为什么“ x * y / z”会导致错误的负值？

Question

System.out.println(1000*16777216/4890);

OUTPUT: -82342

...but should be: 3430923

How can I fix this?

Answer 1

Just add an L to either 1000 or 16777216 (or both)...and that will do the trick.

The reason, like Rod_Algonquin stated, it's a positive overflow of the integer value , and it's happening when you multiply 1000 by 16777216 ; that eventually will be greater than (the max allowed) Integer.MAX_VALUE , so it wraps up and starts from Integer.MIN_VALUE (in your case the value would be -402653184 ; divide it by 4890 and you will get the unexpected result -82342 ).

Have a look (notice the yellow highlighted section; IDEA will warn you with Numeric overflow in expression ):

Answer 2

As pointed out by @Rod_Algonquin , 1000*16777216 exceeds the range of int ( int takes value between -2 ³¹ to 2 ³¹ - 1 ), so it causes overflow.

In order to prevent overflow from happening, you can perform the computation using long by adding the letter L after the number literals like the following:

1000L * 16777216 / 4890

By default, 1000 is an int literal in Java, whereas 1000L is a long literal.

Answer 3

First of all note it's integer arithmetic.Whatever the inbetween calculation would be there stored as integer. 1000*16777216 will give 16777216000 which is out of the range of integer. So now system tries to manage this integer over flow during runtime.As 16777216000 > Integer.MAX_VALUE and now it moves for the negative ranged values to handle the overflow..

Try like this Integer.MAX_VALUE + 1 will give you -2147483648 .Better way to manage this is to use long primitive or you can use double for floating point value.