java - 为java创建词法分析器

Question

我正在尝试为

创建一个词法分析器

cond_exp → 条件 |条件操作 cond_exp

条件 → single_cond | (条件)|条件表达式

single_cond → 变量 |变量' |常数

op → 和 |或 |意味着

常量 → true |假 | !true | !假

变量 → p|q|r

<小时/>

对于 lex 操作中的 switch case，我得到了一个重复的 case 标签，非常感谢您的帮助，这就是我到目前为止所拥有的:

作为示例，这是一个示例字符串:((p'和q)意味着(r或r'))

public class Q4Lexical {

    //character classes
    private static final String Letter="0";
    private static final String opLetter="0";
    private static final String unknown="99";
    private static final String EOF="100";

    //Token classes
    public static final String INT_LIT="10";
    public static final String IDENT="11";

    public static final String pVarible="20";
    public static final String qVarible="21";
    public static final String rVarible="22";

    public static final String andOperation="23";
    public static final String orOperation="24";
    public static final String impliesOperation="25";
    public static final String tConstant="26";
    public static final String fConstant="27";
    public static final String tpConstant="28";
    public static final String fpConstant="29";


    public static final String LEFT_PAREN="30";
    public static final String RIGHT_PAREN="31";

     // Global Variables
    public static int current=0;
    static String StringClass;
    static String lexeme="";
    static String nextChar;
    static int lexLen=0;
    static int token;
    static String nextToken;


    static String expression="((p' and q) implies (r or r'))";

    public static boolean isVarible (String c){
        if((c.equalsIgnoreCase("p")) || (c.equalsIgnoreCase("q")) ||
           (c.equalsIgnoreCase("r")) || (c.equalsIgnoreCase("p'")) ||
           (c.equalsIgnoreCase("q'")) || (c.equalsIgnoreCase("r'")))
                  return true;

        else return false;

    }

    public static boolean isOperation(String c){
     if((c.equalsIgnoreCase("and")) || (c.equalsIgnoreCase("or")) ||
           (c.equalsIgnoreCase("implies")))
                  return true;

        else return false;

    }

    public static boolean isSpace(String c){

        if(c.equalsIgnoreCase(" "))
            return true;
        else return false;

    }

    public static void getString(){
        if(current< expression.length()){
            nextChar= expression.substring(current);
            current++;
           if(isVarible(nextChar))
               StringClass=Letter;
           else if(isOperation(nextChar))
               StringClass=opLetter;
           else 
               StringClass=unknown;
        }
            else
            StringClass = "#";
    }

    public static void getNonBlank(){
        while(isSpace(nextChar))
            getString();

    }

    public static String lookup(String c){
        switch(c){
            case "(":
                addString();
                nextToken= LEFT_PAREN;
                break;
            case ")":
                addString();
                nextToken= RIGHT_PAREN;
                break;
            case "and":
                addString();
                nextToken= andOperation;
                break;
            case "or":
                addString();
                nextToken=orOperation;
                break;
            case "implies":
                addString();
                nextToken=impliesOperation;
                break;
            default:
                addString();
                nextToken=unknown;
                break;
        }
        return nextToken;
    }


    public static void addString(){
        if (lexLen <= 98) 

            lexeme+=nextChar;
        else
            System.out.println("Error - Lexeme is too long");

    }

    public static String lex(){
        lexeme="";
        lexLen=0;
        getNonBlank();

        switch(StringClass){
            case Letter:

                addString();
                getString();
                while(StringClass.equals(Letter) || StringClass.equals(opLetter)){
                    addString();
                    getString();
                }
        switch (nextToken) {
            case "q":
                nextToken=qVarible;
                break;
            case "p":
                nextToken=pVarible;
                break;
            case "r":
                nextToken=rVarible;
                break;
        }
                break;

            case opLetter:
                addString();
                addString();
                while (StringClass.equals(opLetter)){
                    addString();
                    getString();
                }
                nextToken=INT_LIT;
                break;


            case unknown:
                lookup(nextChar);
                getString();
                break;

            case EOF:
                nextToken=EOF;
                break;
        }
        System.out.println("Next Token is : "+nextToken+" Next Lexeme is : "+lexeme);
        return nextToken;


    }

     public static void main(String[] args) {
            getString();
        do {
            lex();
        } while (current != expression.length());
    }
     }

Answer 1

Letter 和 opLetter 都等于同一个东西，“0”，

// character classes
private static final String Letter = "0";
private static final String opLetter = "0";

并且您的编译器会提示，因为不允许两个大小写常量相同。

  switch (StringClass) {
  case Letter:

     // ....

  // not allowed
  case opLetter:

而且它根本没有意义。如果 StringClass 为“0”，哪种情况应该被激活？

解决方案:不要让它们等同于同一事物。更好的是，考虑使用枚举。