表达式的Java语法（仅JDK7）

Question

This is for jdk 7 only. The following grammar is taken from https://docs.oracle.com/javase/specs/jls/se7/html/jls-18.html

Expression: 
    Expression1 [AssignmentOperator Expression1]

Expression1: 
    Expression2 [Expression1Rest]

Expression2:
    Expression3 [Expression2Rest]

Expression2Rest:
    { InfixOp Expression3 }
    instanceof Type

InfixOp: 
|| && | ^ & == != < > <= >= << >> >>> + - * / %

Look at this code snippet

 if (Math.abs(x) >= EDGE || Math.abs(y) >= EDGE)
        clear();

The line inside the parenthesis is an expression (by definition of ParExpression, which is not listed above)

 Math.abs(x) >= EDGE || Math.abs(y) >= EDGE

But I am not able to parse it according to the grammar:

1. ">= EDGE" is an Expression2Rest.
2. "Math.abs(x) >= EDGE" is an Expression2 (since "Math.abs(x)" is an Expression3).
3. Similarly, "Math.abs(y) >= EDGE" is an Expression2.
4. "||" is an infix op, which indicates the next part should be an Expression3.

Here is the problem: "Math.abs(y)>=EDGE" cannot be an Expression3, since it contains the infix op. It is an Expression2 by the grammar. After parsing the example becomes

Expression2 InfixOp Expression2

This does not lead to a valid Expression. What is wrong with the above logic? In addition, currently I use recursion to do the parsing. That's inefficient. Any tutorial for parsing by tree or graph? I need to parse the real java code, the tutorials about parsing simple arithmetic expressions do not help. Thanks.

Answer 1

Read the doc:

• [x] denotes zero or one occurrences of x .
• {x} denotes zero or more occurrences of x .

So:

Expression2:
    Expression3 [Expression2Rest]
Expression2Rest:
    { InfixOp Expression3 }

Means that:

Expression2:
    Expression3 InfixOp Expression3 InfixOp Expression3 InfixOp Expression3 ...

Hence:

Math.abs(x) >=      EDGE  ||      Math.abs(y) >=      EDGE
=========== ------- ===== ------- =========== ------- =====
Expr3       InfixOp Expr3 InfixOp Expr3       InfixOp Expr3

And:

Math         . abs           ( x          )
==========     ==========    ================
Identifier { . Identifier } [IdentifierSuffix]   <-- Primary
                             ( Expression )      <-- Arguments