1000 * 60 * 60 * 24 * 30 results in a negative num

问题:

This question already has answers here:

Why do these two multiplication operations give different results? (2 answers)

Closed 6 years ago.

I'm attempting to calculate 30 days by multiplying milliseconds however the result continually ends up being a negative number for the value of days_30 and I'm not sure why.

Any suggestions are greatly appreciated!

CODE SNIPPET:

// check to ensure proper time has elapsed
                SharedPreferences pref = getApplicationContext()
                        .getSharedPreferences("DataCountService", 0);
                 long days_30 = 1000 * 60 * 60 * 24 * 30;
                 long oldTime = pref.getLong("smstimestamp", 0);
                long newTime = System.currentTimeMillis();
                 if(newTime - oldTime >= days_30){

days_30 value results in: -1702967296

P.S.

 double days_30 = 1000 * 60 * 60 * 24 * 30;
                 double oldTime = pref.getLong("smstimestamp", 0);
                double newTime = System.currentTimeMillis();
                 if(newTime - oldTime >= days_30){

Results in a smaller - but still negative number. -1.702967296E9

回答1:

You are multiplying ints together, and overflow occurs because the maximum integer is 2^31 - 1. Only after the multiplications does it get converted to a long. Cast the first number as a long.

long days_30 = (long) 1000 * 60 * 60 * 24 * 30;

or use a long literal:

long days_30 = 1000L * 60 * 60 * 24 * 30;

That will force long math operations from the start.

回答2:

   long days_30 = 1L * 1000 * 60 * 60 * 24 * 30;

回答3:

Just change your multiplication to long days_30 = 1000L * 60 * 60 * 24 * 30;

回答4:

Java has limitations when it comes to primitive data types. If your long or double are too big, then it will overflow into a negative number. Try using the BigInteger class for storing larger numbers.

Check this out:

How does Java handle integer underflows and overflows and how would you check for it?