lotusswan

相信大家看到過很多比較String和StringBuffer區別的文章，也明白這兩者的區別，然而自從Java 5.0發布以后，我們的比較列表上將多出一個對象了，這就是StringBuilder類。String類是不可變類，任何對String的改變都會引發新的String對象的生成；而StringBuffer則是可變類，任何對它所指代的字符串的改變都不會產生新的對象，可變和不可變類這一對對象已經齊全了，那么為什么還要引入新的StringBuilder類干嗎？相信大家都有此疑問，我也如此。下面，我們就來看看引入該類的原因。

為什么會出現那么多比較String和StringBuffer的文章？原因在于當改變字符串內容時，采用StringBuffer能獲得更好的性能。既然是為了獲得更好的性能，那么采用StringBuffer能夠獲得最好的性能嗎？答案是NO！為什么？如果你讀過《Think in Java》，而且對里面描述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類的區別也在于此，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.jiang.tiger.chap1;

import ?java.util.ArrayList;
import ?java.util.Iterator;
import ?java.util.List;

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：

11359millis?has?elapsed?when?used?String.
344millis?has?elapsed?when?used?StringBuffer.
234millis?has?elapsed?when?used?StringBuilder.
I?play?Bourgeois?guitars?and?Huber?banjos?
I?play?Bourgeois?guitars?and?Huber?banjos?