编译原理使用Java语言编写PL/0编译程序的语法分析程序

下面是使用Java语言编写PL/0编译程序的语法分析程序的示例代码：

import java.util.ArrayList;
import java.util.List;

public class Parser {
    private Lexer lexer;
    private Token token;
    private List<String> errors;

    public Parser(Lexer lexer) {
        this.lexer = lexer;
        this.errors = new ArrayList<>();
        nextToken();
    }

    public List<String> getErrors() {
        return errors;
    }

    private void nextToken() {
        token = lexer.nextToken();
    }

    private void error(String message) {
        errors.add(message);
    }

    private void match(TokenType expected) {
        if (token.getType() == expected) {
            nextToken();
        } else {
            error("Syntax error: " + expected + " expected, but found " + token.getType() + " at line " + token.getLine());
        }
    }

    private void program() {
        block();
        match(TokenType.PERIOD);
    }

    private void block() {
        if (token.getType() == TokenType.CONST) {
            constDeclaration();
        }
        if (token.getType() == TokenType.VAR) {
            varDeclaration();
        }
        while (token.getType() == TokenType.PROCEDURE) {
            procedureDeclaration();
        }
        statement();
    }

    private void constDeclaration() {
        match(TokenType.CONST);
        do {
            match(TokenType.IDENTIFIER);
            match(TokenType.EQUAL);
            match(TokenType.NUMBER);
        } while (token.getType() == TokenType.COMMA);
        match(TokenType.SEMICOLON);
    }

    private void varDeclaration() {
        match(TokenType.VAR);
        do {
            match(TokenType.IDENTIFIER);
        } while (token.getType() == TokenType.COMMA);
        match(TokenType.SEMICOLON);
    }

    private void procedureDeclaration() {
        match(TokenType.PROCEDURE);
        match(TokenType.IDENTIFIER);
        match(TokenType.SEMICOLON);
        block();
        match(TokenType.SEMICOLON);
    }

    private void statement() {
        if (token.getType() == TokenType.IDENTIFIER) {
            assignmentStatement();
        } else if (token.getType() == TokenType.CALL) {
            callStatement();
        } else if (token.getType() == TokenType.IF) {
            ifStatement();
        } else if (token.getType() == TokenType.WHILE) {
            whileStatement();
        } else if (token.getType() == TokenType.READ) {
            readStatement();
        } else if (token.getType() == TokenType.WRITE) {
            writeStatement();
        } else {
            error("Syntax error: statement expected, but found " + token.getType() + " at line " + token.getLine());
        }
    }

    private void assignmentStatement() {
        match(TokenType.IDENTIFIER);
        match(TokenType.ASSIGN);
        expression();
    }

    private void callStatement() {
        match(TokenType.CALL);
        match(TokenType.IDENTIFIER);
    }

    private void ifStatement() {
        match(TokenType.IF);
        condition();
        match(TokenType.THEN);
        statement();
        if (token.getType() == TokenType.ELSE) {
            match(TokenType.ELSE);
            statement();
        }
    }

    private void whileStatement() {
        match(TokenType.WHILE);
        condition();
        match(TokenType.DO);
        statement();
    }

    private void readStatement() {
        match(TokenType.READ);
        match(TokenType.IDENTIFIER);
    }

    private void writeStatement() {
        match(TokenType.WRITE);
        expression();
    }

    private void condition() {
        expression();
        if (isRelationalOperator()) {
            nextToken();
            expression();
        } else {
            error("Syntax error: relational operator expected, but found " + token.getType() + " at line " + token.getLine());
        }
    }

    private void expression() {
        if (token.getType() == TokenType.PLUS || token.getType() == TokenType.MINUS) {
            nextToken();
        }
        term();
        while (token.getType() == TokenType.PLUS || token.getType() == TokenType.MINUS) {
            nextToken();
            term();
        }
    }

    private void term() {
        factor();
        while (token.getType() == TokenType.TIMES || token.getType() == TokenType.SLASH) {
            nextToken();
            factor();
        }
    }

    private void factor() {
        if (token.getType() == TokenType.IDENTIFIER) {
            match(TokenType.IDENTIFIER);
        } else if (token.getType() == TokenType.NUMBER) {
            match(TokenType.NUMBER);
        } else if (token.getType() == TokenType.LEFT_PAREN) {
            match(TokenType.LEFT_PAREN);
            expression();
            match(TokenType.RIGHT_PAREN);
        } else {
            error("Syntax error: factor expected, but found " + token.getType() + " at line " + token.getLine());
        }
    }

    private boolean isRelationalOperator() {
        return token.getType() == TokenType.EQUAL || token.getType() == TokenType.NOT_EQUAL
                || token.getType() == TokenType.LESS_THAN || token.getType() == TokenType.LESS_THAN_OR_EQUAL
                || token.getType() == TokenType.GREATER_THAN || token.getType() == TokenType.GREATER_THAN_OR_EQUAL;
    }

    public void parse() {
        program();
    }
}

这里使用了一个`Lexer`类来实现词法分析，`Token`类来表示识别出的单词。`Parser`类中的各个方法对应了PL/0语言中的各种语法结构，如`program()`对应程序结构、`constDeclaration()`对应常量声明结构等等。`match()`方法用于匹配当前单词类型是否符合预期，若不符合则输出错误信息。`parse()`方法表示开始语法分析过程，调用`program()`方法即可。在语法分析过程中，如果发现了错误，则将错误信息存入`errors`列表中。