一个java实现的简易编译器

import java.util.Scanner; public class Lexer { public static final int EOI = 0; public static final int SEMI = 1; public static final int PLUS = 2; public static final int TIMES = 3; public static final int LP = 4; public static final int RP = 5; public static final int NUM_OR_ID = 6; private int lookAhead = -1; public String yytext = ""; public int yyleng = 0; public int yylineno = 0; private String input_buffer = ""; private String current = ""; private boolean isAlnum(char c) { if (Character.isAlphabetic(c) == true || Character.isDigit(c) == true) { return true; } return false; } private int lex() { while (true) { while (current == "") { Scanner s = new Scanner(System.in); while (true) { String line = s.nextLine(); if (line.equals("end")) { break; } input_buffer += line; } s.close(); if (input_buffer.length() == 0) { current = ""; return EOI; } current = input_buffer; ++yylineno; current.trim(); }//while (current != "") for (int i = 0; i < current.length(); i++) { yyleng = 0; yytext = current.substring(0, 1); switch (current.charAt(i)) { case ';': current = current.substring(1); return SEMI; case '+': current = current.substring(1); return PLUS; case '*': current = current.substring(1);return TIMES; case '(': current = current.substring(1);return LP; case ')': current = current.substring(1);return RP; case '\n': case '\t': case ' ': current = current.substring(1); break; default: if (isAlnum(current.charAt(i)) == false) { System.out.println("Ignoring illegal input: " + current.charAt(i)); } else { while (isAlnum(current.charAt(i))) { i++; yyleng++; } // while (isAlnum(current.charAt(i))) yytext = current.substring(0, yyleng); current = current.substring(yyleng); return NUM_OR_ID; } break; } //switch (current.charAt(i)) }// for (int i = 0; i < current.length(); i++) }//while (true) }//lex() public boolean match(int token) { if (lookAhead == -1) { lookAhead = lex(); } return token == lookAhead; } public void advance() { lookAhead = lex(); } public void runLexer() { while (!match(EOI)) { System.out.println("Token: " + token() + " ,Symbol: " + yytext ); advance(); } } private String token() { String token = ""; switch (lookAhead) { case EOI: token = "EOI"; break; case PLUS: token = "PLUS"; break; case TIMES: token = "TIMES"; break; case NUM_OR_ID: token = "NUM_OR_ID"; break; case SEMI: token = "SEMI"; break; case LP: token = "LP"; break; case RP: token = "RP"; break; } return token; } }