java字符串连接相加实例源码，
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。
测试代码：
Java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
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