爲什麼會出現那麼多比較String和StringBuffer的文章?
原因在於當改變字符串內容時,採用StringBuffer能獲得更好的性能。既然是爲了獲得更好的性能,那麼採用StringBuffer能夠獲得最好的性能嗎?
答案是NO!
爲什麼?
如果你讀過《Think in Java》,而且對裏面描述HashTable和HashMap區別的那部分章節比較熟悉的話,你一定也明白了原因所在。對,就是支持線程同步保證線程安全而導致性能下降的問題。HashTable是線程安全的,很多方法都是synchronized方法,而HashMap不是線程安全的,但其在單線程程序中的性能比HashTable要高。StringBuffer和StringBuilder類的區別也在於此,新引入的StringBuilder類不是線程安全的,但其在單線程中的性能比StringBuffer高。如果你對此不太相信,可以試試下面的例子:
package com.hct.test;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
/**
* @author: chengtai.he
* @created:2009-12-9 上午09:59:57
*/
public class StringBuilderTester {
private static final String base = " base string. ";
private static final int count = 2000000;
public static void stringTest() {
long begin, end;
begin = System.currentTimeMillis();
String test = new String(base);
for (int i = 0; i < count/100; i++) {
test = test + " add ";
}
end = System.currentTimeMillis();
System.out.println((end - begin)
+ " millis has elapsed when used String. ");
}
public static void stringBufferTest() {
long begin, end;
begin = System.currentTimeMillis();
StringBuffer test = new StringBuffer(base);
for (int i = 0; i < count; i++) {
test = test.append(" add ");
}
end = System.currentTimeMillis();
System.out.println((end - begin)
+ " millis has elapsed when used StringBuffer. ");
}
public static void stringBuilderTest() {
long begin, end;
begin = System.currentTimeMillis();
StringBuilder test = new StringBuilder(base);
for (int i = 0; i < count; i++) {
test = test.append(" add ");
}
end = System.currentTimeMillis();
System.out.println((end - begin)
+ " millis has elapsed when used StringBuilder. ");
}
public static String appendItemsToStringBuiler(List list) {
StringBuilder b = new StringBuilder();
for (Iterator i = list.iterator(); i.hasNext();) {
b.append(i.next()).append(" ");
}
return b.toString();
}
public static void addToStringBuilder() {
List list = new ArrayList();
list.add(" I ");
list.add(" play ");
list.add(" Bourgeois ");
list.add(" guitars ");
list.add(" and ");
list.add(" Huber ");
list.add(" banjos ");
System.out.println(StringBuilderTester.appendItemsToStirngBuffer(list));
}
public static String appendItemsToStirngBuffer(List list) {
StringBuffer b = new StringBuffer();
for (Iterator i = list.iterator(); i.hasNext();) {
b.append(i.next()).append(" ");
}
return b.toString();
}
public static void addToStringBuffer() {
List list = new ArrayList();
list.add(" I ");
list.add(" play ");
list.add(" Bourgeois ");
list.add(" guitars ");
list.add(" and ");
list.add(" Huber ");
list.add(" banjos ");
System.out.println(StringBuilderTester.appendItemsToStirngBuffer(list));
}
public static void main(String[] args) {
stringTest();
stringBufferTest();
stringBuilderTest();
addToStringBuffer();
addToStringBuilder();
}
}
上面的程序結果如下:
5266 millis has elapsed when used String.
375 millis has elapsed when used StringBuffer.
281 millis has elapsed when used StringBuilder.
I play Bourgeois guitars and Huber banjos
I play Bourgeois guitars and Huber banjos
從上面的結果來看,這三個類在單線程程序中的性能差別一目瞭然,採用String對象時,即使運行次數僅是採用其他對象的1/100,其執行時間仍然比其他對象高出25倍以上;而採用StringBuffer對象和採用StringBuilder對象的差別也比較明顯,前者是後者的1.5倍左右。由此可見,如果我們的程序是在單線程下運行,或者是不必考慮到線程同步問題,我們應該優先使用StringBuilder類;當然,如果要保證線程安全,自然非StringBuffer莫屬了。
除了對多線程的支持不一樣外,這兩個類的使用幾乎沒有任何差別,上面的例子就是個很好的說明。appendItemsToStringBuiler和appendItemsToStirngBuffer兩個方法除了採用的對象分別爲StringBuilder和StringBuffer外,其他完全相同,而效果也完全相同。
轉載自《再探java基礎——StringBuilder與StringBuffer的區別》