需求：Arrays.asList("AA", "BB") --> "AA,BB"
采用stream的方式
List<String> strList = Arrays.asList("AA","BB","CC");
String newStr = strList.stream().collect(Collectors.joining(","));
System.out.println("Output:" + newStr);// Output:AA,BB,CC
使用String.join()函数，给函数传递一个分隔符合一个迭代器，StringJoiner对象会帮助我们完成所有的事情
String newStr = String.join(",", strList);
System.out.println("Output:" + newStr);// Output:AA,BB,CC
Java 7的实现方法
List strList = Arrays.asList("AA","BB","CC");
StringBuilder sb = newStringBuilder();
for(inti =0; i < strList.size(); i++) {
if(i !=0) {
sb.append(",");
}
sb.append(strList.get(i));
}
System.out.println("Output:"+ sb.toString());// Output:AA,BB,CC
