java.lang.ArrayIndexOutOfBoundsException：2> = 2

Question

I'm brand new to Java and my first assignment was to implement a "for" loop. I wrote this program in C++ and it compiles in Java, but I got an error at runtime. Can anyone tell me what's wrong?

import java.util.Scanner;
import java.util.Vector;

public class GlobalMembersMain
{

    public static Vector<Integer> get_prime_factors(int number)
    {

        Vector<Integer> primefactors = new Vector<Integer>();
        for (int j = 2; j <= number; j++)
        {
            if (number % j == 0)
            {
                primefactors.add(j);
                number = number / j;
                j = 1;
            }
        }
        return primefactors;
    }

    public static void main(String[] args)
    {
        int number;
        int count = 1;
        System.out.print("Enter integer to analyse:");
        System.out.print("\n");
        Scanner scan = new Scanner(System.in);
        number = scan.nextInt();
        Vector<Integer> primefactors = new Vector<Integer>();
        primefactors = get_prime_factors(number);
        System.out.print("Prime factors are ");
        for (int a = 0; a < primefactors.size() + 1; a++)
        {
            if (primefactors.elementAt(a) == primefactors.elementAt(a+1))
            {
                count++;
            }
            else
            {
                System.out.print(primefactors.elementAt(a));
                System.out.print(" (");
                System.out.print(count);
                System.out.print(") ");
                count = 1;
            }
        }
        System.out.print("\n");
    }
}

The output:

Enter integer to analyse:
10
Prime factors are 2 (1) Exception in thread "main" java.lang.ArrayIndexOutOfBoun
dsException: 2 >= 2
        at java.util.Vector.elementAt(Unknown Source)
        at GlobalMembersMain.main(GlobalMembersMain.java:36)

Answer 1

    for (int a = 0; a < primefactors.size() + 1; a++)
    {
        if (primefactors.elementAt(a) == primefactors.elementAt(a+1))
        {
            count++;
        }

Is exceeding the size of the primefactors collection. By 2, in fact.

Change to primefactors.size() - 1 to avoid this error.

Answer 2

Arrays are zero based, which I imagine you are aware of. What you may not be aware of is that in Java a List is backed by an array as well. When you invoke primefactors.size() +1 you are getting one more than you would possibly want. For instance is pf is of size 1 your loop will do the following:

pf.get(0);  //returns the only value in the list  
pf.get(1); // element doesn't exist

Now the other thing is you do not want to use Vector , generally speaking in Java. It is a synchronized collection. What you want is List/ArrayList.

OTHER CODE ISSUES

public static Vector<Integer> get_prime_factors(int number)

this does not need to be static. Also naming convention is camel case in Java so your function name should be getPrimeFactors(int number)

GlobalMembersMain

Should most likely be named GlobalMember as classes are to be singular in nature and I believe you added Main to indicate it was the class that holds the main function.

In your main function you would do this:

GlobalMember member = new GlobalMember();
member.getPrimeFactors(number);

Answer 3

This is where the problem is:

for (int a = 0; a < primefactors.size() + 1; a++)
    {
        if (primefactors.elementAt(a) == primefactors.elementAt(a+1))
        {
            count++;
       }

//...

primefactors.elementAt(a+1) for the last element in your collection will throw the exception (AIOB).

Answer 4

Remember that arrays, lists and vectors in Java are zero-based. In your case, the primefactors vector will consist of two elements, available at index 0 and 1 respectively.

The problem you are facing is that you try to access the element primefactors.elementAt(2) which does not exist.

One problem is the break condition in the loop:

for (int a = 0; a < primefactors.size() + 1; a++) { 
    // ... 
}

The first time, a will be 0, the second time 1 which are both fine. However, the loop will not break the third time, because a will equal 2, which is less than primefactors.size() + 1 . Consequently, there will be a call to primefactors.elementAt(2) which does not exist and the program will blow up.

There is also a second problem inside the loop since you increment the loop variable by one during the comparison:

if (primefactors.elementAt(a) == primefactors.elementAt(a+1)) { 
    // ... 
}

Yet again, your program will fail if you pass 2 as an argument to primefactors.elementAt(...)