一个把List<String>转化为以","隔开的字符串的方法

import java.util.ArrayList;
import java.util.List;
/**
 * 集合操作
 * @author intrl
 * @date 2010-12-15
 * @version 1.0
 */
public class Test {
 
    public static void main(String[] args) {
        List<String> list=new ArrayList<String>();
        list.add("aaa");
        list.add("bbb");
        list.add("ccc");
        System.out.println(listToString(list));//aaa,bbb,ccc
    }
     
    public static String listToString(List<String> stringList){
        if (stringList==null) {
            return null;
        }
        StringBuilder result=new StringBuilder();
        boolean flag=false;
        for (String string : stringList) {
            if (flag) {
                result.append(",");
            }else {
                flag=true;
            }
            result.append(string);
        }
        return result.toString();
    }
}

 方法二:

这代码太垃圾了 
为什么不 
int length = arr.length ;
StringBuffer buf = new StringBuffer("");
if(length > 0){
    buf.append(arr[0]);
}
for(int i = 1 ; i < length ; i++){
   buf.append(',');
   buf.append(arr[i]);
}

 方法三:

 apache common包下的StringUtils的join方法:

 StringUtils.join(list, ",");

 以下其源码:

public static String join(Iterator iterator, String separator) {
 
        // handle null, zero and one elements before building a buffer
        if (iterator == null) {
            return null;
        }
        if (!iterator.hasNext()) {
            return EMPTY;
        }
        Object first = iterator.next();
        if (!iterator.hasNext()) {
            return ObjectUtils.toString(first);
        }
 
        // two or more elements
        StringBuffer buf = new StringBuffer(256); // Java default is 16, probably too small
        if (first != null) {
            buf.append(first);
        }
 
        while (iterator.hasNext()) {
            if (separator != null) {
                buf.append(separator);
            }
            Object obj = iterator.next();
            if (obj != null) {
                buf.append(obj);
            }
        }
        return buf.toString();
    }