6-字符串處理：分割，連接，填充 · guava

# 6-字符串處理：分割，連接，填充 # 6-字符串處理：分割，連接，填充 [原文鏈接](http://code.google.com/p/guava-libraries/wiki/StringsExplained) [譯文鏈接](http://ifeve.com/google-guava-strings) 譯者：沈義揚，校對：丁一 ## 連接器\[Joiner\] 用分隔符把字符串序列連接起來也可能會遇上不必要的麻煩。如果字符串序列中含有null，那連接操作會更難。Fluent風格的[`Joiner`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Joiner.html)讓連接字符串更簡單。 ``` <pre class="calibre11">``` Joiner joiner = Joiner.on("; ").skipNulls(); return joiner.join("Harry", null, "Ron", "Hermione"); ``` ``` 上述代碼返回”Harry; Ron; Hermione”。另外，useForNull(String)方法可以給定某個字符串來替換null，而不像skipNulls()方法是直接忽略null。 Joiner也可以用來連接對象類型，在這種情況下，它會把對象的toString()值連接起來。 ``` <pre class="calibre11">``` Joiner.on(",").join(Arrays.asList(1, 5, 7)); // returns "1,5,7" ``` ``` *警告：joiner實例總是不可變的。用來定義joiner目標語義的配置方法總會返回一個新的joiner實例。這使得joiner實例都是線程安全的，你可以將其定義為static final常量。* ## 拆分器\[Splitter\] JDK內建的字符串拆分工具有一些古怪的特性。比如，String.split悄悄丟棄了尾部的分隔符。問題：”,a,,b,”.split(“,”)返回？ 1. “”, “a”, “”, “b”, “” 2. null, “a”, null, “b”, null 3. “a”, null, “b” 4. “a”, “b” 5. 以上都不對正確答案是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、直白的流暢API模式對這些混亂的特性作了完全的掌控。 ``` <pre class="calibre11">``` Splitter.on(',') .trimResults() .omitEmptyStrings() .split("foo,bar,, qux"); ``` ``` 上述代碼返回Iterable<String>，其中包含”foo”、”bar”和”qux”。Splitter可以被設置為按照任何模式、字符、字符串或字符匹配器拆分。 ### 拆分器工廠 **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)按單個字符拆分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)按正則表達式拆分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)按固定長度拆分；最后一段可能比給定長度短，但不會為空。Splitter.fixedLength(3)### 拆分器修飾符 **方法****描述**[`omitEmptyStrings()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#omitEmptyStrings%28%29)從結果中自動忽略空字符串[`trimResults()`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Splitter.html#trimResults%28%29)移除結果字符串的前導空白和尾部空白[`trimResults(CharMatcher)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6e/common/base/Splitter.html#trimResults%28com.google.common.base.CharMatcher%29)給定匹配器，移除結果字符串的前導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數量如果你想要拆分器返回List，只要使用Lists.newArrayList(splitter.split(string))或類似方法。 *警告：splitter實例總是不可變的。用來定義splitter目標語義的配置方法總會返回一個新的splitter實例。這使得splitter實例都是線程安全的，你可以將其定義為static final常量。* ## 字符匹配器\[CharMatcher\] 在以前的Guava版本中，StringUtil類瘋狂地膨脹，其擁有很多處理字符串的方法：allAscii、collapse、collapseControlChars、collapseWhitespace、indexOfChars、lastIndexNotOf、numSharedChars、removeChars、removeCrLf、replaceChars、retainAllChars、strip、stripAndCollapse、stripNonDigits。所有這些方法指向兩個概念上的問題： 1. 怎么才算匹配字符？ 2. 如何處理這些匹配字符？為了收拾這個泥潭，我們開發了CharMatcher。直觀上，你可以認為一個CharMatcher實例代表著某一類字符，如數字或空白字符。事實上來說，CharMatcher實例就是對字符的布爾判斷——CharMatcher確實也實現了[Predicate<Character>](http://code.google.com/p/guava-libraries/wiki/FunctionalExplained#Predicate)——但類似”所有空白字符”或”所有小寫字母”的需求太普遍了，Guava因此創建了這一API。然而使用CharMatcher的好處更在于它提供了一系列方法，讓你對字符作特定類型的操作：修剪\[trim\]、折疊\[collapse\]、移除\[remove\]、保留\[retain\]等等。CharMatcher實例首先代表概念1：怎么才算匹配字符？然后它還提供了很多操作概念2：如何處理這些匹配字符？這樣的設計使得API復雜度的線性增加可以帶來靈活性和功能兩方面的增長。 ``` <pre class="calibre11">``` String noControl = CharMatcher.JAVA_ISO_CONTROL.removeFrom(string); //移除control字符 String theDigits = CharMatcher.DIGIT.retainFrom(string); //只保留數字字符 String spaced = CharMatcher.WHITESPACE.trimAndCollapseFrom(string, ' '); //去除兩端的空格，并把中間的連續空格替換成單個空格 String noDigits = CharMatcher.JAVA_DIGIT.replaceFrom(string, "*"); //用*號替換所有數字 String lowerAndDigit = CharMatcher.JAVA_DIGIT.or(CharMatcher.JAVA_LOWER_CASE).retainFrom(string); // 只保留數字和小寫字母 ``` ``` 注：CharMatcher只處理char類型代表的字符；它不能理解0x10000到0x10FFFF的Unicode 增補字符。這些邏輯字符以代理對\[surrogate pairs\]的形式編碼進字符串，而CharMatcher只能將這種邏輯字符看待成兩個獨立的字符。 ### 獲取字符匹配器 CharMatcher中的常量可以滿足大多數字符匹配需求： [`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)其他獲取字符匹配器的常見方法包括： **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)枚舉匹配字符。如CharMatcher.anyOf(“aeiou”)匹配小寫英語元音[`is(char)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#is%28char%29)給定單一字符匹配。[`inRange(char, char)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#inRange%28char,%20char%29)給定字符范圍匹配，如CharMatcher.inRange(‘a’, ‘z’)此外，CharMatcher還有[`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[多種多樣的方法](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。其中最常用的羅列如下： **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,%20char%29)把每組連續的匹配字符替換為特定字符。如WHITESPACE.collapseFrom(string, ‘ ‘)把字符串中的連續空白字符替換為單個空格。[`matchesAllOf(CharSequence)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html#matchesAllOf%28java.lang.CharSequence%29)測試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)從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)移除字符序列的前導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,%20java.lang.CharSequence%29)用特定字符序列替代匹配字符。所有這些方法返回String，除了matchesAllOf返回的是boolean。 ## 字符集\[Charsets\] 不要這樣做字符集處理： ``` <pre class="calibre11">``` try { bytes = string.getBytes("UTF-8"); } catch (UnsupportedEncodingException e) { // how can this possibly happen? throw new AssertionError(e); } ``` ``` 試試這樣寫： ``` <pre class="calibre11">``` bytes = string.getBytes(Charsets.UTF_8); ``` ``` [`Charsets`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Charsets.html)針對所有Java平臺都要保證支持的六種字符集提供了常量引用。嘗試使用這些常量，而不是通過名稱獲取字符集實例。 ## 大小寫格式\[CaseFormat\] CaseFormat被用來方便地在各種ASCII大小寫規范間轉換字符串——比如，編程語言的命名規范。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\_UNDERSCORECaseFormat的用法很直接： ``` <pre class="calibre11">``` CaseFormat.UPPER_UNDERSCORE.to(CaseFormat.LOWER_CAMEL, "CONSTANT_NAME")); // returns "constantName" ``` ``` 我們CaseFormat在某些時候尤其有用，比如編寫代碼生成器的時候。