EXPRESSIONS

Prefix Increment Operator

++

15.14.1

of a unary

+

 operating on

q

, or a binary addition of two quantities

p

 and

q

. In C

and C++, the parser handles this problem by performing a limited amount of

semantic analysis as it parses, so that it knows whether

p

 is the name of a type or

the name of a variable.

Java takes a different approach. The result of the

+

 operator must be numeric,

and all type names involved in casts on numeric values are known keywords.

Thus, if

p

 is a keyword naming a primitive type, then

(p)+q

 can make sense only

as a cast of a unary expression. However, if

p

 is not a keyword naming a primitive

type, then

(p)+q

 can make sense only as a binary arithmetic operation. Similar

remarks apply to the

 operator. The grammar shown above splits

CastExpression

into two cases to make this distinction. The nonterminal

UnaryExpression

includes all unary operator, but the nonterminal

UnaryExpressionNotPlusMinus

excludes uses of all unary operators that could also be binary operators, which in

Java are

+

 and

.

The second potential ambiguity is that the expression

(p)++

 could, to a C or

C++ programmer, appear to be either a postfix increment of a parenthesized

expression or the beginning of a cast, for example, in

(p)++q

. As before, parsers

for C and C++ know whether

p

 is the name of a type or the name of a variable. But

a parser using only one token lookahead and no semantic analysis during the

parse would not be able to tell, when

++

 is the lookahead token, whether

(p)

should be considered a

Primary

 expression or left alone for later consideration as

part of a

CastExpression

.

In Java, the result of the

++

 operator must be numeric, and all type names

involved in casts on numeric values are known keywords. Thus, if

p

 is a keyword

naming a primitive type, then

(p)++

 can make sense only as a cast of a prefix

increment expression, and there had better be an operand such as

q

 following the

++

. However, if

p

 is not a keyword naming a primitive type, then

(p)++

 can make

sense only as a postfix increment of

p

. Similar remarks apply to the

 operator.

The nonterminal

UnaryExpressionNotPlusMinus

 therefore also excludes uses of

the prefix operators

++

 and

.

15.14.1   Prefix Increment Operator

++

A unary expression preceded by a

++

 operator is a prefix increment expression.

The result of the unary expression must be a variable of a numeric type, or a com 

pile time error occurs. The type of the prefix increment expression is the type of

the variable. The result of the prefix increment expression is not a variable, but a

value.

At run time, if evaluation of the operand expression completes abruptly, then

the prefix increment expression completes abruptly for the same reason and no

incrementation occurs. Otherwise, the value

1

 is added to the value of the variable

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