Adding element to array provides error codes

Question

I am having trouble with my code. I am trying to make a prime factor program, and I want to remove duplicates. However, they always give me an error when I run the following code, and I am confused on why:

public static void primeFactors(int n) {
    // Create array to filter out duplicates
    List<Integer> primeFactorList = Arrays.asList();
    // Start timer to calculate how long the program takes
    long startTime= System.nanoTime();

    // Print the number of 2s that divide n
    System.out.print("The prime factor(s) of that number:\n");
    // The following is a while loop running when the number is even
    while (n % 2 == 0) {
        primeFactorList.add(2);
        n /= 2;
    }

    // n is now odd, as the even while loop is over.
    // skip one element (Note i = i +2)

    for (int i = 3; i <= Math.sqrt(n); i += 2) {
        // While i divides n, print i and divide n
        while (n % i == 0) {
            primeFactorList.add(i);
            n /= i;
        }
    }

    // This condition is to handle the case when 'n' is a prime number greater than 2
    if (n > 2) {
        primeFactorList.add(n);
    }
    System.out.print(primeFactorList.stream().distinct().collect(Collectors.toList()));
    float endTime= System.nanoTime();
    float totalTime= endTime -startTime;
    System.out.print("\nThis program took " + (totalTime/1000000 + " milliseconds to run"));
}

public static void main(String[] args)
{
    System.out.println("This is a prime factor calculator. Please enter a number: ");
    Scanner intN = new Scanner(System.in);
    int nValue = intN.nextInt();
    if (nValue >= 0){
        int n = nValue;
        primeFactors(n);
    } else {
        System.out.println("Your value is negative, please restart the program and enter a positive integer");
    }

}

The console errors go as followed:

Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.AbstractList.add(AbstractList.java:148)
at java.util.AbstractList.add(AbstractList.java:108)
at com.amitaltman.firstproject.TestingStuff.primeFactors(TestingStuff.java:32)
at com.amitaltman.firstproject.TestingStuff.main(TestingStuff.java:54)

Arrays.asList(); returns a fixed-size list. You can't add to it. — Federico klez Culloca
– Federico klez Culloca, Commented Oct 1, 2020 at 13:26
Make List<Integer> primeFactorList = Arrays.asList(); be List<Integer> primeFactorList = new ArrayList<>();. That should do it, you're using an empty list in that line anyway. — deHaar
– deHaar, Commented Oct 1, 2020 at 13:30

Som · Accepted Answer · 2020-10-01 13:48:37Z

1

You can not dynamically increase the size for the list in that way.

Replace below :

List<Integer> primeFactorList = Arrays.asList();

With Below :

List<Integer> primeFactorList = new ArrayList<Integer>();

This will make your code work.

Example Output

This is a prime factor calculator. Please enter a number:                                                                                                   
45678                                                                                                                                                       
The prime factor(s) of that number:                                                                                                                         
[2, 3, 23, 331]                                                                                                                                             
This program took 0.0 milliseconds to run

answered Oct 1, 2020 at 13:48

Som

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2 Comments

Federico klez Culloca Over a year ago

"You can not dynamically increase the size for the list in that way." The point is not the way you expand the list size, just the fact that a list created that way cannot be expanded, period.

Som Over a year ago

Yes true. That's what I meant. The list created by the person cannot be exapanded.

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