有人可以解释为什么答案是B？ 我正在使用Java，我不明白

Question

Which of the following statements are the same?

(I) x -= x + 4

(II) x = x + 4 - x

(III) x = x - (x + 4)

A. (I) and (II) are the same

B. (I) and (III) are the same

C. (II) and (III) are the same

D. (I), (II), and (III) are the same

Answer 1

x -= y is equivalent to x = x - y

Therefore

x -= x + 4

is equivalent to

x = x - (x+4)

So assuming (II) x = x - (x + 4) was supposed to be (III) x = x - (x + 4) (since you have two options marked as (II) ), (I) and (III) are the same.

Answer 2

It's because of operator precedence. Java evaluates that as if it were

x -= (x+4)

so it first computes (x+4) and then subtracts that from x -- which is what the - part of -= means -- and then updates x , which is what the = part means.

Answer 3

• Case (I) expands to x = x − (x + 4) according to the Java -= operator,
  and mathematically simplifies to x = −4.
• Case (II) mathematically simplifies to x = 4.
• Case (III) mathematically simplifies to x = −4.

Therefore (I) and (III) are the same, which means the answer is (B).

Answer 4

-= is a so called compound assignment.

Those are just short-cuts and combine atomic operations.

x -= y stands for x = xy

x += y stands for x = x+y

x++ stands for x = x+1

x-- stands for x = x-1

There also are ++x and --x which do the same as x++ / x-- except that they return the value of x before it is incremented / decremented.

Official Java tutorial:

"You can also combine the arithmetic operators with the simple assignment operator to create compound assignments. For example, x+=1; and x=x+1; both increment the value of x by 1."

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op1.html

I think the same works for *= , /= and %=