compare_recursion.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 100 行

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

package comparerecursion;
import java.io.*;
import exception.SyntaxException;
import exception.LexException;
/**
 *
 * @author zouhao
 */
public class Compare_Recursion {
    /*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 * 带递归的版本
 * @author zouhao
 */

	int lookahead;
        int index;
        String input;
        boolean end;
        //BufferedReader keyboard;
	public Compare_Recursion() throws IOException {
                //keyboard = new BufferedReader(new InputStreamReader(System.in));
                input = "1+2+3+4+5+6+7+8+9e";
                index = 0;
                lookahead = input.charAt(index);
                end = false;
        }

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

	void rest() throws IOException {
            if(!end){
            try{  
                    if(lookahead == 'e')end = true;
                    else if (lookahead == '+') {
                            match('+');
                            term();
                            //System.out.write('+');
                            rest();
                    } else if (lookahead == '-') {
                            match('-');
                            term();               term();
                            //System.out.write('-');
                            rest();
                    } 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{

                }
        }
        
}