问题描述

编译原理scanner的java代码

package lexer;

public class Token {

public final int tag;public Token(int t) {    tag = t;}public String toString() {    return """" + (char) tag;}

}

package lexer;

public class Tag {
public final static int AND = 256 BASIC = 257 BREAK = 258 DO = 259
ELSE = 260 EQ = 261 FALSE = 262 GE = 263 ID = 264 IF = 265
INDEX = 266 LE = 267 MINUS = 268 NE = 269 NUM = 270 OR = 271
REAL = 272 TEMP = 273 TRUE = 274 WHILE = 275;
}

package lexer;

public class Real extends Token {
public final float value;

public Real(float v) {    super(Tag.REAL);    value = v;}public String toString() {    return """" + value;}

}

package lexer;

public class Num extends Token {

public final int value;public Num(int v) {    super(Tag.NUM);    value = v;}public String toString() {    return """" + value;}

}

package lexer;

public class Word extends Token {
public String lexeme = """";

public Word(String s int tag) {    super(tag);    lexeme = s;}public String toString() {    return lexeme;}public static final Word and = new Word(""&&"" Tag.AND) or = new Word(""||""        Tag.OR) eq = new Word(""=="" Tag.EQ) ne = new Word(""!="" Tag.NE)        le = new Word(""<="" Tag.LE) ge = new Word("">="" Tag.GE)        minus = new Word(""minus"" Tag.MINUS) True = new Word(""true""                Tag.TRUE) False = new Word(""false"" Tag.FALSE)        temp = new Word(""t"" Tag.TEMP);

}

package lexer;

import java.io.IOException;
import java.util.Hashtable;

public class Lexer {

public static int line = 1;char peek = ' ';Hashtable words = new Hashtable();void reserve(Word w) {    words.put(w.lexeme w);}public Lexer() {    reserve(new Word(""if"" Tag.IF));    reserve(new Word(""else"" Tag.ELSE));    reserve(new Word(""while"" Tag.WHILE));    reserve(new Word(""do"" Tag.DO));    reserve(new Word(""break"" Tag.BREAK));    reserve(Word.True);    reserve(Word.False);}public void readch() throws IOException {    peek = (char) System.in.read();}boolean readch(char c) throws IOException {    readch();    if (peek != c) {        return false;    }    peek = ' ';    return true;}public Token scan() throws IOException {    for (;; readch()) {        if (peek == ' ' || peek == 't')            continue;        else if (peek == 'n') {            line += 1;        } else {            break;        }    }    switch (peek) {    case '&':        if (readch('&'))            return Word.and;        else            return new Token('&');    case '|':        if (readch('|'))            return Word.or;        else            return new Token('|');    case '=':        if (readch('='))            return Word.eq;        else            return new Token('=');    case '!':        if (readch('='))            return Word.ne;        else            return new Token('!');    case '<':        if (readch('='))            return Word.le;        else            return new Token('<');    case '>':        if (readch('='))            return Word.ge;        else            return new Token('>');    }    if (Character.isDigit(peek)) {        int v = 0;        do {            v = 10 * v + Character.digit(peek 10);            readch();        } while (Character.isDigit(peek));        if (peek != '.')            return new Num(v);        float x = v;        float d = 10;        for (;;) {            readch();            if (!Character.isDigit(peek))                break;            x = x + Character.digit(peek 10) / d;            d = d * 10;        }        return new Real(x);    }    if (Character.isLetter(peek)) {        StringBuffer b = new StringBuffer();        do {            b.append(peek);            readch();        } while (Character.isLetterOrDigit(peek));        String s = b.toString();        Word w = (Word) words.get(s);        if (w != null)            return w;        w = new Word(s Tag.ID);        words.put(s w);        return w;    }    Token tok = new Token(peek);    peek = ' ';    return tok;}public void out() {    System.out.println(words.size());}public char getPeek() {    return peek;}public void setPeek(char peek) {    this.peek = peek;}

}

package main;

import java.io.IOException;
import java.util.Hashtable;

import lexer.Lexer;
import lexer.Token;

public class Main {
public static void main(String[] args) throws IOException {
Lexer lexer = new Lexer();
char c;
do {
Token token=lexer.scan();
switch (token.tag) {
case 270:
case 272:
System.out.println(""(NUM +token.toString()+"")"");
break;
case 264:
System.out.println(""(ID +token.toString()+"")"");
break;
case 256:
case 257:
case 258:
case 259:
case 260:
case 265:
case 274:
case 275:
System.out.println(""(KEY +token.toString()+"")"");
break;
case 13:
break;
default:
System.out.println(""(SYM +token.toString()+"")"");
break;
}

    } while (lexer.getPeek()!='n');}

}

怎么把上面从控制台输入的代码改成读取txt文件输入

解决方案

http://outofmemory.cn/code-snippet/2242/java-usage-BufferedReader-duqu-console-out