Question

I’m new to Java and just code a program with BigInteger.

public static void main(String[] args) {
  BigInteger n = new BigInteger("5");
  BigInteger i = new BigInteger("2");
  while (lesserOrEqual(i,n) {
    System.out.println("n.mod(i) = "+n.mod(i));
    if (n.mod(i) == ZERO) {
      n = n.divide(i);
    }
    else {
      i.add(ONE);
  }
  System.out.println("n = "+n);
  System.out.println("i = "+i);
}


public static boolean lesserOrEqual(BigInteger m, BigInteger n) `{
  if (m.compareTo(n) == -1 || m.compareTo(n) == 0)
    return true;
  return false;
}

ZERO and ONE are defined of the type BigInteger 0, 1, respectively.

I want “i=2” to divide “n=5”, if “n mod i == 0”, else “i++”, until “n” to be lesser or equal to “i”.

I think the output must be

n.mod(i) = 1
n = 5
i = 3
n.mod(i) = 2
n = 5
i = 4
n.mod(i) = 1
n = 5
i = 5
n.mod(i) = 0
n = 1
i = 5

and with the equivalent code with primitive type int, I have the result as expected.

but this with BigInteger goes to infinite loop

n.mod(i) = 1
n = 5
i = 2
...

Does anyone know why it is so?

Answer 1

The BigInteger class represents integers as immutable objects.

There are two points here.

1. Don’t use == to test if two BigIntegers are equal
2. To change the value of a BigInteger variable you must do i = i.add(ONE); and not just i.add(ONE);.

The reason not to use == to compare BigIntegers is because instead of checking for numerical equality you are checking that they are the same object in memory.

Consider with Strings.

String a = "a";
String b = "b";

String x = a + b;
String y = "ab";

In the above example x.equals(y) is true because they contain the same number of characters in exactly the same order. However, x == y is not true because they are different objects in memory.

The reason you need to to assign the result of arithmetic operations to a variable is because BigInteger is immutable. Thus arithmetic operations cannot change the value of the object it is operating on, but it can create a new BigInteger (which it returns). Which is why you must assign the result of the arithmetic operation to the variable you want it saved in.

As an aside a shortcut for your lesserThanOrEqual to is this.

boolean result = m.compareTo(n) <= 0;

Basically

• m == n becomes m.compareTo(n) == 0
• m != n becomes m.compareTo(n) != 0
• m < n becomes m.compareTo(n) < 0
• m > n becomes m.compareTo(n) > 0
• m <= n becomes m.compareTo(n) <= 0
• m >= n becomes m.compareTo(n) >= 0