Java Multiple Two numbers issue with negative numbers [duplicate]

The maximum postive value an int can hold is 2147483647 beyond which the value goes to the other end (i.e. it starts from the negative end). You can understand it from the following demo:

public class Main { public static void main(String[] args) { int i = -2147483648; int j = i * -1; System.out.println("j=" + j); System.out.println(Integer.MAX_VALUE); System.out.println(Integer.MAX_VALUE + 1); System.out.println(Integer.MIN_VALUE); // Some more examples for you to understand this concept better System.out.println(Integer.MAX_VALUE + 2); System.out.println(Integer.MAX_VALUE + 3); } } 

Output:

j=-2147483648 2147483647 -2147483648 -2147483648 -2147483647 -2147483646 

After -2147483648 is multipled with -1, it becomes 2147483648 but an int variable can not hold this much big positive value; so, it will start from the negative end (i.e. Integer.MIN_VALUE).

The number 2147483648 does not exists. The biggest value of an int is 2147483647, which is 1 smaller than your expected result. The multiplication causes an overflow, which 'rolls back' the number to the smallest negative value, and continues the calculation from there. (With other words: 2147483647+1=-2147483648 (smallest negative)) Since the result would only be 1 over the maximum value, there is no additional action required and the minimal int value is returned.

If you want to fix this issue, use 'long' instead of 'int' for your variables. You can also use more complex classes like BigDecimal, or write a custom multiplication function for byte arrays.

Note: no matter what numeric type you use, as long as the memory used for representing the number is finite you can run into similar issues. Although under normal circumstances it is unlikely, even for a 32-bit integer (int).