长过int除法

Question

之前有人问过这个问题。我不同意这个答案。为什么除法会这样？请看 long aslong=n/(a*b)

这一行

仅供参考。几年前的类似问题。 Unexpected result in long/int division

long n=8589934592l; // =2^33 
int nasint=(int)n;  // it can't fit into int so it's zero.          
int a=2097152; int b=1024;
int asint=(int) n/(a*b); // n is cast into zero. so answer is zero.
long aslong=n/(a*b); // don't the int get cast into long?
long aslong2=n/((long)a*(long)b); // as expected

System.out.println("long n=" + n + " as int=" + nasint); // long n=8589934592 as int=0
System.out.println("asint=" + asint); // asint=0
System.out.println("aslong=" + aslong); // aslong=-4
System.out.println("aslong2=" + aslong2); // aslong2=4

Answer 1

Java 语言规范说明如下。

15.7.3.“计算尊重括号和优先级”：

The Java programming language respects the order of evaluation indicated explicitly by parentheses and implicitly by operator precedence.

15.7.2.“在操作前评估操作数”：

The Java programming language guarantees that every operand of an operator (except the conditional operators &&, ||, and ? :) appears to be fully evaluated before any part of the operation itself is performed.

这意味着 (a*b) 在 n/(a*b) 中的除法运算符之前计算 ，因为 a 和 b是 int 个变量，乘法是 int。然后将结果进行“Widening Primitive Conversion”到long的除法。

既然我引用了 JLS，那我也引用一下这个吧（特别感谢 David Wallace 的关注）:

3.10.1.“整数文字”：

An integer literal is of type long if it is suffixed with an ASCII letter L or l (ell); otherwise it is of type int (§4.2.1).

The suffix L is preferred, because the letter l (ell) is often hard to distinguish from the digit 1 (one).

长过int除法

Long over int division

java

division