我发现我和你的差不多，就是在处理输入输出上不一样，今天弄了一天的RE了
package com.whutjava;

import java.io.BufferedInputStream;
import java.util.Scanner;
import java.util.Stack;

/**
 * 浏览器对象类
 * @author whut
 *
 */
public class Main {

	/**
	 * @param args
	 */
	public static void  main(String[] args){
		
		String homePage = "http://www.acm.org/";
		Main browse = new Main(homePage);
		
		Scanner cin = new Scanner(new BufferedInputStream(System.in)); 
		while(true){
			String command = "";
			command = cin.next();
			if(Main.COMMAND_VISIT.equals(command)){
				command = command + " " + cin.next();
			}
			browse.browse(command);
			
			if(browse.isClose()){
				break;
			}else if(browse.isErrorCommand()){
				System.out.println(Main.IGNORED);
			}else if(browse.isIgnored()){
				System.out.println(Main.IGNORED);
			}else {
				System.out.println(browse.currentPage);
			}
		}
	}
	
	private String currentPage;

	public static final String COMMAND_BACK = "BACK";
	public static final String COMMAND_FORWARD = "FORWARD";
	public static final String COMMAND_VISIT = "VISIT";
	public static final String COMMAND_QUIT = "QUIT";
	
	public static final String IGNORED = "Ignored";
	
	private boolean close = false;
	private boolean errorCommand = false;
	private boolean ignored = false;
	
	public boolean isIgnored() {
		return ignored;
	}

	public void setIgnored(boolean ignored) {
		this.ignored = ignored;
	}

	public boolean isErrorCommand() {
		return errorCommand;
	}

	public void setErrorCommand(boolean errorCommand) {
		this.errorCommand = errorCommand;
	}

	public boolean isClose() {
		return close;
	}

	public void setClose(boolean close) {
		this.close = close;
	}

	/**
	 * URL 存放堆栈, for forward
	 */
	private Stack<String> forwardStack;
	
	/**
	 * 浏览历史堆栈， for back
	 */
	private Stack<String> backStack;
	
	/**
	 * 使用主页初始化的浏览器对象
	 * @param homeURL
	 */
	public Main(String homeURL) {
		super();
		forwardStack = new Stack<String>();
		backStack = new Stack<String>();
		
		currentPage = homeURL;
	}
	
	/**
	 * 浏览行为，输入命令，返回浏览器动作
	 * @param command:浏览命令
	 * @return
	 */
	public void browse(String command){
		this.errorCommand = false;
		this.ignored = false;
		if(COMMAND_BACK.equals(command)){
			//1,处理返回操作,查找浏览历史堆栈
			if(this.backStack.empty()){
				ignored = true;
			}else{
				//历史堆栈出栈, 当前页进栈
				this.forwardStack.push(this.currentPage);
				this.currentPage = this.backStack.pop();
			}
		}else if(COMMAND_FORWARD.equals(command)){
			//1,处理返回操作,查找浏览urlStack
			if(this.forwardStack.empty()){
				ignored = true;
			}else{
				//urlStack堆栈出栈
				this.backStack.push(this.currentPage);
				this.currentPage = this.forwardStack.pop();
			}
		}else if(COMMAND_QUIT.equals(command)){
			this.close = true;
			return;
		}else{
			//处理VISIT 命令和 非法命令
			if(command != null && command.startsWith(COMMAND_VISIT+" http://"))
			{
				String url = command.substring(COMMAND_VISIT.length()+1);
				if(url.length() > 70 || url.indexOf(" ") >= 0)
				{
					//按规定url地址过长，被忽略,或者包含有空格
					errorCommand = true;
					return;
				}
				//当前页面进历史栈，前进栈清空
				this.backStack.push(this.currentPage);
				this.currentPage = url;
				this.forwardStack.clear();
				return;
			}
			//非法命令
			errorCommand = true;
			return;
		}
	}

}