java实现词法分析程序

Java实现词法分析程序可以使用正则表达式和有限状态自动机（DFA）来识别和处理输入的代码。

以下是一个简单的Java程序，用于从输入代码中提取标识符、关键字和运算符等词汇元素：

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Lexer {
    private static final String[] KEYWORDS = {"int", "float", "double", "if", "else", "for", "while", "do", "switch", "case", "break", "continue", "return"};
    private static final String IDENTIFIER_REGEX = "[a-zA-Z][a-zA-Z0-9]*";
    private static final String INTEGER_REGEX = "\\d+";
    private static final String FLOAT_REGEX = "\\d+\\.\\d+";
    private static final String OPERATOR_REGEX = "[+\\-*/%]|==|!=|<=|>=|<|>";
    private static final String WHITESPACE_REGEX = "\\s+";
    
    public static void main(String[] args) {
        String code = "int x = 10;\nfloat y = 3.14;\nif (x < y) {\n    System.out.println(\"x is less than y\");\n} else {\n    System.out.println(\"x is greater than or equal to y\");\n}";
        Pattern pattern = Pattern.compile(IDENTIFIER_REGEX + "|" + INTEGER_REGEX + "|" + FLOAT_REGEX + "|" + OPERATOR_REGEX + "|" + WHITESPACE_REGEX);
        Matcher matcher = pattern.matcher(code);
        while (matcher.find()) {
            String token = matcher.group();
            if (token.matches(IDENTIFIER_REGEX)) {
                if (isKeyword(token)) {
                    System.out.println(token + " is a keyword");
                } else {
                    System.out.println(token + " is an identifier");
                }
            } else if (token.matches(INTEGER_REGEX)) {
                System.out.println(token + " is an integer");
            } else if (token.matches(FLOAT_REGEX)) {
                System.out.println(token + " is a float");
            } else if (token.matches(OPERATOR_REGEX)) {
                System.out.println(token + " is an operator");
            } else if (token.matches(WHITESPACE_REGEX)) {
                // do nothing
            } else {
                System.out.println("Unknown token: " + token);
            }
        }
    }
    
    private static boolean isKeyword(String token) {
        for (String keyword : KEYWORDS) {
            if (keyword.equals(token)) {
                return true;
            }
        }
        return false;
    }
}

该程序使用正则表达式将输入代码按照标识符、整数、浮点数、运算符和空格进行匹配，并对每个匹配到的词汇元素进行分类和输出。其中，关键字列表通过数组 `KEYWORDS` 存储，识别标识符时需要判断是否为关键字。