EXPRESSIONS

Primary Expressions

15.7

15.7   Primary Expressions

Primary expressions include most of the simplest kinds of expressions, from

which all others are constructed: literals, field accesses, method invocations, and

array accesses. A parenthesized expression is also treated syntactically as a pri 

mary expression.

Primary:

PrimaryNoNewArray

ArrayCreationExpression

PrimaryNoNewArray:

Literal

this

(

Expression

 )

ClassInstanceCreationExpression

FieldAccess

MethodInvocation

ArrayAccess

As programming language grammars go, this part of the Java grammar is

unusual, in two ways. First, one might expect simple names, such as names of

local variables and method parameters, to be primary expressions. For technical

reasons, names are lumped together with primary expressions a little later when

postfix expressions are introduced ( 15.13).

The technical reasons have to do with allowing left to right parsing of Java

programs with only one token lookahead. Consider the expressions

(z[3])

 and

(z[])

. The first is a parenthesized array access ( 15.12) and the second is the

start of a cast ( 15.15). At the point that the look ahead symbol is

[

, a left to right

parse will have reduced the

z

 to the nonterminal

Name

. In the context of a cast we

prefer not to have to reduce the name to a

Primary

, but if

Name

 were one of the

alternatives for

Primary

, then we could not tell whether to do the reduction (that

is, we could not determine whether the current situation would turn out to be a

parenthesized array access or a cast) without looking ahead two tokens, to the

token following the

[

. The Java grammar presented here avoids the problem by

keeping

Name

 and

Primary

 separate and allowing either in certain other syntax

rules (those for

MethodInvocation

,

ArrayAccess

,

PostfixExpression

, but not for

FieldAccess,

 because this is covered by

Name

). This strategy effectively defers the

question of whether a

Name

 should be treated as a

Primary

 until more context can

be examined. (Other problems remain with cast expressions; see  19.1.5.)

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