StringBuilder有多消耗性能?

在看KMP算法时,想要简单的统计一下执行时间和性能。 

得出的结论是： Java的String的indexOf方法性能最好，其次是KMP算法，其次是传统的BF算法，当然，对比有点牵强，SUN的算法也使用Java来实现、用的看着不像是KMP，还需要详细研究一下。

测试代码如下所示：

package com.test.test.kmp;import java.util.Random;public class KMPTest {public static void main(String[] args) {test();}//public static void test() {//int allLen = 8000000;int subLen = 11;int charLen = 4;String cl = buildString(charLen);int times = 50;//testMatch(allLen, subLen, cl, times);}public static void testMatch(int allLen, int subLen, String cl,  int times){int allBF = 0;int allString = 0;int allKMP = 0;int allGC = 0;int allbuild = 0;int alltoArray = 0;long start = System.currentTimeMillis();System.out.println("--------------------------");for (int i = 0; i < times; i++) {// 1. 构造字符串的损耗long buildStart = System.currentTimeMillis();String sub = buildString(subLen, cl);String all = buildString(allLen, sub) +sub;long buildEnd = System.currentTimeMillis();allbuild += (buildEnd - buildStart);// 2. toCharArray的损耗long toArrayStart = System.currentTimeMillis();char[] allArray = all.toCharArray();char[] subArray = sub.toCharArray();long toArrayEnd = System.currentTimeMillis();alltoArray += (toArrayEnd - toArrayStart);//long startBF = System.currentTimeMillis();int indexBF = bfIndexOf(all, sub);long endBF = System.currentTimeMillis();//long timeoutBF = endBF - startBF;allBF += timeoutBF;System.gc();allGC += (System.currentTimeMillis() - endBF);//long startString = System.currentTimeMillis();int indexString = stringIndexOf(all, sub);long endString = System.currentTimeMillis();//long timeoutString = endString - startString;allString += timeoutString;System.gc();allGC += (System.currentTimeMillis() - endString);//long startKMP = System.currentTimeMillis();//int indexKMP = kmpIndexOf(all, sub);int indexKMP = KMP_Index(allArray, subArray);long endKMP = System.currentTimeMillis();//long timeoutKMP = endKMP - startKMP;allKMP += timeoutKMP;System.gc();allGC += (System.currentTimeMillis() - endKMP);////System.out.println("all="+all.substring(0,100)+" ......");//System.out.println("sub="+sub);////System.out.println("indexBF="+indexBF+";耗时: "+timeoutBF+"ms");//System.out.println("indexString="+indexString+";耗时: "+timeoutString+"ms");if(indexBF == indexString && indexKMP == indexString){//System.out.println("!!!!对比相等。");} else {throw new RuntimeException("对比出错.");}//if(i % 20 == 10){System.out.println(i+"次bfIndexOf= "+allBF+"ms");System.out.println(i+"次stringIndexOf= "+allString+"ms");System.out.println(i+"次KMPIndexOf= "+allKMP+"ms");System.out.println(i+"次allbuild= "+allbuild+"ms");System.out.println(i+"次alltoArray= "+alltoArray+"ms");System.out.println(i+"*3次,GC= "+allGC+"ms");long end = System.currentTimeMillis();long allTime = end-start;long lossTime = allTime - (allBF+allString+allKMP+allGC);System.out.println(i+"次,所有总计耗时: "+(allTime)+"ms; 损耗:" + lossTime +"ms; 损耗比: " +((0.0+lossTime)/allTime * 100)+"%");System.out.println("--------------------------");}}//System.out.println(times+"次bfIndexOf;总计耗时: "+allBF+"ms");System.out.println(times+"次stringIndexOf;总计耗时: "+allString+"ms");System.out.println(times+"次KMPIndexOf;总计耗时: "+allKMP+"ms");System.out.println(times+"次allbuild= "+allbuild+"ms");System.out.println(times+"次alltoArray= "+alltoArray+"ms");System.out.println(times+"*3次,GC;总计耗时: "+allGC+"ms");long end = System.currentTimeMillis();long allTime = end-start;long lossTime = allTime - (allBF+allString+allKMP+allGC);System.out.println(times+"次,所有总计耗时: "+(allTime)+"ms; 损耗:" + lossTime +"ms; 损耗比: " +((0.0+lossTime)/allTime * 100)+"%");System.out.println("--------------------------");}//// 构建字符public static String buildString(int len){return buildString(len, null);}public static String buildString(int len, String sub){//final int TYPE_NUMBER = 0;final int TYPE_LOWERCASE = 1;final int TYPE_UPPERCASE = 2;//long seed = System.nanoTime();Random random = new Random(seed);////StringBuilder builder = new StringBuilder();for (int i = 0; i < len; i++) {//int type = random.nextInt(3);// 0-2int cvalue = 0;if(TYPE_NUMBER == type){cvalue = '0' + random.nextInt(10);// 0~(n-1)} else if(TYPE_LOWERCASE == type){cvalue = 'a' + random.nextInt(26);// 0~(n-1)} else if(TYPE_UPPERCASE == type){cvalue = 'A' + random.nextInt(26);// 0~(n-1)} else {throw new RuntimeException(Random.class.getName()+"#newxtInt(int);Wrong!");}////cvalue = 'A' + random.nextInt(26);// 0~(n-1)//char c = (char)cvalue;if(null != sub && sub.length() > 1){int index = random.nextInt(sub.length());c = sub.charAt(index);}//String s = String.valueOf(c);builder.append(c);//}//return builder.toString();}/** * Java自带的方法,内部实现了 * @param all * @param sub * @return */public static int stringIndexOf(String all, String sub){// 防御式编程if(null == all || null== sub){return -1;}// 调用Java的String方法return all.indexOf(sub);}/** * BF算法 * @param all * @param sub * @return */public static int bfIndexOf(String all, String sub){// 防御式编程if(null == all || null== sub){return -1;}//int lenAll = all.length();int lenSub = sub.length();int i = 0;while (i < lenAll) {// 从i下标开始对比int j = 0;while (j < lenSub) {//char all_i = all.charAt(i);char sub_j = sub.charAt(j);if(all_i == sub_j){// 相等则继续对比下一个字符i++;j++; // 这个增长很重要} else {// 否则跳出内层循环break;}}// 如果 j 增长到和 lenSub相等,则匹配成功if(lenSub == j){return i - lenSub;} else {i = 1+i - j; // 回溯 i}}//return -1;}/** * KMP 算法 * @param all * @param sub * @return */public static int kmpIndexOf(String all, String sub){//char[] allArray = all.toCharArray();  char[] subArray = sub.toCharArray();  //return KMP_Index(allArray, subArray);}    /**      * 获得字符串的next函数值      *       * @param t      *            字符串      * @return next函数值      */      public static int[] next(char[] t) {          int[] next = new int[t.length];          next[0] = -1;          int i = 0;          int j = -1;          while (i < t.length - 1) {              if (j == -1 || t[i] == t[j]) {                  i++;                  j++;                  if (t[i] != t[j]) {                      next[i] = j;                  } else {                      next[i] = next[j];                  }              } else {                  j = next[j];              }          }          return next;      }        /**      * KMP匹配字符串      *       * @param allArray      *            主串      * @param subArray      *            模式串      * @return 若匹配成功，返回下标，否则返回-1      */  public static int KMP_Index(char[] allArray, char[] subArray) {          int[] next = next(subArray);          int i = 0;          int j = 0;          while (i <= allArray.length - 1 && j <= subArray.length - 1) {              if (j == -1 || allArray[i] == subArray[j]) {                  i++;                  j++;              } else {                  j = next[j];              }          }          if (j < subArray.length) {              return -1;          } else              return i - subArray.length; // 返回模式串在主串中的头下标      }  }

测试的一个结果如下所示：

--------------------------10次bfIndexOf= 94ms10次stringIndexOf= 56ms10次KMPIndexOf= 76ms10次allbuild= 5870ms10次alltoArray= 137ms10*3次,GC= 155ms10次,所有总计耗时: 6388ms; 损耗:6007ms; 损耗比: 94.03569192235442%--------------------------30次bfIndexOf= 365ms30次stringIndexOf= 222ms30次KMPIndexOf= 295ms30次allbuild= 16452ms30次alltoArray= 395ms30*3次,GC= 452ms30次,所有总计耗时: 18184ms; 损耗:16850ms; 损耗比: 92.66388033435987%--------------------------50次bfIndexOf;总计耗时: 550ms50次stringIndexOf;总计耗时: 335ms50次KMPIndexOf;总计耗时: 450ms50次allbuild= 26501ms50次alltoArray= 637ms50*3次,GC;总计耗时: 734ms50次,所有总计耗时: 29211ms; 损耗:27142ms; 损耗比: 92.91705179555647%--------------------------

从中可以看出来，使用StringBuilder构造字符串，800万*50次append大约消耗了26秒。换算下来每秒钟是1600万次。。。

看来是需要写一个专门计时的函数，本来Junit是有自己的统计的，但是样本不太好做。

如此看来，数据的准备，也就是测试用例的选取很关键,可能需要先生成并写入到文本文件中。