# 6-字符串处理：分割，连接，填充
[原文链接](http://code.google.com/p/guava-libraries/wiki/StringsExplained) [译文链接](http://ifeve.com/google-guava-strings) 译者：沈义扬，校对：丁一
## 连接器[Joiner]
用分隔符把字符串序列连接起来也可能会遇上不必要的麻烦。如果字符串序列中含有null，那连接操作会更难。Fluent风格的[`Joiner`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Joiner.html)让连接字符串更简单。
```
Joiner joiner = Joiner.on("; ").skipNulls();
return joiner.join("Harry", null, "Ron", "Hermione");
```
上述代码返回”Harry; Ron; Hermione”。另外，useForNull(String)方法可以给定某个字符串来替换null，而不像skipNulls()方法是直接忽略null。 Joiner也可以用来连接对象类型，在这种情况下，它会把对象的toString()值连接起来。
```
Joiner.on(",").join(Arrays.asList(1, 5, 7)); // returns "1,5,7"
```
_警告：joiner实例总是不可变的。用来定义joiner目标语义的配置方法总会返回一个新的joiner实例。这使得joiner实例都是线程安全的，你可以将其定义为static final常量。_
## 拆分器[Splitter]
JDK内建的字符串拆分工具有一些古怪的特性。比如，String.split悄悄丢弃了尾部的分隔符。 问题：”,a,,b,”.split(“,”)返回？
1. “”, “a”, “”, “b”, “”
2. null, “a”, null, “b”, null
3. “a”, null, “b”
4. “a”, “b”
5. 以上都不对
正确答案是5：””, “a”, “”, “b”。只有尾部的空字符串被忽略了。 [`Splitter`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html)使用令人放心的、直白的流畅API模式对这些混乱的特性作了完全的掌控。
```
Splitter.on(',')
.trimResults()
.omitEmptyStrings()
.split("foo,bar,, qux");
```
上述代码返回Iterable<String>，其中包含”foo”、”bar”和”qux”。Splitter可以被设置为按照任何模式、字符、字符串或字符匹配器拆分。
### 拆分器工厂
| **方法** | **描述** | **范例** |
|:--- |:--- |:--- |
| [`Splitter.on(char)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#on%28char%29) | 按单个字符拆分 | Splitter.on(‘;’) |
| [`Splitter.on(CharMatcher)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#on%28com.google.common.base.CharMatcher%29) | 按字符匹配器拆分 | Splitter.on(CharMatcher.BREAKING_WHITESPACE) |
| [`Splitter.on(String)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#on%28java.lang.String%29) | 按字符串拆分 | Splitter.on(“, “) |
| [`Splitter.on(Pattern)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#on%28java.util.regex.Pattern%29) [`Splitter.onPattern(String)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#onPattern%28java.lang.String%29) | 按正则表达式拆分 | Splitter.onPattern(“\r?\n”) |
| [`Splitter.fixedLength(int)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#fixedLength%28int%29) | 按固定长度拆分；最后一段可能比给定长度短，但不会为空。 | Splitter.fixedLength(3) |
### 拆分器修饰符
| **方法** | **描述** |
|:--- |:--- |
| [`omitEmptyStrings()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#omitEmptyStrings%28%29) | 从结果中自动忽略空字符串 |
| [`trimResults()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#trimResults%28%29) | 移除结果字符串的前导空白和尾部空白 |
| [`trimResults(CharMatcher)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…e/common/base/Splitter.html#trimResults%28com.google.common.base.CharMatcher%29) | 给定匹配器，移除结果字符串的前导匹配字符和尾部匹配字符 |
| [`limit(int)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#limit%28int%29) | 限制拆分出的字符串数量 |
如果你想要拆分器返回List，只要使用Lists.newArrayList(splitter.split(string))或类似方法。 _警告：splitter实例总是不可变的。用来定义splitter目标语义的配置方法总会返回一个新的splitter实例。这使得splitter实例都是线程安全的，你可以将其定义为static final常量。_
## 字符匹配器[CharMatcher]
在以前的Guava版本中，StringUtil类疯狂地膨胀，其拥有很多处理字符串的方法：allAscii、collapse、collapseControlChars、collapseWhitespace、indexOfChars、lastIndexNotOf、numSharedChars、removeChars、removeCrLf、replaceChars、retainAllChars、strip、stripAndCollapse、stripNonDigits。 所有这些方法指向两个概念上的问题：
1. 怎么才算匹配字符？
2. 如何处理这些匹配字符？
为了收拾这个泥潭，我们开发了CharMatcher。
直观上，你可以认为一个CharMatcher实例代表着某一类字符，如数字或空白字符。事实上来说，CharMatcher实例就是对字符的布尔判断——CharMatcher确实也实现了[Predicate<Character>](http://code.google.com/p/guava-libraries/wiki/FunctionalExplained#Predicate)——但类似”所有空白字符”或”所有小写字母”的需求太普遍了，Guava因此创建了这一API。
然而使用CharMatcher的好处更在于它提供了一系列方法，让你对字符作特定类型的操作：修剪[trim]、折叠[collapse]、移除[remove]、保留[retain]等等。CharMatcher实例首先代表概念1：怎么才算匹配字符？然后它还提供了很多操作概念2：如何处理这些匹配字符？这样的设计使得API复杂度的线性增加可以带来灵活性和功能两方面的增长。
```
String noControl = CharMatcher.JAVA_ISO_CONTROL.removeFrom(string); //移除control字符
String theDigits = CharMatcher.DIGIT.retainFrom(string); //只保留数字字符
String spaced = CharMatcher.WHITESPACE.trimAndCollapseFrom(string, ' ');
//去除两端的空格，并把中间的连续空格替换成单个空格
String noDigits = CharMatcher.JAVA_DIGIT.replaceFrom(string, "*"); //用*号替换所有数字
String lowerAndDigit = CharMatcher.JAVA_DIGIT.or(CharMatcher.JAVA_LOWER_CASE).retainFrom(string);
// 只保留数字和小写字母
```
注：CharMatcher只处理char类型代表的字符；它不能理解0x10000到0x10FFFF的Unicode 增补字符。这些逻辑字符以代理对[surrogate pairs]的形式编码进字符串，而CharMatcher只能将这种逻辑字符看待成两个独立的字符。
### 获取字符匹配器
CharMatcher中的常量可以满足大多数字符匹配需求：
| [`ANY`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#ANY) | [`NONE`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#NONE) | [`WHITESPACE`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#WHITESPACE) | [`BREAKING_WHITESPACE`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#BREAKING_WHITESPACE) |
|:--- |:--- |:--- |:--- |:--- |
| [`INVISIBLE`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#INVISIBLE) | [`DIGIT`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#DIGIT) | [`JAVA_LETTER`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#JAVA_LETTER) | [`JAVA_DIGIT`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#JAVA_DIGIT) |
| [`JAVA_LETTER_OR_DIGIT`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#JAVA_LETTER_OR_DIGIT) | [`JAVA_ISO_CONTROL`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#JAVA_ISO_CONTROL) | [`JAVA_LOWER_CASE`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#JAVA_LOWER_CASE) | [`JAVA_UPPER_CASE`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#JAVA_UPPER_CASE) |
| [`ASCII`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#ASCII) | [`SINGLE_WIDTH`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#SINGLE_WIDTH) |
其他获取字符匹配器的常见方法包括：
| **方法** | **描述** |
|:--- |:--- |
| [`anyOf(CharSequence)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#anyOf%28java.lang.CharSequence%29) | 枚举匹配字符。如CharMatcher.anyOf(“aeiou”)匹配小写英语元音 |
| [`is(char)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#is%28char%29) | 给定单一字符匹配。 |
| [`inRange(char, char)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#inRange%28char, char%29) | 给定字符范围匹配，如CharMatcher.inRange(‘a’, ‘z’) |
此外，CharMatcher还有[`negate()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#negate%28%29)、[`and(CharMatcher)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#and%28com.google.common.base.CharMatcher%29)和[`or(CharMatcher)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#or%28com.google.common.base.CharMatcher%29)方法。
### 使用字符匹配器
CharMatcher提供了[多种多样的方法](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#method_summary)操作CharSequence中的特定字符。其中最常用的罗列如下：
| **方法** | **描述** |
|:--- |:--- |
| [`collapseFrom(CharSequence, char)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#collapseFrom%28java.lang.CharSequence, char%29) | 把每组连续的匹配字符替换为特定字符。如WHITESPACE.collapseFrom(string, ‘ ‘)把字符串中的连续空白字符替换为单个空格。 |
| [`matchesAllOf(CharSequence)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#matchesAllOf%28java.lang.CharSequence%29) | 测试是否字符序列中的所有字符都匹配。 |
| [`removeFrom(CharSequence)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#removeFrom%28java.lang.CharSequence%29) | 从字符序列中移除所有匹配字符。 |
| [`retainFrom(CharSequence)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#retainFrom%28java.lang.CharSequence%29) | 在字符序列中保留匹配字符，移除其他字符。 |
| [`trimFrom(CharSequence)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#trimFrom%28java.lang.CharSequence%29) | 移除字符序列的前导匹配字符和尾部匹配字符。 |
| [`replaceFrom(CharSequence, CharSequence)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#replaceFrom%28java.lang.CharSequence, java.lang.CharSequence%29) | 用特定字符序列替代匹配字符。 |
所有这些方法返回String，除了matchesAllOf返回的是boolean。
## 字符集[Charsets]
不要这样做字符集处理：
```
try {
bytes = string.getBytes("UTF-8");
} catch (UnsupportedEncodingException e) {
// how can this possibly happen?
throw new AssertionError(e);
}
```
试试这样写：
```
bytes = string.getBytes(Charsets.UTF_8);
```
[`Charsets`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Charsets.html)针对所有Java平台都要保证支持的六种字符集提供了常量引用。尝试使用这些常量，而不是通过名称获取字符集实例。
## 大小写格式[CaseFormat]
CaseFormat被用来方便地在各种ASCII大小写规范间转换字符串——比如，编程语言的命名规范。CaseFormat支持的格式如下：
| **格式** | **范例** |
|:--- |:--- |
| [`LOWER_CAMEL`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CaseFormat.html#LOWER_CAMEL) | lowerCamel |
| [`LOWER_HYPHEN`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CaseFormat.html#LOWER_HYPHEN) | lower-hyphen |
| [`LOWER_UNDERSCORE`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CaseFormat.html#LOWER_UNDERSCORE) | lower_underscore |
| [`UPPER_CAMEL`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CaseFormat.html#UPPER_CAMEL) | UpperCamel |
| [`UPPER_UNDERSCORE`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CaseFormat.html#UPPER_UNDERSCORE) | UPPER_UNDERSCORE |
CaseFormat的用法很直接：
```
CaseFormat.UPPER_UNDERSCORE.to(CaseFormat.LOWER_CAMEL, "CONSTANT_NAME")); // returns "constantName"
```
我们CaseFormat在某些时候尤其有用，比如编写代码生成器的时候。
- Google Guava官方教程（中文版）
- 1-基本工具
- 1.1-使用和避免null
- 1.2-前置条件
- 1.3-常见Object方法
- 1.4-排序: Guava强大的”流畅风格比较器”
- 1.5-Throwables：简化异常和错误的传播与检查
- 2-集合
- 2.1-不可变集合
- 2.2-新集合类型
- 2.3-强大的集合工具类：java.util.Collections中未包含的集合工具
- 2.4-集合扩展工具类
- 3-缓存
- 4-函数式编程
- 5-并发
- 5.1-google Guava包的ListenableFuture解析
- 5.2-Google-Guava Concurrent包里的Service框架浅析
- 6-字符串处理：分割，连接，填充
- 7-原生类型
- 9-I/O
- 10-散列
- 11-事件总线
- 12-数学运算
- 13-反射