本节介绍Guava的utilities中的工具包，

1. Joiner
2. Spliter
3. Preconditions
4. Objects
5. Strings
6. MoreObjects
7. ComparisonChain
8. CharMatcher
9. StopWatcher
10. ServerLoader

1. Joiner

负责将可迭代的集合，按照指定的分隔符，Join成一个字符串，其分隔符是指定的。

private final List<String> str1 = Arrays.asList( "1", "2", "3", "4", "5");
private final List<String> str2null = Arrays.asList( "1", "2", "3", "4", null);

//用法1
String result = Joiner.on("#").join(str1);
assertThat(result, equalTo("1#2#3#4#5"));

//用法2
String result = Joiner.on("#").join(str2null);
assertThat(result, equalTo("1#2#3#4"));  //会报错，因为join默认不能包含null

//用法3，忽略null值
String result = Joiner.on("#").skipNulls().join(str2null);
assertThat(result, equalTo("1#2#3#4"));

//用法4，使用默认值代替null值
String result = Joiner.on("#").useForNull("DEFAULT").join(str2null);
assertThat(result, equalTo("1#2#3#4#DEFAULT"));

//用法5，appendTo到StringBuilder中
final StringBuilder builder = new StringBuilder();
StringBuilder resultBuilder = Joiner.on("#").useForNull("DEFAULT").appendTo(builder, str2null);
assertThat(resultBuilder, sameInstance(builder));
assertThat(resultBuilder.toString(), equalTo("1#2#3#4#DEFAULT"));
assertThat(builder.toString(), equalTo("1#2#3#4#DEFAULT"));

//用法6，appendTo到文件流中
//注意try..catch..用法
try (FileWriter writer = new FileWriter(new File(fileNameStr))) {
      //写入文件
     Joiner.on("#").useForNull("DEFAULT").appendTo(writer, str2null);
     //判断文件是否存在
     assertThat(Files.isFile().test(new File(fileNameStr)), equalTo(true));
} catch (IOException e) {
     fail("Error.");
}

//用法7，对map进行join
private final Map<String, String> stringMap = of("1", "2", "3", "4");
String mapStr=Joiner.on('#').withKeyValueSeparator("=").join(stringMap);
assertThat(mapStr, equalTo("1=2#3=4"));

2. Spliter

spliter工作和joiner工作相反，将字符串按照指定的字符进行分割，

//不忽略空值的分割
List<String> result = Splitter.on("|").splitToList("1|2");
assertThat(result, notNullValue());
assertThat(result.size(), equalTo(2));
assertThat(result.get(0), equalTo("1"));
assertThat(result.get(1), equalTo("2"));

//忽略空值的语法 omitEmptyStrings，
List<String> result = Splitter.on("|").splitToList("1|2|||");
assertThat(result, notNullValue());
assertThat(result.size(), equalTo(5));
result = Splitter.on("|").omitEmptyStrings().splitToList("1|2|||");
assertThat(result, notNullValue());
assertThat(result.size(), equalTo(2));

//对切分后的每个结果进行trim两边的空格，语法，trimResults
List<String> result = Splitter.on("|").omitEmptyStrings().splitToList("1 | 2|||");
assertThat(result, notNullValue());
assertThat(result.size(), equalTo(2));
assertThat(result.get(0), equalTo("1 "));
assertThat(result.get(1), equalTo(" 2"));
result = Splitter.on("|").trimResults().omitEmptyStrings().splitToList("1 | 2|||");
assertThat(result.get(0), equalTo("1"));
assertThat(result.get(1), equalTo("2"));

//按照长度进行切分
List<String> result = Splitter.fixedLength(4).splitToList("aaaabbbbccccdddd");
assertThat(result, notNullValue());
assertThat(result.size(), equalTo(4));
assertThat(result.get(0), equalTo("aaaa"));
assertThat(result.get(3), equalTo("dddd"));

//分割之后，限制最大的分割数量
List<String> result = Splitter.on("#").limit(3).splitToList("1#2#3#4#5");
assertThat(result, notNullValue());
assertThat(result.size(), equalTo(3));
assertThat(result.get(0), equalTo("1"));
assertThat(result.get(1), equalTo("2"));
assertThat(result.get(2), equalTo("3#4#5"));

//正则切分
List<String> result = Splitter.on(Pattern.compile("\\|")).trimResults().omitEmptyStrings().splitToList("1| 2|||");
assertThat(result, notNullValue());
assertThat(result.size(), equalTo(2));
assertThat(result.get(0), equalTo("1"));
assertThat(result.get(1), equalTo("2"));

//切分成Map
Map<String, String> result = Splitter.on(Pattern.compile("\\|")).trimResults()
                                             .omitEmptyStrings().withKeyValueSeparator("=").split("1=2| 3=4|||");
assertThat(result, notNullValue());
assertThat(result.size(), equalTo(2));
assertThat(result.get("1"),equalTo("2"));
assertThat(result.get("3"),equalTo("4"));