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.