如果你看一下 BigInteger （随JDK提供），在我看来
multiply（..）有 O（n ^ 2）（实际上该方法是 multiplyToLen（..））。其他方法的代码有点复杂，但你可以看到自己。

If you look at the code for BigInteger (provided with JDK), it appears to me that multiply(..) has O(n^2) (actually the method is multiplyToLen(..)). The code for the other methods is a bit more complex, but you can see yourself.

注意：这是针对Java 6.我假设它在Java中没有区别7。

Note: this is for Java 6. I assume it won't differ in Java 7.