java - subtracting longs goes wrong -

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long freesize = ((main.maxspace-main.usedspace)*1000*1000);  maxspace = 20000 usedspace = 8

--> freesize = -1482836480

why result negative?

change type of maxspace , usedspace int long. if can't change code 

long freesize = 1000l*1000*(main.maxspace - main.usedspace);

so result calculated long, not int.

now calculated 

main.maxspace-main.usedspace              -> 19992 (main.maxspace-main.usedspace)*1000       -> 19992000 (main.maxspace-main.usedspace)*1000*1000l -> 19992000000

problem here operating on integers, result must integer, max value of integer 

2147483647 19992000000 out of range

so java take last 32 bits of result , change integer

10010100111100111011011011000000000 -> 19992000000    10100111100111011011011000000000 -> -1482836480

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