Is it possible to multiple a BigInteger by a Double in Java?

Question

I have a very large number ( number1 ) stored as a BigInteger , and a double (number2). I plan to multiply number1 and number2 , and store the result as a double.

Using the multiply() method has not helped me achieve this. What is the way forward?

Answer 1

In order to preserve the arbitrary precision as long as possible, do the multiplication in BigDecimal , and then convert the result to double , like this:

BigDecimal tmp = new BigDecimal(myBigInteger);
tmp = tmp.multiply(new BigDecimal(myDouble));
double res = tmp.doubleValue();

Answer 2

The simplest solution is probably big.doubleValue() * myDouble .

This won't be particularly fast, unfortunately, since BigInteger.doubleValue() has a notably slow implementation. (It might be faster in the future...perhaps if Oracle applies my patch .)

Alternately, you can round a double directly to a BigInteger using Guava's DoubleMath.roundToBigInteger(double, RoundingMode) .

Answer 3

在BigInteger上调用.doubleValue() ，并将它们乘以双精度数。

Answer 4

Why is BigInteger#multiply() not helpful? There are really only two reasonable answers:

BigInteger a = /* whatever */;
double b = /* whatever */

// either
double result = a.multiply(new BigInteger(b)).doubleValue();
// or
double result = a.doubleValue() * b;