看高手代码--从小case学大道理

看到这个帖子，自己也写了一个测试例子，但是结果好像不一样，是不是我的例子写的有问题吗？

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class TestGetMessage {
    private static final String STATUS_305 = "Use Proxy";
    private static final String STATUS_307 = "Temporary Redirect";
    private static final String STATUS_400 = "Bad Request";
    private static final String STATUS_401 = "Unauthorized";
    private static final String STATUS_402 = "Payment Required";
    private static final String STATUS_403 = "Forbidden";
    private static final String STATUS_404 = "Not Found";
    private static final String STATUS_405 = "Method Not Allowed";
    private static final String STATUS_406 = "Not Acceptable";
    private static final String STATUS_407 = "Proxy Authentication Required";
    private static final String STATUS_408 = "Request Timeout";
    
    static Map<String, String> httpStatusCodeMappings = new ConcurrentHashMap<String, String>();
    static {
        httpStatusCodeMappings.put("sc.305", STATUS_305);
        httpStatusCodeMappings.put("sc.307", STATUS_307);
        httpStatusCodeMappings.put("sc.400", STATUS_400);
        httpStatusCodeMappings.put("sc.401", STATUS_401);
        httpStatusCodeMappings.put("sc.402", STATUS_402);
        httpStatusCodeMappings.put("sc.403", STATUS_403);
        httpStatusCodeMappings.put("sc.404", STATUS_404);
        httpStatusCodeMappings.put("sc.405", STATUS_405);
        httpStatusCodeMappings.put("sc.406", STATUS_406);
        httpStatusCodeMappings.put("sc.407", STATUS_407);
        httpStatusCodeMappings.put("sc.408", STATUS_408);
    }
    
    public static String getSwitchMessage(int status){
        switch (status) {
            case 305: return STATUS_305;
            case 404: return STATUS_404;
    	}
		return null; 
    }
    
    public static String getMapMessage(int status){
        return httpStatusCodeMappings.get("sc."+ status);
    }


	public static void main(String[] args) {
		int status = 404;
		long loopNum = 100000000L;
		
		long switchStartTime = System.currentTimeMillis();
		for(long i = 0; i < loopNum; i++){
			getSwitchMessage(status);
		}
		System.out.println("switch:"+(System.currentTimeMillis() - switchStartTime));
		
		long mapStartTime = System.currentTimeMillis();
		for(long i = 0; i < loopNum; i++){
			getSwitchMessage(status);
		}
		System.out.println("map:"+(System.currentTimeMillis() - mapStartTime));
	}
}

首先，谁说hash表比case慢一万倍的？？
其实case的代码实现是 if ..else if形式的，复杂度是O(n)
hashMap 的复杂度是 O(1),当然里面的实现有函数调用，（函数调用的效率我没测试过，不过据《代码大全2》里面的资料，java的函数调用所消耗的时间和 普通赋值语句相同。） 就是说hashMap实际操作时间不过 十几个 语句周期。

代码最大的问题是 明明是一堆静态常量，为什么要用 ConcurrentHashMap？？

感觉实际编码的过程是： 开始不知道为什么使用了一个大而全，含义复杂的map，（所以做成同步，key中出现了字符串拼装），后来发现效率不够，于是打补丁，就出现了那堆case。 没什么好说的，不过是一堆临时代码，真正应该做的是重构应该重新定义Map的结构。

其实这种东西效率最高的是 以空间换时间：
String[] httpStatusCode = new int[600];
httpStatusCode[404] = "Not Found";
………………
只是多使用了 600 * 8 = 4.8k空间。

或者最普通的方式，使用：
com.google.common.collect.ImmutableMap

public static int getSwitchMessage(int status){   
        switch (status) {   
            case 305: return 1;   
            case 404: return 2;   
        }   
        return 0;    
    }   
       
    public static int getMapMessage(int status){   
        httpStatusCodeMappings.get("sc."+ status);
        return 0;
    } 

哥们，虽然你有5颗星，但这次不得不反对你了。个人觉得：这是一个非常不错的Tips，不至于看不懂吧？我写的话肯定会在switch语句块前注明为什么要这样写。考虑另外一个问题：

		List someList = new ArrayList();
		try {
			someList = someObject.somdMethod();
			// more
		} catch (Exception e) {
			// do sth
		}

		return someList;

new ArrayList()就是没有必要的，当然啦，也不会太影响性能的，也不会产生可读性的问题，但是这样写真的应该吗？作为程序员，本身就应该精益求精，如果实际中我写了我粘上的这种类似的代码，我感到羞愧，不为别的，因为我想做到更好。

看到这个帖子，自己也写了一个测试例子，但是结果好像不一样，是不是我的例子写的有问题吗？

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class TestGetMessage {
    private static final String STATUS_305 = "Use Proxy";
    private static final String STATUS_307 = "Temporary Redirect";
    private static final String STATUS_400 = "Bad Request";
    private static final String STATUS_401 = "Unauthorized";
    private static final String STATUS_402 = "Payment Required";
    private static final String STATUS_403 = "Forbidden";
    private static final String STATUS_404 = "Not Found";
    private static final String STATUS_405 = "Method Not Allowed";
    private static final String STATUS_406 = "Not Acceptable";
    private static final String STATUS_407 = "Proxy Authentication Required";
    private static final String STATUS_408 = "Request Timeout";
    
    static Map<String, String> httpStatusCodeMappings = new ConcurrentHashMap<String, String>();
    static {
        httpStatusCodeMappings.put("sc.305", STATUS_305);
        httpStatusCodeMappings.put("sc.307", STATUS_307);
        httpStatusCodeMappings.put("sc.400", STATUS_400);
        httpStatusCodeMappings.put("sc.401", STATUS_401);
        httpStatusCodeMappings.put("sc.402", STATUS_402);
        httpStatusCodeMappings.put("sc.403", STATUS_403);
        httpStatusCodeMappings.put("sc.404", STATUS_404);
        httpStatusCodeMappings.put("sc.405", STATUS_405);
        httpStatusCodeMappings.put("sc.406", STATUS_406);
        httpStatusCodeMappings.put("sc.407", STATUS_407);
        httpStatusCodeMappings.put("sc.408", STATUS_408);
    }
    
    public static String getSwitchMessage(int status){
        switch (status) {
            case 305: return STATUS_305;
            case 404: return STATUS_404;
    	}
		return null; 
    }
    
    public static String getMapMessage(int status){
        return httpStatusCodeMappings.get("sc."+ status);
    }


	public static void main(String[] args) {
		int status = 404;
		long loopNum = 100000000L;
		
		long switchStartTime = System.currentTimeMillis();
		for(long i = 0; i < loopNum; i++){
			getSwitchMessage(status);
		}
		System.out.println("switch:"+(System.currentTimeMillis() - switchStartTime));
		
		long mapStartTime = System.currentTimeMillis();
		for(long i = 0; i < loopNum; i++){
			getSwitchMessage(status);
		}
		System.out.println("map:"+(System.currentTimeMillis() - mapStartTime));
	}
}