substring,subsequence,charAt执行效率的不同

 package com.java.tencent;
 
 public class T_2_longestPalindrome {
 
     public String test1(String s){
         long startTime=System.currentTimeMillis();
         int len = s.length();
         int tmp = 0;
         String result = "";
         for(int i=0;i<len;i++){
             if(tmp+i-len>0){
                 break;
             }
             int lenj = len-1;
             for(int j=lenj;j>=i;j--){
                 if(tmp>j-i+1){
                     break;
                 }
                 String ch = s.substring(i,i+1);
                 String chj = s.substring(j,j+1);
                 if(ch.equals(chj)){
                     String str = s.substring(i,j+1);
                     if(tmp>str.length()){
                         continue;
                     }
                     boolean bl = true;
                     int ln = str.length();
                     int lenm = ln/2+1;
                     for(int m=0;m<lenm;m++){
                         CharSequence start = str.subSequence(m, m+1);
                         CharSequence end = str.subSequence(ln-m-1,ln-m);
                         if(!start.equals(end)){
                             bl = false;
                             break;
                         }
                     }
                     if(bl && ln>tmp){
                         result = str;
                         tmp = ln;
                     }
                 }
             }
         }
         System.out.println(result);
         long endTime=System.currentTimeMillis();
         System.out.println("程序运行时间： "+(endTime-startTime)+"ms");
         return result;
     }
 
     public String test2(String s){
         long startTime=System.currentTimeMillis();
         int len = s.length();
         int tmp = 0;
         String result = "";
         for(int i=0;i<len;i++){
             if(tmp+i-len>0){
                 break;
             }
             for(int j=len-1;j>=i;j--){
                 if(tmp>j-i+1){
                     break;
                 }
                 String str = s.substring(i,j+1);
                 boolean bl = true;
                 for(int m=0;m<(str.length()/2+1);m++){
                     Character start = str.charAt(m);
                     Character end = str.charAt(str.length()-m-1);
                     if(!start.equals(end)){
                         bl = false;
                         break;
                     }
                 }
                 if(bl && str.length()>tmp){
                     result = str;
                     tmp = str.length();
                     break;
                 }
             }
         }
         System.out.println(result);
         long endTime=System.currentTimeMillis();
         System.out.println("程序运行时间： "+(endTime-startTime)+"ms");
         return result;
     }
 
     public static void main(String[] args) {
         T_2_longestPalindrome lp = new T_2_longestPalindrome();
         String s = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabcaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
         lp.test2(s);
     }
 
 }

以上写了一个返回最长回文子串的程序（1000个a，回文字串就是如abcddcba、bcb，左右两边相同，当然a一个字符也是），之前使用的test1总是500以上的执行时间，时间总是无法降低，最后问题落到了两个截取字串位置上，经过修改，test2成功降低执行时间。

test1 使用 sub 截取字符串，执行时间500+ms

test2 使用 charAt，执行时间180ms

以上可以明显看出执行效率的不同。