为 java 创建词法分析器

Question

我正在尝试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） |cond_exp

single_cond →变量 | 变量' | 常量

OP → 和 | 或 | 暗示

常数 → 真 | 假 | ！真 | 假

变量 → p|q|r

---

对于 lex 操作中的开关大小写，我得到了一个重复的案例标签，我将不胜感激，这就是我到目前为止所拥有的：

举个例子，这是一个示例字符串：（（p' 和 q） 暗示 （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

字母和opLetter都等于同一个东西，"0"，

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

并且您的编译器正在抱怨，因为您不允许两个大小写常量相同。

  switch (StringClass) {
  case Letter:
     // ....
  // not allowed
  case opLetter:

另外，它只是没有意义。如果 StringClass 为"0"，则应激活哪种情况？

解决方案：不要让它们等于同一件事。更好的是，考虑使用枚举。