eval.java

用编译原理方法计算表达式的值,例如(2+3)*84+3这样的复杂表达式 深入体现了面向对象的方法,代码可以很容易进行overwrite

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
/**
 * 
 * Course - CS601 OO Programming
 * Instructor - Terence Parr
 * Assignment - 4
 *
 * Eval class reads from standard in, writes to standard out, and errors out to 
 * standard error.   The class passes the stream (stdin) to {@link EvalParser}
 * 
 * The class is able to evaluate expressions of the following nature 
 * 
 * 5+2
 * or
 * 5*4+3
 * or
 * 5*5*7*3+12
 * 
 * The class respects multiplicative precedence
 * 
 * so 5+3*3 evaluates to 45 not 24
 */
public class Eval {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		BufferedReader reader = null;
		try {
			reader = new BufferedReader(new InputStreamReader(System.in));
		
			EvalParser p = new EvalParser(new TokenBuffer(new Lexer(reader, 4), 4));
			System.out.println(p.expression());
		} catch (IllegalArgumentException ex) {
			System.err.println("Expression error: " + ex.getMessage());
		} finally {
			if (reader != null) {
				try { reader.close(); } 
				catch (IOException ex) { 
					System.err.println("Error closing standard in stream: " + ex.getMessage());
					ex.printStackTrace(System.err);
				}
			}
		}
	}
}