BigInteger addition always 0

问题

I have the following issue: when trying to add to a sum of BigIntegers the outcome remains 0.

Here is the code:

 public void NumberOfOutcomes(int x, int y){
   BigInteger first = BigInteger.valueOf(0);
   BigInteger second = BigInteger.valueOf(0);
   for(int i = 0; i <= (x / 2); i++){
       first.add( fac(x - i).divide((fac(x - 2*i).multiply(fac(i)))) );
       System.out.println("First " + first.add( fac(x - i).divide((fac(x - 2*i).multiply(fac(i)))) ));
    }

   for(int i = 0; i <= (y / 2); i++){
       second.add( fac(y - i).divide((fac(y - 2*i).multiply(fac(i)))) );
       System.out.println("Second " + second.add( fac(y - i).divide((fac(y - 2*i).multiply(fac(i)))) ));
    } 
   System.out.println("First " + first);
   System.out.println("Second " + second);
   System.out.println(first.multiply(second));
   }

Here fac is the factorial function.

Here is what comes on the terminal:

points1.NumberOfOutcomes(2, 3)
First 1
First 1
Second 1
Second 2
First 0
Second 0
0

回答1:

This is because BigInteger is immutable which means that its value does not change. So first.add(x) will create a new BigInteger containing the computations result, i.e. just reassign the result to first, like first = first.add(...).

回答2:

The add method of BigInteger class returns the sum of the operand and the value already stored in the the object itself. But it does not store the result in the object (first or second). It doesn't work in the same way as

first += value;

actually, you have to make it look like this:

first = first.add(value);

来源：https://stackoverflow.com/questions/30097093/biginteger-addition-always-0

标签

java

add

biginteger

factorial