lexer and parser 的java简单实现

A pragmatic benefit of using a dedicated upfront lexing phase is that you don’t couple the subsequent parser with lexical detail. This is useful during early programming language development, when the lexical and syntactic details are still changing frequently
使用专用的前期lexing阶段的一个实际好处是，您不会将后续解析器与词法细节相结合。这在早期编程语言开发期间很有用，因为词法和句法细节仍然经常变化。
参考

1. Why separate lexing and parsing?
2. https://stackoverflow.com/questions/2842809/lexers-vs-parsers
3. https://www.zhihu.com/question/31065265

1. Lexer

github：compile/src/lexer
java实现一个词法分析器，参考link可以识别加法与乘法中的token。

token	种别码
EOI	0
SEMI（;）	1
PLUS(+)	2
TIMES(*)	3
LP( ( )	4
RP( ) )	5
NUM	6
INT	7
EQ( =)	8
ID	9

运行src/lexer
input：

int a = 1 ;
int b = a + 1;
end

output：

实现思路就是根据那个表，用switch…case…框架和最长匹配原则实现了词法分析。

2.parse

Grammar:

statements -> expression ;
	   | expression ; statements

expression -> term 
 	   |term + expression
  
term -> factor 
           | factor * term
 
 factor -> NUM_OR_ID
	   | LP expression RP

github：compile/src/lexer
语法太过简单，用递归下降算法可得：

input:

1+2+3
end

output

input :

213+123+23989+4546;
34545*233+8980;
end		

output: