Why am I getting strange output?

Question

This program finds prime factors and prints them out like this: factor (order) factor (order)

import java.util.Scanner; import java.util.ArrayList; public class Factorise2 { public static ArrayList<Integer> get_prime_factors(int number) { //Get the absolute value so that the algorithm works for negative numbers int absoluteNumber = Math.abs(number); ArrayList<Integer> primefactors = new ArrayList<Integer>(); ArrayList<Integer> newprimes = new ArrayList<Integer>(); int b; int c = (int)Math.sqrt(absoluteNumber); //Get the square root so that we can break earlier if it's prime for (int j = 2; j <= absoluteNumber;) { //Test for divisibility by j, and add to the list of prime factors if it's divisible. if (absoluteNumber % j == 0) { primefactors.add(j); absoluteNumber /= j; c = (int)Math.sqrt(absoluteNumber); } else { for (int a = 0; a < newprimes.size();) { //Change j to the next prime b = newprimes.get(a); if (j % b == 0) { j++; a = 0; } else { a++; } } newprimes.add(j); } if (j > c) { primefactors.add(absoluteNumber); break; } } return primefactors; } public static void main(String[] args) { //Declare and initialise variables int number; int count = 1; Scanner scan = new Scanner(System.in); //Get a number to work with System.out.println("Enter integer to analyse:"); number = scan.nextInt(); //Get the prime factors of the number ArrayList<Integer> primefactors = get_prime_factors(number); //Group the factors together and display them on the screen System.out.print("Prime factors of " + number + " are "); primefactors.add(0); for (int a = 0; a < primefactors.size() - 1; a++) { if (primefactors.get(a) == primefactors.get(a+1)) { count++; } else { System.out.print(primefactors.get(a) + " (" + count + ") "); count = 1; } } } }

It works for the most part, but when I input 131033809, I get a strange output:

C:\Users\Eamon>java Factorise2 Enter integer to analyse: 4 Prime factors of 4 are 2 (2) C:\Users\Eamon>java Factorise2 Enter integer to analyse: 128 Prime factors of 128 are 2 (7) C:\Users\Eamon>java Factorise2 Enter integer to analyse: 9 Prime factors of 9 are 3 (2) C:\Users\Eamon>java Factorise2 Enter integer to analyse: 121 Prime factors of 121 are 11 (2) C:\Users\Eamon>java Factorise2 Enter integer to analyse: 131033809 Prime factors of 131033809 are 11447 (1) 11447 (1)

Why doesn't it write

11447 (2)

You seriously believe that there is even the slightest chance that this is a bug in Java rather than in your understanding of how Java works? Really?? — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Nov 17, 2012 at 19:56
Did you even look at the code? There's not much places it could have gone wrong on my end. I flagged you for being unconstructive. — Eamon Moloney
– Eamon Moloney, Commented Nov 17, 2012 at 20:01
Your hubris though needs to be addressed as it will prevent you from solving this. The odds of your finding a true Java bug given your current coding level are astronomically small, so small that you really must look at your question along the lines of "exactly what about Java am I not understanding". You may think of this as being unconstructive, but how you look at a problem matters and will effect how you solve it. You need to do some debugging of your code either with a debugger or with println statements because you must accept that the bug is yours until strongly proven otherwise. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Nov 17, 2012 at 20:06
That's why I asked a question. I asked "Is there a bug in Java?", I wasn't saying there was one. Instead of answering my question you went and took the piss out of it. — Eamon Moloney
– Eamon Moloney, Commented Nov 17, 2012 at 20:19

jma127 · Accepted Answer · 2012-11-17 20:02:18Z

11

In Java, the Integer datatype is an object. Thus, == operations will only return true if the object identities are the same: note, this does not concern whether the values are actually equal. You should use:

if (primefactors.get(a).equals(primefactors.get(a+1)))

answered Nov 17, 2012 at 20:02

jma127

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2 Comments

Daniel Fischer Over a year ago

You should mention the cache for Integers of small absolute value - by default -128 to 127 - as the reason why == worked for small prime factors.

Sbodd Over a year ago

Read the linked answer - basically, Java guarantees that for small values of Integer, etc, == always works; but there's no such guarantee for larger ones.

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