Google Guava常用类Joiner

例：

Joiner joiner = Joiner.on("; ").skipNulls();
  . . .
 return joiner.join("Harry", null, "Ron", "Hermione"); 
 // Harry; Ron; Hermione

如果参数有null且没有用skipNulls()或useForNull(String)会抛NullPointerException
A appendTo(A appendable, Iterable<?> parts)
Appendable接口 可追加
实现类 BufferedWriter, CharArrayWriter, CharBuffer, FileWriter, FilterWriter, LogStream, OutputStreamWriter, PipedWriter, PrintStream, PrintWriter, StringBuffer, StringBuilder, StringWriter, Writer
用法

        String[] arr = {"a","b","c"};
        StringBuilder ab = new StringBuilder("start: ");
        StringBuilder ac = Joiner.on("").appendTo(ab,arr);
        System.out.println(ab); // start: abc
        System.out.println(ac); // start: abc

A appendTo(A appendable, @Nullable Object first, @Nullable Object second, Object… rest)
向appendable参数后追加first、second…所有参数

String join(Iterable<?> parts)
将可迭代的参数中所有子元素连接。

static Joiner on(char separator)
为static对象设置分隔符
Joiner实例不可变，即用final修饰，一旦初始化不可变，所以分步调用joiner的静态函数是不起作用的。如：

 Joiner joiner = Joiner.on(',');
 joiner.skipNulls(); // does nothing!
 return joiner.join("wrong", null, "wrong");

这样使得joiner线程安全，并且返回的都是final static常量

Joiner skipNulls()

Joiner useForNull(String nullText)

Joiner.MapJoiner withKeyValueSeparator(String keyValueSeparator)

Map<Integer, String> map = new HashMap<Integer, String>();
    map.put(1, "a");
    map.put(2, "b");
    Joiner.MapJoiner joiner = Joiner.on(";").withKeyValueSeparator("->");
    System.out.println(joiner.join(map)); // 1->a;2->b

总结：