Search:

Nope, just re-assign it to the same variable. The operation will return a new BigInteger already.

BigInteger a = new BigInteger("1");
a = a.add(a);

I would recommend against using BigDecimal for this kind of operation, us a BigInteger instead.

Second, you're update loop never re-assigns i. BigDecimals and BigIntegers are immutable and need to...

I don't see why you would be running out of memory, 100! should fit in ~66 bytes, and even in it's string representation is ~158 characters long.

The JVM's default heap size is quite "small"...

Using BigDecimals and BigIntegers in Java has always been a pain.

You're last bit of code is actually computing sum = sum * sum * i, not sum = sum * i

That's probably the easiest method for isolating decimal digits in a BigDecimal.

What overall problem are you trying to solve? Perhaps there's a better way to solve that.