Question

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How can I check if multiplying two numbers in Java will cause an overflow?

Suppose I have a Java class method, which uses * and + operations.

int foo(int a, int b) {
  ... // some calculations with + and * 
}

How to make sure that no overflow occurs in foo?

I guess I can either use BigDecimal or replace all + and * with “wrappers” like:

int sum(int a, int b) {
   int c = a + b;
   if (a > 0 && b > 0 && c < 0) 
     throw new MyOverfowException(a, b)
   return c;
}

int prod(int a, int b) {
   int c = a * b;
   if (a > 0 && b > 0 && c < 0) 
     throw new MyOverfowException(a, b)
   return c;
}

Are there better ways to make sure that no int overflow occurs in a Java method ?

Answer 1

It is a difficult problem from an engineering perspective.

The Secure Coding site recommends:

• use of preconditions; i.e. range-check the inputs so that overflow is impossible,
• doing each individual arithmetic operation using the next larger primitive integer type and explicitly checking for overflow, or
• using BigInteger.

This Dr Dobbs article suggests creating a library of primitive arithmetic methods that do each primitive operation with an explicit overflow check. (You could view this as an implementation of bullet point #2 above.) But the authors go further by suggesting that you use bytecode rewriting to replace arithmetic bytecodes with calls to the equivalent methods which incorporate overflow checks.

Unfortunately, there is no way to enable overflow checking natively in Java. (But the same applies in lots of other languages; e.g. C, C++ … )