What does it mean for an expression to contain “at most one side effect, as its outermost operation”?

Question

In Java Language Spex 15.7:

Code is usually clearer when each expression contains at most one side effect, as its outermost operation

What does it mean?

Answer 1

It means that each expression should do one task at a time.

Consider the following two declarations:

int a = 10;
int b = 20;

Now the task is to add these two ints and increment b by 1. There are two way to do it.

int c = a + b++;

and

int c = a + b;
b++;

JLS prefers and recommends the latter one.

Answer 2

What this means is that:

int x = someFunction(a, b);

is clearer when someFunction(a, b) doesn't have any side-effect i.e. it doesn't change anything. Rather the only change in the above is the assignment to x.

Another example would be use of prefix/postfix incrementers.

int x = a + b;

is clearer than

int x = (a++) + (++b);

since only x is assigned to. In the second example a and b are changed in the same statement.

By limiting the side effects, you can more easily reason about the functioning of the code, and/or re-order statement invocations, including parallelising them e.g. in the below, if the methods don't have side-effects, then you can invoke the methods a(), b() and c() representing the arguments in any order, and/or in parallel.

int res = f(a(), b(), c());

Answer 3

Side-effect of an expression is mostly an assignment to variable during evaluation of the expression.

Notice the code:

int x = 5, y = 7;
while ((z = x-- + --y) > 0 ) {
    console.out("What is 'z' now? " + z);
    console.out("How many times will this be printed?");
}

Evaluation of the condition has 3 side-effects:

• decrementing x
• decrementing y
• assignment to z

Looks twisted, isn't it?