04_字符串连接

1.字符串连接操作符"+",不适合用在大规模字符串连接场合，由于字符串不可变，2个字符串连接在一起时，它们内容都要被拷贝。

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测试代码：

public class _51StringConcatenation {private static final int NUM_ITEMS = 5000;private static final String LINE_FOR_ITEM = "DJLKFLDLJKFKJLDKJFKJDKFLKDLKL" +"LKJFLKDLKFLKJDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDLKKFDFDJFKLKLDFDFKDF";/** * @param args */public static void main(String[] args) {statement();statement2();String result = statement3();System.out.println(NUM_ITEMS*LINE_FOR_ITEM.length() == result.length());}private static String statement(){String result = "";long start = System.currentTimeMillis();for(int i=0;i<NUM_ITEMS;i++){result += LINE_FOR_ITEM;}System.out.println("String concatention Using Time:"+(System.currentTimeMillis() - start));return result;}private static String statement2(){StringBuilder result = new StringBuilder();long start = System.currentTimeMillis();for(int i=0;i<NUM_ITEMS;i++){result.append(LINE_FOR_ITEM);}System.out.println("String Builder Using Time:"+(System.currentTimeMillis() - start));return result.toString();}private static String statement3(){StringBuilder result = new StringBuilder(NUM_ITEMS*LINE_FOR_ITEM.length());long start = System.currentTimeMillis();for(int i=0;i<NUM_ITEMS;i++){result.append(LINE_FOR_ITEM);}System.out.println("String Builder When Allocat Size Using Time:"+(System.currentTimeMillis() - start));return result.toString();}}

? 打印结果:

?

String concatention Using Time:35172

String Builder Using Time:47

String Builder When Allocat Size Using Time:0

true

具有初始大小StringBuilder性能最好，其次StringBuilder，“+”号连接最差，
测试jdk1.6测试，jdk1.6在编译时，已经把"+"号连接转换成StringBuilder，不知道为何性能还是这么差