StringBuilder与StringBuffer的区别
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如果你读过《Think in Java》,而且对里面描述HashTable和HashMap区别的那部分章节比较熟悉的话,你一定也明白了原因所在。对,就是支持线程同步保证线程安全而导致性能下降的问题。HashTable是线程安全的,很多方法都是synchronized方法,而HashMap不是线程安全的,但其在单线程程序中的性能比HashTable要高。StringBuffer和StringBuilder类的区别也在于此,新引入的StringBuilder类不是线程安全的,但其在单线程中的性能比StringBuffer高。如果你对此不太相信,可以试试下面的例子:
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(); }}
上面的程序结果如下:
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
从上面的结果来看,这三个类在单线程程序中的性能差别一目了然,采用String对象时,即使运行次数仅是采用其他对象的1/100,其执行时间仍然比其他对象高出25倍以上;而采用StringBuffer对象和采用StringBuilder对象的差别也比较明显,前者是后者的1.5倍左右。由此可见,如果我们的程序是在单线程下运行,或者是不必考虑到线程同步问题,我们应该优先使用StringBuilder类;当然,如果要保证线程安全,自然非StringBuffer莫属了。
除了对多线程的支持不一样外,这两个类的使用几乎没有任何差别,上面的例子就是个很好的说明。appendItemsToStringBuiler和appendItemsToStirngBuffer两个方法除了采用的对象分别为StringBuilder和StringBuffer外,其他完全相同,而效果也完全相同。
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