lexemreader.java

是有关解释器的.用JAVA编写.可以解释一般的JAVA程序.

import java.io.*;

/** Internal class: Used by the scanner * */
public class LexemReader {
    private final int TABSKIP = 4; // number of characters to be counted for a
                                   // tabular character

    // possible to make this a variable and pass the value to the
    // Scanner/LexemReader constructor

    private Reader reader; // character stream containg the source

    private int line; // current line within file

    private int pos; // position of the current character within the current
                     // line

    private int lookAhead = -2; // -2 ==> no character has been peeked at

    LexemReader(String fileName) throws ScannerException {
        try {
            reader = new FileReader(fileName);
        } catch (FileNotFoundException e) {
            throw new ScannerException(line, pos, "File <" + fileName
                    + "> not found");
        }

        line = 1;
        pos = 0;
    }

    int line() {
        return line;
    }

    int pos() {
        return pos;
    }

    // read a character from the stream without interpreting it
    int fetchCharacter() throws ScannerException {
        int ch;

        try {
            ch = reader.read();
        } catch (IOException e) {
            throw new ScannerException(line, pos, "IO Error");
        }

        return ch;
    }

    // enable peeking at the very next character
    // store it so that it will be used as the next logical character
    char followingCharacter() throws ScannerException {
        lookAhead = fetchCharacter();
        return (char) lookAhead;
    }

    // read the next character, interpreting it in terms of updating the
    // position
    // take a possible previous lookahead read into account
    // this and the next method return an integer result because EOF is returned
    // as -1
    int nextCharacter() throws ScannerException {
        int ch;

        if (lookAhead != -2) {
            ch = lookAhead;
            lookAhead = -2;
        } else
            ch = fetchCharacter();

        // update position markers
        switch (ch) {
        case '\t':
            pos += TABSKIP;
            break;

        case '\r':
            break;

        case '\n':
            line++;
            pos = 0;
            break;

        default:
            pos++;
        }

        return ch;
    }

    // read the next character skipping all whitespace in the process
    int nextNonWhiteCharacter() throws ScannerException {
        int ch;

        do {
            ch = nextCharacter();
        } while (isWhiteSpace((char) ch));

        return ch;
    }

    // answer whether the argument is a whitespace character
    private boolean isWhiteSpace(char c) {
        switch (c) {
        case ' ':
        case '\t':
        case '\r':
        case '\n':
            return true;

        default:
            return false;
        }
    }
}