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分享小段代码求优化
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
用new string的话要60毫秒,
而直接用test的话只要16毫秒,
问题是我必需从bytes中取得后面两位的数据, 前面两位没有用,
有什么办法优化吗?
调试发现是new string比较耗时间,
比如以下代码
StringBuilder sb = new StringBuilder();
long start = System.currentTimeMillis();
byte[] bytes = {1,2,3,4};
for (int i = 0; i < 100000; i++) {
String string = new String(bytes, 2, 2);
// String string = "test";
sb.append(string);
}
long end = System.currentTimeMillis();
System.out.println(end - start);用new string的话要60毫秒,
而直接用test的话只要16毫秒,
问题是我必需从bytes中取得后面两位的数据, 前面两位没有用,
有什么办法优化吗?
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lost_guy_in_scut 2012-12-12
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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);
aotian16 2012-12-12
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不能改结构,
位数不固定, 只有前面两位是无效数据
wapigzhu 2012-12-12
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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);
}
JiinYuu 2012-12-12
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能改bytes的来源的方法么，把他的返回值改为char[]；
如果不能改，你每次取bytes里面的位数都确定么，每次都是最后两个么？
aotian16 2012-12-12
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是比较长的, = =!!!
JiinYuu 2012-12-12
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如果bytes长度不多，可以复制下；如果很长，可能就得不偿失了。
aotian16 2012-12-12
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这个好啊, 可惜我取到的值已经是byte数组了
难道我自己复制下?
JiinYuu 2012-12-12
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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
aotian16 2012-12-12
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1楼的我试过了, 没有什么效果
2楼的, 问题是byte数组不是固定的, 也就是string也是变化的
faping_cao 2012-12-12
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String string = new String(bytes, 2, 2);
放在for外面，
还是说bytes每次不一样？
linao 2012-12-12
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创建String对象放在for循环外，append 100000次就可以了
花谢尊前不敢香 2012-12-12
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StringBuilder sb = new StringBuilder();
long start = System.currentTimeMillis();
byte[] bytes = {1,2,3,4};
String string=null;
for (int i = 0; i < 100000; i++) {
string = new String(bytes, 2, 2);
sb.append(string);
}
long end = System.currentTimeMillis();
System.out.println(end - start);
永恒的罗纳尔多 2012-12-12
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/**
* @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;
}
aotian16 2012-12-12
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貌似还是复制最快了, 大家帮我试试
package pkg;
public class TestBytes {
/**
* @param args
*/
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
byte[] bytes = {1, 2,
97, 98, 97, 98, 97, 98, 97, 98, 97, 98,
97, 98, 97, 98, 97, 98, 97, 98, 97, 98,
97, 98, 97, 98, 97, 98, 97, 98, 97, 98,
97, 98, 97, 98, 97, 98, 97, 98, 97, 98,};
System.out.print(foo1(sb, bytes));
sb.delete(0, sb.length());
System.out.print(",");
System.out.print(foo2(sb, bytes));
sb.delete(0, sb.length());
System.out.print(",");
System.out.print(foo3(sb, bytes));
sb.delete(0, sb.length());
System.out.print(",");
System.out.print(foo4(sb, bytes));
sb.delete(0, sb.length());
System.out.println();
}
/**
* @param sb
* @param bytes
*/
private static long foo4(StringBuilder sb, byte[] bytes) {
long start = System.currentTimeMillis();
for (int i = 0; i < 100000; i++) {
for (int j = 0; j < bytes.length - 2; j++) {
sb.append(bytes[j + 2]);
}
}
long end = System.currentTimeMillis();
return end - start;
}
/**
* @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;
}
/**
* @param sb
* @param bytes
*/
private static long foo2(StringBuilder sb, byte[] bytes) {
long start = System.currentTimeMillis();
String string = null;
for (int i = 0; i < 100000; i++) {
string = new String(bytes, 2, 40);
sb.append(string);
}
long end = System.currentTimeMillis();
return end - start;
}
/**
* @param sb
* @param bytes
*/
private static long foo1(StringBuilder sb, byte[] bytes) {
long start = System.currentTimeMillis();
String string = "abababababababababababababababababababab";
for (int i = 0; i < 100000; i++) {
sb.append(string);
}
long end = System.currentTimeMillis();
return end - start;
}
}
