Java long assignment confusing

Question

Why does this java code

long a4 = 1L;
long a3 = 1;
long a2 = 100L * 1024 * 1024 * 1024;
long a1 = 100 * 1024 * 1024 * 1024;
System.out.println(a4);
System.out.pr         


        

Answer 1

107374182400 is exactly 25 times the full range of an integer (2^32), which means that if you try to fit it into an integer it will overflow. And because it would fit exactly 25 times it ends up precisely on 0 (this is a coincidence and other huge multiplications may end up either positive or negative). And you are using an integer right up to the point you cast to long

long a1 = 100 * 1024 * 1024 * 1024;

is equivalent to

int temp = 100 * 1024 * 1024 * 1024;
long a1 = (long)temp;

If you put a single long in the expression, it is forced to use long maths not integer maths which removes the problem