String 时间类型怎么进行比较大小？

package com.zz.bridge.impl;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;

/**
 * 测试类
 * Copyright 2015年6月9日
 * created by txxs
 * all right reserved
 */
public class BridgeTest {
	public static void main(String args[]){
		DateFormat df = new SimpleDateFormat("yyyyMM");
		//获取Calendar实例
		Calendar currentTime = Calendar.getInstance();
		Calendar compareTime = Calendar.getInstance();
	    try {
	    	//把字符串转成日期类型
			currentTime.setTime(df.parse("201406"));
			compareTime.setTime(df.parse("201506"));
		} catch (ParseException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	    //利用Calendar的方法比较大小
	    if (currentTime.compareTo(compareTime) > 0) {
	    	System.out.println("前者时间小");
		}else{
			System.out.println("后边时间大");
		}
	    //转成数字后比较大小
		int startTime = Integer.parseInt("201406");
		int endTime = Integer.parseInt("201506");
		System.out.println(endTime-startTime);
	}
}