【一】 for循环方式

StringBuffer sb = new StringBuffer();
for(int i=0;i<=list.size()-1;i++){
  if(i<list.size()-1){
     sb.append(list.get(i) + ",");
    }else {
     sb.append(list.get(i));
     }
 }
}
//或者
for(int i=0;i<=list.size()-1;i++){
  sb.append(list.get(i));
  if(i != list.size()-1){
  sb.append(",");
  }
}

【二】apache工具类中StringUtils方式

import org.apache.commons.lang.StringUtils;
String join = StringUtils.join(list, ",");

【三】list.stream的方式

String join = list.stream().map(String::valueOf).collect(Collectors.joining(","));

【四】lambda循环的方式

StringBuffer sb = new StringBuffer();
list.stream().forEach(str->{
    sb.append(str).append(",");
});
sb.deleteCharAt(sb.length()-1);
return sb.toString();

【五】谷歌工具类

import com.google.common.base.Joiner;
String result = Joiner.on(",").join(list);

【六】String自带的join方法

String join = String.join(",", list);