Java Puzzler - 将 double 转换为 int

Question

int anInt = 1;
double aDouble = 2.5;

anInt = anInt + aDouble; // Error - need to cast double to int

anInt += aDouble; // This is ok. Why?

anInt = aDouble; // This is also an error.

anInt = 1 + aDouble; // This is also an error.

所以我的问题是:为什么执行 anInt += aDouble 不是编译错误？

Answer 1

四种情况中的三种正确报告错误。复合赋值是该规则的唯一异常(exception)。 Java 语言规范第 15.26.2 部分解释了原因:

15.26.2 Compound Assignment Operators

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.

For example, the following code is correct:

short x = 3;
x += 4.6;

and results in x having the value 7 because it is equivalent to:

short x = 3;
x = (short)(x + 4.6);

如您所见，通过隐式插入强制转换避免了错误。