[英]Java: += equivalence
Is: 方法是:
x -= y;
equivalent to: 相当于:
x = x - y;
No, they are NOT equivalent the way you expressed them. 不,它们与你表达它们的方式不相同。
short x = 0, y = 0;
x -= y; // This compiles fine!
x = x - y; // This doesn't compile!!!
// "Type mismatch: cannot convert from int to short"
The problem with the third line is that -
performs what is called "numeric promotion" ( JLS 5.6 ) of the short
operands, and results in an int
value, which cannot simply be assigned to a short
without a cast. 第三行的问题是
-
执行short
操作数的所谓“数字提升”( JLS 5.6 ),并产生一个int
值,不能简单地将其赋值给没有强制转换的short
。 Compound assignment operators contain a hidden cast! 复合赋值运算符包含隐藏的强制转换
The exact equivalence is laid out in JLS 15.26.2 Compound Assignment Operators : 确切的等价在JLS 15.26.2化合物分配操作符中列出 :
A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T)((E1) op (E2)) , where T is the type of E1 , except that E1 is evaluated only once.
形式E1 op = E2的复合赋值表达式等效于E1 =(T)((E1)op(E2)) ,其中T是E1的类型,除了E1仅被评估一次。
So to clarify some of the subtleties: 所以澄清一些细微之处:
int x = 5; x *= 2 + 1; // x == 15, not 11
int i = 0; i += 3.14159; // this compiles fine!
arr[i++] += 5; // this only increments i once
Java also has *=
, /=
, %=
, +=
, -=
, <<=
, >>=
, >>>=
, &=
, ^=
and |=
. Java还有
*=
, /=
, %=
, +=
, -=
, <<=
, >>=
, >>>=
, &=
, ^=
和|=
。 The last 3 are also defined for booleans ( JLS 15.22.2 Boolean Logical Operators ). 最后3个也是为布尔值定义的( JLS 15.22.2布尔逻辑运算符 )。
Yes, it is. 是的。 This syntax is the same in most C-derived languages.
在大多数C派生语言中,此语法是相同的。
Not exactly. 不完全是。 The reason it was introduced in C was to allow the programmer to do some optimizations the compiler couldn't.
它在C中引入的原因是允许程序员做一些编译器无法做到的优化。 For example:
例如:
A[i] += 4
used to be compiled much better than 曾经比编译好得多
A[i] = A[i] + 4
by the compilers of the time. 由当时的编译器。
And, if "x" has side effects, eg "x++" then it is wildly different. 而且,如果“x”有副作用,例如“x ++”那么它就会大不相同。
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