Any function that returns any type of value can be used in a boolean expression, so side-effects are a concern.

For example,
say that a method `computeMaximum()`

computes the maximum value of some variables,
stores the result in `maximum`

and returns
that value:

int maximum; // set bycomputeMaximum(). . . if ( sum < 100 && computeMaximum() < 500 ) { result = 2 * maximum ; // maximum might not be computed. } . . .

There is a problem here.
The method that follows `&&`

sets `maximum`

(as a side effect)
only when
`sum`

is greater than 100.
The assignment statement will sometimes put
the wrong value in `result`

.
You should arrange the expression like this:

int maximum; . . . if ( computeMaximum() < 500 && sum < 100 ) { result = 2 * maximum ; } . . .

With this arrangement the side effect will always happen.
The two `if`

statements look almost identical;
however, the first one is a bug (probably).
Bugs like this can be hard to find.

The best solution is to write `computeMaximum()`

so that it has no side effect,
then use it like this:

int maximum; . . . maximum = computeMaximum(); if ( maximum < 500 && sum < 100 ) { result = 2 * maximum ; } . . .