问题:

Possible Duplicate:
Java += operator

In Java, this is not valid (doesn't compile), as expected:

long lng = 0xffffffffffffL;
int i;
i = 5 + lng;    //"error: possible loss of magnitude"

But this is perfectly fine (?!)

long lng = 0xffffffffffffL;
int i = 5;
i += lng;       //compiles just fine

This is obviously a narrowing operation, that can possibly exceed the int range. So why doesn't the compiler complain?

回答1:

i += lng; compound assignment operator cast's implicitly.

i+=lng; 
is same as 
i = int(i+lng);

FROM JLS:

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.

回答2:

This is defined in the JLS #15.26.2:

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.

In other words, i += lng performs a cast implicitly.

回答3:

The compiler does not complain because, according to JLS §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.

Thus,