以下语句如何工作？

Question

int x = 10;
x += x++;

System.out.println(x);

why the answer of above statement is 20 ?

Answer 1

The operator += is an addition assignment operator. Like Alya said above, x += x++ is equivalent to x = x + x++ , which in your case is x = 10 + 10 . However, it's a very messy statement and I'll explain why towards the end of this post.

Now, you're probably thinking "Why is it 20 and not 21 (10 + 11) since you have the ++?" and that's valid. There's actually a difference between a post-increment and a pre-increment. x++ is the post-increment and will actually evaluate the value of x first and THEN increment x, while ++x is the pre-increment which will increment x and THEN evaluate the value of x.

For example, x = 10; System.out.println(x++); System.out.println(x); x = 10; System.out.println(x++); System.out.println(x); will print 10 and then print 11 because the first print line prints x and THEN performs the ++ calculation, making x 11 which the next line prints. Conversely, x = 10; System.out.println(++x); System.out.println(x); x = 10; System.out.println(++x); System.out.println(x); will print 11 on both print statements.

Going back to why I said x += x++; is very messy is because technically the ++ operator isn't performed in this case. x++ is technically the same as x=x+1 and remembering that x+=y is the same as x = x+y) , the line x += x++; is kind of like saying x = x + (x = x + 1); which is kind of weird looking because you do 2 assignment statements in one and won't actually "work how you want it". Back to your example int x = 10; x += x++; int x = 10; x += x++; if you print x, you will get 20 even though you could look at it as: x is now the value of x + the value of x, then finally + 1 to it. But unfortunately, that's not how it works.

To solve your problem, if you change your code from a post-increment to a pre-increment, then it should work, ie: x+=++x; will print your 11 but I would argue the that's quite unreadable and a bit confusing. x+=x; x++; System.out.println(x); is easier to follow.

Answer 2

x++ will execute first. It returns x and then increments x by 1 .

Finally, the += operator will add to x the return value of x++ , which was 10 .

Thus, x will be 20 and it will overwrite the changes to x by the statement x++ .

Answer 3

So first x is initialized to be 10 . Then the x++ has higher precedence so that gets carried out first. the " ++ " is a post-increment in this case (because it is after the variable as opposed to pre-increment which would be ++x ). Post-increment means "first use the variable then increment it by one" so in this case it first uses x to be 10 then increments it to 11 after it is used. Then we look at the " += " which is short hand for " x = x+x++ ". so we have x = 10+10 which = 20 . If you were to carry this out again it would equal x = 20+20 = 40 .

In this particular case, the x++ isn't necessary as x is reassigned the value after it is incremented each time.

Answer 4

int x = 10; x += x++;

will equal to x=x+x

where x++ mean use the x value then increament it , so it's value will be 10

so the result will equal 20

---

if you want to see the change of the x , see this example:

    int x = 10;
    int y = 10;
    y +=x++;
    System.out.println(y);
    System.out.println(x);

will print :

y=20
x=11////////////according to x++ and without to overwrite it 

Answer 5

//
// Shows how increments work.
//
int i = 0;
System.out.println(i);

i++; // Add one
System.out.println(i);

i += 2; // Add two
System.out.println(i);

    i += 3; // Add three
System.out.println(i);

++i; // Add one
System.out.println(i);

i += i; // Added itself
System.out.println(i);

//
// Uses increments and assigns.
//
int v = 0;
v = i++; // Increment after value copy
System.out.println(v);
System.out.println(i);

v = ++i; // Increment before value copy
System.out.println(v);
System.out.println(i);

//Output

0 - 1 3 6 7 14 14 15 16 16

Answer 6

  x+=x++ first assigns the value to x and then increments (post-increment)
  x+=++x first increments then assign the value to x (pre increment)

there are two types of increments/decrements in programming

  1. pre-increment/decrement
  2. post-increment/decrement

In programming both of these have same operations but differ in there nature as they both used for increment or decrement; they can be written as,

  x+=1; (increment by 1)
  x-=1; (decrement by 1)

you can use a variable instead in the above cases as well