英文:
Java primitive variable reassignment during expression
问题
我在测试中遇到了一个让我感到困惑的问题（以下附有代码）。简单说，我认为变量被重新赋值，然后作为值添加回表达式中（使输出为"8, 10"），但似乎原始值在某种程度上并未更改。我漏掉了什么？
附言：如果存在类似的问题，我很抱歉，我找不到类似的问题（可能太明显了:P）。
class InitTest{
public static void main(String[] args){
int a = 10;
int b = 20;
a += (a = 4);
b = b + (b = 5);
System.out.println(a + ", " + b);
}
}
英文:
I had a question in my test that I got confused about (code attached below). To put it shortly, I thought that the variables are reassigned and then added back as a value to the expression (making the output "8, 10") but seems like the original value somehow is not changed. What am I missing?
p.s. Sorry if a similar question exists, I couldn't find one (probably its too obvious :P).
class InitTest{
public static void main(String[] args){
int a = 10;
int b = 20;
a += (a = 4);
b = b + (b = 5);
System.out.println(a + ", " + b);
}
}
答案1
得分: 1
a += (a = 4);
上述代码在逻辑上等同于以下内容：
a = a + (a = 4);
如果我们用现有的变量值替换 `a`，则简化为：
a = 10 + 4 = 14
我们可以对变量 `b` 做同样的操作：
b = b + (b = 5) = 20 + 5 = 25
我们得到这个结果是因为运算符优先级。加法运算符 `+` 的优先级高于赋值运算符 `=`，这是由 Java [运算符优先级表](https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html) 定义的。
可以看出，加法赋值运算符 `+=` 与赋值运算符共享相同的优先级，在这种情况下，表达式从左到右进行评估。
---
相反，如果表达式是：
a = (a = 4) + a;
b = (b = 5) + b;
那么它将得到您期望的输出（`a = 8`，`b = 10`），因为在计算表达式时会先计算左操作数，然后再计算右操作数。我会尝试查找这在 Java 语言规范中的具体规定位置。
英文:
a += (a = 4);
The above is logically equivalent to the following:
a = a + (a = 4);
If we substitute in the existing value for a, then this simplifies to:
a = 10 + 4 = 14
We can do the same for b:
b = b + (b = 5) = 20 + 5 = 25
We see this result because of operator precedence. The addition operator, +, has a higher precedence than the assignment operator, =, as defined by the Java operator precedence table.
You can see that the addition-assignment operator, +=, shares the same precedence with the assignment operator, in which case the expression is evaluated from left to right.
If, instead, the expressions were:
a = (a = 4) + a;
b = (b = 5) + b;
Then it would result in the output that you expect (a = 8, b = 10), as the left operand is computed before the right operand (when evaluating the expression). I'll try to locate where this is specified in the Java Language Specification.
答案2
得分: -1
在Java中，当将一个值赋给一个变量时，首先计算的是等号（=）左边的部分，然后是右边的部分。因此，当你写下 a += (a = 4);，这等同于 a = a + (a=4);，这与 a = a + 4; 相同，对于变量b也是相同的。
英文:
In Java when assigning a value to a variable, the first thing that is calculated is the left part of the = sign and then the right. Therefore, when you write a += (a = 4); which is equivalant to a = a + (a=4) which is the same as a = a + 4 , same for b.
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