Question

I play around with java.math.BigInteger. Here is my java class,

public class BigIntegerTest{
   public static void main(String[] args) {
     BigInteger fiveThousand = new BigInteger("5000");
     BigInteger fiftyThousand = new BigInteger("50000");
     BigInteger fiveHundredThousand = new BigInteger("500000");
     BigInteger total = BigInteger.ZERO;
     total.add(fiveThousand);
     total.add(fiftyThousand);
     total.add(fiveHundredThousand);
     System.out.println(total);
 }
}

I think the result is 555000. But the actual is 0. Why ?

Answer 1

BigInteger objects are immutable. Their values cannot be changed, once created.

When you call .add a new BigInteger object is created and returned, and must be stored if you want to access its value.

BigInteger total = BigInteger.ZERO;
total = total.add(fiveThousand);
total = total.add(fiftyThousand);
total = total.add(fiveHundredThousand);
System.out.println(total);

(It’s fine to say total = total.add(...) because it’s just removing the reference to the old total object and reassigning it the reference to the new one created by .add).