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

Question

I want to handle the special case where multiplying two numbers together causes an overflow. The code looks something like this:

int a = 20; long b = 30;  // if a or b are big enough, this result will silently overflow long c = a * b; 

That's a simplified version. In the real program a and b are sourced elsewhere at runtime. What I want to achieve is something like this:

long c; if (a * b will overflow) {     c = Long.MAX_VALUE; } else {     c = a * b; } 

How do you suggest I best code this?

Update: a and b are always non-negative in my scenario.

Answer 1

Java 8 has Math.multiplyExact, Math.addExact etc. for ints and long. These throw an unchecked ArithmeticException on overflow.

Answer 2

If a and b are both positive then you can use:

if (a != 0 && b > Long.MAX_VALUE / a) {     // Overflow } 

If you need to deal with both positive and negative numbers then it's more complicated:

long maximum = Long.signum(a) == Long.signum(b) ? Long.MAX_VALUE : Long.MIN_VALUE;  if (a != 0 && (b > 0 && b > maximum / a ||                b < 0 && b < maximum / a)) {     // Overflow } 

Here's a little table I whipped up to check this, pretending that overflow happens at -10 or +10:

a =  5   b =  2     2 >  10 /  5 a =  2   b =  5     5 >  10 /  2 a = -5   b =  2     2 > -10 / -5 a = -2   b =  5     5 > -10 / -2 a =  5   b = -2    -2 < -10 /  5 a =  2   b = -5    -5 < -10 /  2 a = -5   b = -2    -2 <  10 / -5 a = -2   b = -5    -5 <  10 / -2