除以双，0为结果？

Question

很明显

 int x = 3/4; // x is 0 here

但我不明白为什么以下情况也是如此：

double x = 3/4; // x is also 0 here

通过以下我能够得到我想要的结果：

 double x = (double) 3/4; // x is 0.75 here, but why do i have to cast?