Java Reference
In-Depth Information
As you can see from studying the table in Display 3.6, most binary operators
associate from left to right. But the assignment operators associate from right to left.
So the expression
numberl = number2 = number3
means
numberl = (number2 = number3)
which in turn is interpreted as the following fully parenthesized expression:
(number1 = (number2 = number3))
However, this fully parenthesized expression may not look like it means anything until
we explain a bit more about the assignment operator.
Although we do not advocate using the assignment operator
=
as part of a complex
expression, it is an operator that returns a value, just as
+
and
*
do. When an
assignment operator
=
is used in an expression, it changes the value of the variable on
the left-hand side of the assignment operator and also returns a value—namely, the new
value of the variable on the left-hand side of the expression. So
(number2 = number3)
sets
number2
equal to the value of
number3
and returns the value of
number3
. Thus,
numberl = number2 = number3
which is equivalent to
(numberl = (number2 = number3))
sets both
number2
and
number1
equal to the value of
number3
. It is best to not use
assignment statements inside of expressions, although simple chains of assignment
operators such as the following are clear and acceptable:
numberl = number2 = number3;
Although we discourage using expressions that combine the assignment operator
and other operators in complicated ways, let's try to parenthesize one just for practice.
Consider the following:
numberl = number2 = number3 + 7 * factor
The operator of highest precedence is
*
, and the operator of next-highest precedence is
+
, so this expression is equivalent to
numberl = number2 = (number3 + (7 * factor))
which leaves only the assignment operators to group. They associate right to left, so the
fully parenthesized equivalent version of our expression is
(numberl = (number2 = (number3 + (7 * factor))))
