小段代码求优化
我的一个程序里有个方法比较耗时,
调试发现是new string比较耗时间,

比如以下代码

[解决办法]
StringBuilder sb = new StringBuilder();

long start = System.currentTimeMillis();
char[] bytes = {1,2,3,4};
for (int i = 0; i < 100000; i++) {
sb.append(bytes, 2, 2);
}
long end = System.currentTimeMillis();

System.out.println(end - start);

只要5ms
int是可以直接当做char的
[解决办法]

public static void main(String[] args) {
long start = System.currentTimeMillis();
byte[] bytes = { 1, 2, 3, 4 };
byte[] target = new byte[1024 * 1024];
int currentIndex = 0;
for (int i = 0; i < 10000000; i++) {
if(currentIndex + 2 > target.length){
byte[] newTarget = new byte[target.length * 3 / 2];
System.arraycopy(target, 0, newTarget, 0, currentIndex);
target = newTarget;
}
System.arraycopy(bytes, 2, target, currentIndex, 2);
currentIndex += 2;
}
String str = new String(target, 0, currentIndex);
System.out.println(str.length());
long end = System.currentTimeMillis(); 
 
System.out.println(end - start);
}

次数改成10000000了，你那个方法要2s多，这个600多ms
[解决办法]

StringBuilder sb = new StringBuilder();
        long start = System.currentTimeMillis();
        byte[] bytes = {1,2,3,4};
        String string=null;
        byte[]  des = new byte[2]; 
        for (int i = 0; i < 100000; i++) {
        System.arraycopy(bytes, 2, des, 0, 2);
            string =Arrays.toString(des);
            sb.append(string);
        }
        long end = System.currentTimeMillis();
        System.out.println(end - start);

[解决办法]
 /**
     * @param sb
     * @param bytes
     */
    private static long foo3(StringBuilder sb, byte[] bytes) {
        long start = System.currentTimeMillis();
 
        for (int i = 0; i < 100000; i++) {
            char[] chars = new char[40];
            for (int j = 0; j < chars.length; j++) {
                chars[j] = (char) bytes[j + 2];
            }
            sb.append(chars);
 
        }
        long end = System.currentTimeMillis();
 
        return end - start;
    }

如果byte[]是由中文字符串获取的，将byte直接转换为char是错误的做法，

    private static String foo( byte[] bytes) {

        int time = 100000;
        int size = 0;
        byte[] buff = new byte[time<<2];
        for (int i = 0; i < time; i++) {
        int contentSize = bytes.length-2;
            if(buff.length-size<contentSize){
            int tmpLength = buff.length;
            while(tmpLength-size<contentSize){
            tmpLength=tmpLength<<1; 
 
            }
            byte[] tmp = new byte[tmpLength];
            System.arraycopy(buff,0,tmp,0,size);
            buff=tmp;
            }
            System.arraycopy(bytes, 2, buff, size, contentSize);
            size  +=contentSize;
        }
        String result = new String(buff,0,size);
       return result;      
  
    }