i and i=i++ in for loop java

Question

What is the logic behind this behaviour?

 int i=0;
    for(int k=0;k<10;k++){
    i++;
    }
    System.out.println("i="+i);

Output=10; //Exepcted



 int i=0;
    for(int k=0;k<10;k++){
    i=i++;
    }
    System.out.println("i="+i);

Output=0; //Surprised :) 

Can anybody throw some light on above functionality?

Solution 1

See this brilliant answer:

x = x++;

is equivalent to

int tmp = x;
x++;
x = tmp;

From this question.

Solution 2

i = i++ is a postfix increment operator - it increments i, then returns it to its original value (because the i++ essentially "returns" the value of i before it was incremented.)

i = ++i would work since it's a prefix increment operator, and would return the value of I after the increment. However, you probably just want to do i++ there without any extra assignment as you do in the first run - it's (essentially) shorthand as it is for i = i+1.

Solution 3

What is happening happens because java is pass-by-value.

In the first loop, i is getting incremented in the i++ statement, however, in the second loop what is happening is that i gets pointed to a new memory location that stores the value of i (in this case 0) and then increments the old location.

To visualise:

i => 0x00000001 // 0

for() {
    i => 0x00000002 <- 0  // store old i value (0) in new location
    0x00000001++          // Increment the value stored at the old location

    // Cause there is no longer a reference to 0x00000001, 
    // it will get garbage collected and you will be left with
    // i => 0x00000002

And it will keep doing that, assigning the old value to a new location and incrementing the old value for each pass of the loop

Solution 4

i = i++;

is equivalent to,

int temp = i; // temp = 0
i++; // i=1
i = temp; // i = 0

Solution 5

i=i++; will never increment i because the ++ is processed after the i=i.

you could see it like this:

int i=0;
for(int k=0;k<10;k++){
    int j = 0;
    i = j;
    j = j + 1;
}

View more solutions