postfix.java

来自「Java编写后缀表达式计算器, 用于输出生成后缀表达式, 程序包含完整的Docu」· Java 代码 · 共 96 行

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

package postfix;
import java.io.*;
import exception.SyntaxException;
import exception.LexException;
import exception.ExceptionContainer;
/**
 * 老师给的代码，经过必要的改造
 * @author zouhao
 */

class Parser {
	static int lookahead;
        int index;
        String input;
        BufferedReader keyboard;
	public Parser() throws IOException {
                keyboard = new BufferedReader(new InputStreamReader(System.in));
                input = keyboard.readLine();
                index = 0;
                lookahead = input.charAt(index);
        }

	void expr() throws IOException {
		term();
		rest();
	}

	void rest() throws IOException {
            try{
                while(lookahead != '\n'){
                    if (lookahead == '+') {
                            match('+');
                            term();
                            System.out.write('+');
                            if(lookahead != '\n'){continue;}
                            else {break;}
                    } else if (lookahead == '-') {
                            match('-');
                            term();
                            System.out.write('-');
                            if(lookahead != '\n'){continue;}
                            else {break;}
                    } else if(Character.isDigit((char)lookahead)){
                            throw new SyntaxException(index,"miss operator");
                    } else throw new LexException(index);
                    }
                }
                catch (SyntaxException e){}
                catch (LexException e){}
        }
       	

	void term() throws IOException {
		try
                    {if (lookahead == '+'||lookahead == '-') {
                        throw new SyntaxException(index,"miss operand");
                    }else if(Character.isDigit((char)lookahead)) {
                   	System.out.write((char)lookahead);
			match(lookahead);
                    }
                    else  throw new LexException(index);
                }
                catch (SyntaxException e){}
                catch (LexException e){}
	}

	void match(int t) throws IOException {
                if (lookahead == t) {
                    if(index<input.length()-1)
                    { 
                        index++;
                        lookahead = input.charAt(index);
                    }
                    else lookahead = '\n';
                 }
                else{

                }
        }
}
public class Postfix {
    public static void main(String[] args) throws IOException {
		System.out.println("Input an infix expression and output its postfix notation:");
		Parser parser = new Parser();
                parser.expr();
                ExceptionContainer temp = ExceptionContainer.getinstance();
                temp.printexception(parser.input);
		System.out.println("\nEnd of program.");
	}
}