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

Question

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

Solution 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.

Solution 2

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

Solution 3

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

Comments

[Rohit Jain]

Check out this post

[Steve Kuo]

A long is good enough in this case

[Steve Kuo]

If "Computers have limitations when it comes to numbers", then how would BigInteger help?

[rgettman]

If a double goes beyond its range, then POSITIVE_INFINITY (or NEGATIVE_INFINITY) results, not an overflow.

[But I'm Not A Wrapper Class]

I'm sorry for the unclear explanation. Java limits primitive data types. For example, longs and doubles are 64 bits (excluding over head). However, if you use a Java object (or a class that extends from Object), the size can as large as the computer's memory is (or whatever the remaining space is before it crashes lol).