java.lang.Appendable接口

    Appendable接口的实现类的对象能够被添加 char 序列和值。如果某个类的实例打算接收取自 java.util.Formatter 的格式化输出，那么该类必须实现 Appendable 接口。
    要添加的字符应该是有效的 Unicode 字符。
    Appendable 对于多线程访问而言没必要是安全的。线程安全由扩展和实现此接口的类负责。

    所有已知实现类：
    BufferedWriter, CharArrayWriter, CharBuffer, FileWriter, FilterWriter, LogStream, OutputStreamWriter, PipedWriter, PrintStream, PrintWriter, StringBuffer, StringBuilder, StringWriter, Writer

    所期望用法的示例：

   StringBuilder sb = new StringBuilder();                         // StringBuilder实现了Appendable接口
   // Send all output to the Appendable object sb
   Formatter formatter = new Formatter(sb, Locale.US);

   // Explicit argument indices may be used to re-order output.
   // 将格式化后的字符串放入sb
   formatter.format("%4$2s %3$2s %2$2s %1$2s", "a", "b", "c", "d")
   // -> " d  c  b  a"

   // Optional locale as the first argument can be used to get
   // locale-specific formatting of numbers.  The precision and width can be
   // given to round and align the value.
   formatter.format(Locale.FRANCE, "e = %+10.4f", Math.E);
   // -> "e =    +2,7183"

   // The '(' numeric flag may be used to format negative numbers with
   // parentheses rather than a minus sign.  Group separators are
   // automatically inserted.
   formatter.format("Amount gained or lost since last statement: $ %(,.2f", balanceDelta);
   // -> "Amount gained or lost since last statement: $ (6,217.58)"

 常见格式化请求的便捷方法是按照如下调用格式来阐明的：

   // Writes a formatted string to System.out.
   System.out.format("Local time: %tT", Calendar.getInstance());
   // -> "Local time: 13:34:18"

   // Writes formatted output to System.err.
   System.err.printf("Unable to open file '%1$s': %2$s",
                     fileName, exception.getMessage());
   // -> "Unable to open file 'food': No such file or directory"

 与 C 语言的 sprintf(3) 类似，可以使用静态方法 String.format(String,Object...) 来格式化 Strings：

   // Format a string containing a date.
   import java.util.Calendar;
   import java.util.GregorianCalendar;
   import static java.util.Calendar.*;

   Calendar c = new GregorianCalendar(1995, MAY, 23);
   String s = String.format("Duke's Birthday: %1$tm %1$te,%1$tY", c);
   // -> s == "Duke's Birthday: May 23, 1995"

    java.lang.Appendable接口方法

1. Appendable append(char c)                                   向此 Appendable 添加指定字符。
2. Appendable append(CharSequence csq)           向此 Appendable 添加指定的字符序列。
3. Appendable append(CharSequence csq, int start, int end)          向此 Appendable 添加指定字符序列的子序列。