使用Java语言编写PL/0编译程序的词法分析程序。

下面是一个简单的 Java 词法分析程序，可以识别 PL/0 语言的关键字、标识符、数字和符号。

import java.io.*;

public class Lexer {
    private static final int LETTER = 0;
    private static final int DIGIT = 1;
    private static final int UNKNOWN = 99;
    private static final int MAX_LEXEME_LEN = 100;

    private static final int EOF = -1;

    private static final int INT_LIT = 10;
    private static final int IDENT = 11;
    private static final int ASSIGN_OP = 20;
    private static final int ADD_OP = 21;
    private static final int SUB_OP = 22;
    private static final int MULT_OP = 23;
    private static final int DIV_OP = 24;
    private static final int LEFT_PAREN = 25;
    private static final int RIGHT_PAREN = 26;
    private static final int SEMICOLON = 27;
    private static final int PERIOD = 28;

    private static final String[] RESERVED_WORDS = {
            "begin", "call", "const", "do", "end",
            "if", "odd", "procedure", "read", "then",
            "var", "while", "write"
    };

    private static final int[] RESERVED_WORD_TOKENS = {
            21, 22, 23, 24, 25,
            26, 8, 27, 28, 29,
            30, 31, 32
    };

    private static int charClass;
    private static char[] lexeme = new char[MAX_LEXEME_LEN];
    private static char nextChar;
    private static int lexemeLen;
    private static int token;
    private static FileInputStream fis;

    private static void addChar() {
        if (lexemeLen + 1 >= MAX_LEXEME_LEN) {
            System.err.println("Error: lexeme is too long.");
            System.exit(1);
        }
        lexeme[lexemeLen++] = nextChar;
        lexeme[lexemeLen] = '\0';
    }

    private static void getChar() {
        try {
            int c = fis.read();
            if (c != EOF) {
                nextChar = (char) c;
                if (Character.isLetter(nextChar)) {
                    charClass = LETTER;
                } else if (Character.isDigit(nextChar)) {
                    charClass = DIGIT;
                } else {
                    charClass = UNKNOWN;
                }
            } else {
                charClass = EOF;
            }
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    private static void getNonBlank() {
        while (Character.isWhitespace(nextChar)) {
            getChar();
        }
    }

    private static int lookup(char[] s) {
        for (int i = 0; i < RESERVED_WORDS.length; i++) {
            if (RESERVED_WORDS[i].equals(new String(s))) {
                return RESERVED_WORD_TOKENS[i];
            }
        }
        return IDENT;
    }

    private static int lex() {
        lexeme = new char[MAX_LEXEME_LEN];
        lexemeLen = 0;
        getNonBlank();
        switch (charClass) {
            case LETTER:
                addChar();
                getChar();
                while (charClass == LETTER || charClass == DIGIT) {
                    addChar();
                    getChar();
                }
                token = lookup(lexeme);
                break;
            case DIGIT:
                addChar();
                getChar();
                while (charClass == DIGIT) {
                    addChar();
                    getChar();
                }
                token = INT_LIT;
                break;
            case UNKNOWN:
                switch (nextChar) {
                    case ':':
                        addChar();
                        getChar();
                        if (nextChar == '=') {
                            addChar();
                            getChar();
                            token = ASSIGN_OP;
                        } else {
                            token = UNKNOWN;
                        }
                        break;
                    case '+':
                        addChar();
                        getChar();
                        token = ADD_OP;
                        break;
                    case '-':
                        addChar();
                        getChar();
                        token = SUB_OP;
                        break;
                    case '*':
                        addChar();
                        getChar();
                        token = MULT_OP;
                        break;
                    case '/':
                        addChar();
                        getChar();
                        token = DIV_OP;
                        break;
                    case '(':
                        addChar();
                        getChar();
                        token = LEFT_PAREN;
                        break;
                    case ')':
                        addChar();
                        getChar();
                        token = RIGHT_PAREN;
                        break;
                    case ';':
                        addChar();
                        getChar();
                        token = SEMICOLON;
                        break;
                    case '.':
                        addChar();
                        getChar();
                        token = PERIOD;
                        break;
                    default:
                        addChar();
                        getChar();
                        token = UNKNOWN;
                        break;
                }
                break;
            case EOF:
                token = EOF;
                lexeme[0] = 'E';
                lexeme[1] = 'O';
                lexeme[2] = 'F';
                lexeme[3] = '\0';
                break;
        }
        System.out.printf("Token: %d, Lexeme: %s\n", token, new String(lexeme));
        return token;
    }

    public static void main(String[] args) {
        if (args.length != 1) {
            System.err.println("Usage: Lexer <input_file>");
            System.exit(1);
        }
        try {
            fis = new FileInputStream(args[0]);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
            System.exit(1);
        }
        getChar();
        while (token != EOF) {
            lex();
        }
    }
}

使用该词法分析程序，可以通过以下命令执行：

javac Lexer.java
java Lexer <input_file>

其中 `<input_file>` 是要分析的 PL/0 源代码文件。程序会输出每个 Token 的类型和对应的词素。