使用Joiner

典型的使用原生的java處理拼接字符串的方法：

    public String buildString(List<String> stringList, String delimiter){
         StringBuilder builder = new StringBuilder();
         for (String s : stringList) {
             if(s !=null){
                 builder.append(s).append(delimiter);
             }
        }
        builder.setLength(builder.length() – delimiter.length());
        return builder.toString();
   }

注意：這里需要移除最后一個(gè)分隔符。
原生的java實(shí)現(xiàn)其實(shí)也不是很復(fù)雜，但是使用Google Guava類庫(kù)中的Joiner來實(shí)現(xiàn)的話更加簡(jiǎn)潔：

    Joiner.on("|").skipNulls().join(stringList);

上面的skipNulls方法是忽略了stringList中為null的空值。當(dāng)然，Joiner中也可以指定空值得替代，比如如下的代碼：

     Joiner.on("|").useForNull("no value").join(stringList);

有幾點(diǎn)需要說明：Joiner類不僅能處理String類型的數(shù)據(jù)，還能處理數(shù)組，迭代對(duì)象以及任何對(duì)象的可變參數(shù)類。其實(shí)也是調(diào)用了Object的toString()方法。另外如果空值存在集合中，并且skipNulls活著useForNull都沒有指定的話，NullPointerException異常將會(huì)拋出。還有，一旦Joiner對(duì)象創(chuàng)建，將會(huì)是不變的。比如如下的useForNull將不會(huì)起任何作用：

     Joiner stringJoiner = Joiner.on("|").skipNulls();
     //the useForNull() method returns a new instance of the Joiner!
     stringJoiner.useForNull("missing");
     stringJoiner.join("foo","bar",null);

Joiner類不僅可以返回String對(duì)象，還可以返回StringBuilder對(duì)象：

      StringBuilder stringBuilder = new StringBuilder();
      Joiner joiner = Joiner.on("|").skipNulls();
     //returns the StringBuilder instance with the values foo,bar,baz appeneded with "|" delimiters
     joiner.appendTo(stringBuilder,"foo","bar","baz")

Joiner類可以作用于實(shí)現(xiàn)了Appendable的所有類：

     FileWriter fileWriter = new FileWriter(new File("path")):
     List<Date> dateList = getDates();
     Joiner joiner = Joiner.on("#").useForNulls(" ");
     //returns the FileWriter instance with the valuesappended into it
     joiner.appendTo(fileWriter,dateList);

Joiner類也可以作用于Map對(duì)象：

    public void testMapJoiner() {
         //Using LinkedHashMap so that the original order is preserved
         String expectedString = "Washington D.C=Redskins#New York City=Giants#Philadelphia=Eagles#Dallas=Cowboys";
         Map<String,String> testMap = Maps.newLinkedHashMap();
         testMap.put("Washington D.C","Redskins");
         testMap.put("New York City","Giants");
         testMap.put("Philadelphia","Eagles");
         testMap.put("Dallas","Cowboys");
         String returnedString = Joiner.on("#").withKeyValueSeparator("=").join(testMap);
         assertThat(returnedString,is(expectedString));
    }