标签：log 对象 执行 因此 target return 修改 java字符串 mic
JDK 1.8(Java 8)里新增String.join()方法用于字符串连接。本文基于《Java实现String.join()和效率比较》一文，分析和比较四种自定义实现与String.join()方法的效率，并纠正原文的一些错误。
代码示例如下：
1 public class Test { 2 public static void main(String[] args) { 3 String[] strOri = {"a","b","c","d","e","f","g","h"}; //同new string[]{"a","b","c","d","e","f","g","h"} 4 String strRes = ""; int loops = 100000; 5 6 Date date = new Date(); 7 8 for(int i = 0; i < loops; i++) {strRes = join1(strOri, ":");} date = recordTime(date, 1); //1 9 for(int i = 0; i < loops; i++) {strRes = join2(strOri, ":");} date = recordTime(date, 2); 10 for(int i = 0; i < loops; i++) {strRes = join3(strOri, ":");} date = recordTime(date, 3); 11 for(int i = 0; i < loops; i++) {strRes = join4(strOri, ":");} date = recordTime(date, 4); 12 for(int i = 0; i < loops; i++) {strRes = join5(strOri, ":");} date = recordTime(date, 5); 13 14 long startTime = System.currentTimeMillis(); //2 15 for(int i = 0; i < loops; i++) {strRes = join5(strOri, ":");} 16 long endTime = System.currentTimeMillis(); System.out.println("5c:{" + strRes + "} costs " + (endTime-startTime) + "ms"); 17 18 startTime = System.nanoTime(); 19 for(int i = 0; i < loops; i++) {strRes = join5(strOri, ":");} 20 endTime = System.nanoTime(); System.out.println("5n:{" + strRes + "} costs " + (endTime-startTime) + "ns"); 21 } 22 23 private static void recordTime_Wrong(Date date, int no) { 24 System.out.println(no + ": costs " + (new Date().getTime()-date.getTime()) + "ms"); 25 date = new Date(); 26 } 27 private static Date recordTime(Date date, int no) { 28 System.out.println(no + ": costs " + (new Date().getTime()-date.getTime()) + "ms"); 29 return new Date(); 30 } 31 32 private static String join1(String[] strOri, String delimiter) { 33 StringBuffer sb = new StringBuffer(); //3 34 for(String s : strOri) { 35 sb.append(s+delimiter); //4 36 } 37 return sb.toString().substring(0, sb.toString().length()-1); 38 } 39 40 private static String join2(String[] strOri, String delimiter) { 41 StringBuffer sb = new StringBuffer(); 42 for(String s : strOri) { 43 sb.append(s+delimiter); 44 } 45 String s = sb.toString(); 46 return s.substring(0, s.length()-1); 47 } 48 49 private static String join3(String[] strOri, String delimiter) { 50 StringBuffer sb = new StringBuffer(); 51 for(int i = 0; i < strOri.length; i++) { 52 if (i != strOri.length-1) { 53 sb.append(strOri[i]+delimiter); 54 } else { 55 sb.append(strOri[i]); 56 } 57 } 58 return sb.toString(); 59 } 60 61 private static String join4(String[] strOri, String delimiter) { 62 StringBuilder stringBuilder = new StringBuilder(); 63 for (int i = 0; i < strOri.length-1; i++) { 64 stringBuilder.append(strOri[i]).append(delimiter); 65 } 66 stringBuilder.append(strOri[strOri.length-1]); 67 return stringBuilder.toString(); 68 } 69 70 private static String join5(String[] strOri, String delimiter) { 71 return String.join(delimiter, strOri); //5 72 } 73 }
选取三次运行输出结果如下：
1: costs 930ms
2: costs 902ms
3: costs 637ms
4: costs 230ms
5: costs 364ms
5c:{a:b:c:d:e:f:g:h} costs 413ms
5n:{a:b:c:d:e:f:g:h} costs 286466296ns
1: costs 834ms
2: costs 788ms
3: costs 576ms
4: costs 248ms
5: costs 350ms
5c:{a:b:c:d:e:f:g:h} costs 384ms
5n:{a:b:c:d:e:f:g:h} costs 283256112ns
1: costs 774ms
2: costs 728ms
3: costs 605ms
4: costs 297ms
5: costs 417ms
5c:{a:b:c:d:e:f:g:h} costs 280ms
5n:{a:b:c:d:e:f:g:h} costs 279838638ns
可见，join4()执行最快，其次是join5()。join1()和join2()执行效率接近，前者调用两次toString()，故效率略低。
总结如下：
1. 原文recordTime(即本文recordTime_Wrong)方法中，无法通过"date = new Date()"修改外部的date引用(根因详见《java中的传值与传引用》)。这会导致每次调用recordTime()时，起始时间始终是"Date date = new Date()"获得的对象(表现为join*耗时递增)。
2. 查看Java源码可知，new Date()其实就是调用System.currentTimeMillis()：
1 public Date() { 2 this(System.currentTimeMillis()); //相当于Date(System.currentTimeMillis()) 3 }
可以使用new Date().getTime()获取当前时间戳(毫秒)。注意，该毫秒数一般以1970-01-01 00:00:00为参考点，但东八区要加上时区，即以1970-01-01 08:00:00为参考时间。此外，通过getTime()获取毫秒数效率不如System.currentTimeMillis()，后者返回自1970年1月1日0时起的毫秒数。
System.nanoTime()的计时精度不保证一定高于System.currentTimeMillis()，但可保证数值递增(后者相减时可能产生负值)。
若要对代码进行更准确的计时，可参考《How do I write a correct micro-benchmark in Java?》一文。
3. StringBuffer对象是线程安全的，其方法都是同步的(synchronized)。临时变量应使用StringBuilder(效率更高)，避用StringBuffer。
4. 在循环内部，不要使用append(a+b)的形式，而应改为append(a).append(b)。
5. String.join()内部使用StringBuilder实现，因此join5()性能接近join4()。当然，String.join()的功能比join4()更多。
标签：log 对象 执行 因此 target return 修改 java字符串 mic
原文地址：http://www.cnblogs.com/clover-toeic/p/7894437.html
