用StringJoiner 做字符串拼接

• API 包括 setEmptyValue：设置空值 toString：转换成 String add：添加字符串 merge：从另一个 StringJoiner 合并 length：长度（包括前缀后缀）

StringJoiner stringJoiner = new StringJoiner(",").add("hello").add("world").add("moximoxi");

输出结果：hello,world,moximoxi

StringJoiner stringJoiner = new StringJoiner(",", "[", "]");
 stringJoiner.add("hello");
 stringJoiner.add("world");
 stringJoiner.add("moximoxi");

输出结果：[hello,world,moximoxi]

• JDK1.8 String也封装了stringJoiner 让拼接操作变得更简单

String str = String.join(",", "hello", "world", "moximoxi");