accepted Answer · 2022-06-18 17:52:30Z

1

First, to answer your question, because a collection (like ArrayList) can only contain object instances. That means you must use the Integer wrapper type instead of the int primitive type. However, in Java 8+, there are simple ways to perform that conversion. I would also strongly recommend a try-with-Resources over manually closing the BufferedReader. I also simplified the code a little. Like,

public static int[] readCSV() {
    List<Integer> entries = new ArrayList<>();
    File file = new File("someDataFile.csv");
    try (BufferedReader br = new BufferedReader(new FileReader(file))) {
        String line;
        while ((line = br.readLine()) != null) {
            String[] row = line.split("\\s*,\\s*"); // Consume white space
            for (String value : row) {
                entries.add(Integer.parseInt(value));
            }
        }
    } catch (IOException ioe) {
        ioe.printStackTrace();
    }
    return entries.stream().mapToInt(Integer::intValue).toArray();
}

answered Jun 18, 2022 at 17:52

Elliott Frisch

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2 Comments

DimitrisMel Over a year ago

Thanks for that. Is the stream solution better than the for loop solution? Does it do it faster or with less memory?

Elliott Frisch Over a year ago

@DimitrisMel That number (3692851740) is outside the range of an int. Sounds like you want to long(s). It uses less code. Less code is better than more code. Faster or less memory are not the only metrics. Good luck!

Unmitigated · Accepted Answer · 2022-06-18 17:48:29Z

0

List#toArray always returns an Object[]. The closest you can get is entries.toArray(new Integer[0]) to get an Integer[].

To get an int[] you can use the Streams API or loop over the List and copy it over to an array.

Integer[] arr = list.toArray(new Integer[0]);
int[] arr2 = list.stream().mapToInt(i -> i).toArray();

answered Jun 18, 2022 at 17:48

Unmitigated

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