string stringbuffer stringbuilder区别

string 是不可变类，内部数据结构为final char数组，字符串拼接时会创建新的string对象，从而导致性能问题，性能会差别2000倍左右（基准stringbuilder执行时间）
stringbuffer是可变类，并且是线程安全的，synchronized方法构成，内部数据结构为char数组，其字符串拼接是基于char数组的拼接，期间并不创建新对象（对比string），但由于synchronized标识方法签名，导致一定效率下降，其性能为2倍左右（基准stringbuilder执行时间）
stringbuilder同样是可变类，内部数据结构跟stringbuffer相同，不同点是没有synchronized方法签名，其设计思路与hashtable hashmap类似，为了对线程安全不敏感的地方提供了一种提升效率的可能（亦或者单线程调用本身就是线程安全的），其性能相对为最高

测试代码 借鉴于：http://www.cnblogs.com/Fskjb/archive/2010/04/19/1715176.html

package daily.y2016.m3.d07.a1;

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();
}
}