為Java創建詞法分析器

Question

我正在嘗試創建一個詞法分析器

cond_exp→條件| 條件op cond_exp

條件→single_cond | （條件）| cond_exp

single_cond→變量| 變量” | 不變

op→和| 或| 暗示

常數→true | 錯誤 ！true | ！假

變量→p | q | r

---

對於lex操作中的switch case，我得到一個重復的case標簽，非常感謝您的幫助，這是我到目前為止所擁有的：

作為示例，這是一個示例字符串：（（（p'和q）暗含（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都等於同一事物“ 0”，

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

並且您的編譯器在抱怨，因為不允許兩個大小寫常量相同。

  switch (StringClass) {
  case Letter:

     // ....

  // not allowed
  case opLetter:

加上它只是沒有意義。 如果StringClass為“ 0”，應激活哪種情況？

解決方案：不要讓他們等於同一件事。 更好的是，考慮使用枚舉。