How is if/while condition evaluated when we use assignments instead of comparison?

by
  • = is assignment operator,
  • == is comparison operator.

But assignment operator

x = y

not only assigns value from y to variable x, but it also returns that value.

Thanks to that we can write code like

x = y = z = 1;
//equivalent of:
x = (y = (z = 1));

(although it is not recommended as it can be confusing, especially for new Java programmers)

As you see 1 is first assigned to variable z, then expression z = 1 returns 1 which can be assigned to variable y. Then again assigning 1 to y returns 1 which can be assigned to variable x.

Because of that returning mechanism it is possible to write code like if (b = true) since true will be assigned to b and then returned. Since if(..) expected boolean for its condition, and found one code compiled fine.

In other words if(b=true){...} is very similar to if(true){b=true; ...}. This means that such if will always execute code from true branch (since that is what we ware assigning to b).

BONUS: How to prevent this typo?

  • omit ==true and ==false parts.

    • In case of if(b==true) we can write if(b) since (b == true) will always give same result as already stored in b.
    • In case of if(b==false) we can write if(!b).

  • use Yoda conditions if(true == b){..} where value is used before/on left side and variable on right side of ==.
    Even if by mistake we will write = instead of == we will end up with true = b which will end up as compilation error since we can’t assign anything to value like true (just like we can’t compile 2=3; which would attempt to assign 3 to 2 which makes no sense). We can only assign values to variables.

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