数学表达式的正确优先顺序是什么 [英] What is the right precedence of the math expression

问题描述

在此Java表达式中，数学运算的正确顺序是什么?

What is the correct sequence of the math operations in this expression in Java:

    a + b  * c / ( d - e )
1.    4    1   3     2
2.    4    2   3     1

我知道两个答案的结果是相同的.但是我想充分理解java编译器的逻辑.在此示例中，首先执行的是乘法或括号中的表达式?包含相关说明的文档链接.

I understand that result is the same in both answers. But I would like to fully understand the java compiler logic. What is executed first in this example - multiplication or the expression in parentheses? A link to the documentation that covers that would be helpful.

更新:谢谢大家的回答.你们中的大多数人都写道，括号中的表达式首先被求值.在查看了Grodriguez提供的参考之后，我创建了一些测试:

UPDATE: Thank you guys for the answers. Most of you write that the expression in parentheses is evaluated first. After looking at the references provided by Grodriguez I created little tests:

int i = 2;
System.out.println(i * (i=3)); // prints '6'
int j = 2;
System.out.println((j=3) * j); // prints '9'

有人可以解释为什么这些测试产生不同的结果吗?如果首先对括号中的表达式求值，我会期望得到相同的结果-9.

Could anybody explain why these tests produce different results? If the expression in parentheses is evaluated the first I would expect the same result - 9.

推荐答案

到目前为止，几乎所有人都对运算符优先级的求值顺序感到困惑.在Java中，优先级规则使表达式等同于以下内容:

Almost everybody so far has confused order of evaluation with operator precedence. In Java the precedence rules make the expression equivalent to the following:

a + (b  * c) / ( d - e )

因为*和/具有相同的优先级，并且保持关联性.

because * and / have equal precedence and are left associative.

评估顺序严格定义为首先是左操作数，然后是右操作数，然后是(||和&&除外).因此，评估顺序为:

The order of evaluation is strictly defined as left hand operand first, then right, then operation (except for || and &&). So the order of evaluation is:

  a
      b
      c
    *
      d
      e
    -
  /
+

评估顺序在页面上.缩进反映了语法树的结构

order of evaluation goes down the page. Indentation reflects the structure of the syntax tree

修改

回应Grodriguez的评论.以下程序:

In response to Grodriguez's comments. The following program:

public class Precedence 
{
    private static int a()
    {
        System.out.println("a");
        return 1;
    }   
    private static int b()
    {
        System.out.println("b");
        return 2;
    }
    private static int c()
    {
        System.out.println("c");
        return 3;
    }
    private static int d()
    {
        System.out.println("d");
        return 4;
    }
    private static int e()
    {
        System.out.println("e");
        return 5;
    }

    public static void main(String[] args) 
    {
        int x = a() + b() * c() / (d() - e());
        System.out.println(x);
    } 
}

产生输出

a
b
c
d
e
-5

清楚地表明乘法是在减法之前 执行的.

which clearly shows the multiplication is performed before the subtraction.

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