Note: Originally I posted C# code in this answer for purposes of illustration, since C# allows you to pass int parameters by reference with the ref keyword. I've decided to update it with actual legal Java code using the first MutableInt class I found on Google to sort of approximate what ref does in C#. I can't really tell if that helps or hurts the answer. I will say that I personally haven't done all that much Java development; so for all I know there could be much more idiomatic ways to illustrate this point.
Perhaps if we write out a method to do the equivalent of what x++ does it will make this clearer.
public MutableInt postIncrement(MutableInt x) {
int valueBeforeIncrement = x.intValue();
x.add(1);
return new MutableInt(valueBeforeIncrement);
}
Right? Increment the value passed and return the original value: that's the definition of the postincrement operator.
Now, let's see how this behavior plays out in your example code:
MutableInt x = new MutableInt();
x = postIncrement(x);
postIncrement(x) does what? Increments x, yes. And then returns what x was before the increment. This return value then gets assigned to x.
So the order of values assigned to x is 0, then 1, then 0.
This might be clearer still if we re-write the above:
MutableInt x = new MutableInt(); // x is 0.
MutableInt temp = postIncrement(x); // Now x is 1, and temp is 0.
x = temp; // Now x is 0 again.
Your fixation on the fact that when you replace x on the left side of the above assignment with y, "you can see that it first increments x, and later attributes it to y" strikes me as confused. It is not x that is being assigned to y; it is the value formerly assigned to x. Really, injecting y makes things no different from the scenario above; we've simply got:
MutableInt x = new MutableInt(); // x is 0.
MutableInt y = new MutableInt(); // y is 0.
MutableInt temp = postIncrement(x); // Now x is 1, and temp is 0.
y = temp; // y is still 0.
So it's clear: x = x++ effectively does not change the value of x. It always causes x to have the values x0, then x0 + 1, and then x0 again.
Update: Incidentally, lest you doubt that x ever gets assigned to 1 "between" the increment operation and the assignment in the example above, I've thrown together a quick demo to illustrate that this intermediate value does indeed "exist," though it will never be "seen" on the executing thread.
The demo calls x = x++; in a loop while a separate thread continuously prints the value of x to the console.
public class Main {
public static volatile int x = 0;
public static void main(String[] args) {
LoopingThread t = new LoopingThread();
System.out.println("Starting background thread...");
t.start();
while (true) {
x = x++;
}
}
}
class LoopingThread extends Thread {
public @Override void run() {
while (true) {
System.out.println(Main.x);
}
}
}
Below is an excerpt of the above program's output. Notice the irregular occurrence of both 1s and 0s.
Starting background thread...
0
0
1
1
0
0
0
0
0
0
0
0
0
0
1
0
1
x++is post-increment;x=is assignment of result; the result ofx++is the originalx(and there is a side-effect of increment, but that doesn't change the result), So this can be interpreted asvar tmp = x; x++; x = tmp;– Marc Gravell♦