CODEWITHSUNDEEP
javalanguage-lawyerjls
user24723440
user24723440

Reputation: 563

In Java, how can a name become a member of a Primary expression?

First, let's take a look at a very simple piece of code.

class S { int x = 0; } 
class Test1 { 
    public static void main(String[] args) {
        S s = new S(); 
        System.out.println("s.x=" + s.x);
    } 
}

s.x is a field access expression.

The following is the relevant syntax excerpted by me from the Java Language Specification:

15.11:

FieldAccess:

Primary . Identifier

super . Identifier

TypeName . super. Identifier

15.8: Primary:

PrimaryNoNewArray

ArrayCreationExpression

PrimaryNoNewArray:

Literal

ClassLiteral

this TypeName . this

( Expression )

ClassInstanceCreationExpression

FieldAccess

ArrayAccess

MethodInvocation

MethodReference

If only looking at the syntax excerpted above, then s.x seemingly does not conform to the syntax.The reason is that s seemingly does not belong to any member of the Primary expressions.

Fortunately, the specification gives an explanation later, and this is exactly where my doubt lies：

This part of the grammar of the Java programming language 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 grouped together with primary expressions a little later when postfix expressions are introduced (§15.14).

"names are grouped together with primary expressions a little later when postfix expressions are introduced",My understanding of this sentence is that a name will become a member of the primary expressions under certain circumstances.

15.14:

Postfix expressions include uses of the postfix ++ and -- operators. Names are not considered to be primary expressions (§15.8), but are handled separately in the grammar to avoid certain ambiguities. They become interchangeable only here, at the level of precedence of postfix expressions.

My question is: 1.Is my above understanding correct?

2."They become interchangeable only here, at the level of precedence of postfix expressions", (1).What does this sentence mean? (2).Can you explain, based on my example, why s in s.x becomes a member of the primary expressions here?(On the premise that my understanding is correct.) (3).What is the relationship between this and the level of precedence of postfix expressions? (4).What does "interchangeable" refer to?

Thank you for your reading.

Upvotes: -1

Views: 111

Answers (1)

user2357112
user2357112

Reputation: 281757

s.x is not a primary expression, and does not "become a member of the primary expressions" at any point. s.x is parsed as an ExpressionName:

ExpressionName:
  Identifier
  AmbiguousName . Identifier

AmbiguousName:
  Identifier
  AmbiguousName . Identifier

which is one of the options for a PostfixExpression, alongside Primary:

PostfixExpression:
  Primary
  ExpressionName
  PostIncrementExpression
  PostDecrementExpression

When the JLS says that names and primary expressions "become interchangeable" at the level of precedence of postfix expressions, it means that both names and primary expressions can be parsed as postfix expressions. It does not mean that names become primary expressions.

Upvotes: 4

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