The BigInteger class is a very handy tool for representing immutable arbitrary-precision integers.

This class also contains methods (originating from java.lang.Number) that are useful for converting BigInteger into a primitive type such as byte, long, or double. However, these methods can produce unexpected results and confusion. For example, let's assume that we have BigInteger that wraps Long.MAX_VALUE:

BigInteger nr = BigInteger.valueOf(Long.MAX_VALUE);

Let's convert this BigInteger into a primitive long via the BigInteger.longValue() method:

long nrLong = nr.longValue();

So far, everything has worked as expected since the Long.MAX_VALUE is 9,223,372,036,854,775,807 and the nrLong primitive variable has exactly this value.

Now, let's try to convert this BigInteger class into a primitive int value via the BigInteger.intValue() method:

int nrInt = nr.intValue();

This time, the nrInt primitive variable will have a value of -1 (the same result will produce shortValue() and byteValue()). Conforming to the documentation, if the value of BigInteger is too big to fit in the specified primitive type, only the low-order n bits are returned (n depends on the specified primitive type). But if the code is not aware of this statement, then it will push values as -1 in further computations, which will lead to confusion.

However, starting with JDK 8, a new set of methods was added. These methods are dedicated to identifying the information that's lost during the conversion from BigInteger into the specified primitive type. If a piece of lost information is detected, ArithmeticException will be thrown. This way, the code signals that the conversion has encountered some issues and prevents this unpleasant situation.

These methods are longValueExact(), intValueExact(), shortValueExact(), and byteValueExact():

long nrExactLong = nr.longValueExact(); // works as expected
int nrExactInt = nr.intValueExact();    // throws ArithmeticException

Notice that intValueExact() did not return -1 as intValue(). This time, the lost information that was caused by the attempt of converting the largest long value into int was signaled via an exception of the ArithmeticException type.