常用Java工具类

Ignacio Coloma 说道：作为一个软件开发者，缺乏想象力是最严重的罪过之一。我们经常把事情重复做一遍又一遍，但是我们很少改变这种方式，至少我是这样。经过这些年开发，在我的工具箱里面有了一些每个项目我都需要用到的工具，烦人的重复工作不再是我的事。 

下面这些工具也许你已经用到，让我来仔细介绍它们： 

StringUtils  

这是象面包和奶油一样必须的通用语言库，这个实用工具类包括一些很早以前在String中未包含的重要方法。 

 StringUtils.isEmpty( null ) && StringUtils.isEmpty( "" );  // true
 StringUtils.isBlank("   \n\t" );                        // true
 StringUtils.substringAfterLast("foo.bar.baz" ,  "." );    // "baz"
 StringUtils.substringBeforeLast("foo.bar.baz" ,  "." );   // "foo.bar"
 StringUtils.split("foo.bar.baz" ,  '.' );                 // { "foo", "bar", "baz" }
 StringUtils.split("foo,  bar,baz" ,  ", " );              // { "foo", "bar", "baz" }
 StringUtils.leftPad("1" ,  3 ,  '0' );                      // "001"

IOUtils and FileUtils  

在一种当你需要手动操作多个文件罕见情况下必须具备的工具，这两个工具很相似(FileUtils操作文件，IOUtils操作InputStream和Reader classes)，和捆绑常用IO.  

 File file1;
 File file2;
 InputStream inputStream;
 OutputStream outputStream;

 // copy one file into another
 FileUtils.copyFile(file1, file2);
 IOUtils.copy(inputStream, outputStream);

 // read a file into a String
 String s1 = FileUtils.readFileToString(file1);
 String s2 = IOUtils.toString(inputStream);

 // read a file into a list of Strings, one item per line
 List<String> l1 = FileUtils.readLines(file1);
 List<String> l2 = IOUtils.readLines(inputStream);

 // put this in your finally() clause after manipulating streams
 IOUtils.closeQuietly(inputStream);

 // return the list of xml and text files in the specified folder and any subfolders
 Collection<File> c1 = FileUtils.listFiles(file1, { "xml" ,  "txt"  },  true );

 // copy one folder and its contents into another
 FileUtils.copyDirectoryToDirectory(file1, file2);

 // delete one folder and its contents
 FileUtils.deleteDirectory(file1);

Google collections  

这是我所知道的最好的扩展实现包，其中一些被社区叫嚣着要加入JDK：  

 // create an ArrayList with three arguments
 List<String> list = Lists.newArrayList("foo" ,  "bar" ,  "baz" );

 // notice that there is no generics or class cast,
 // and still this line does not generate a warning.
 Set<String> s = Sets.newConcurrentHashSet();

 // intersect and union are basic features of a Set, if you ask me
 Set<String> s = Sets.intersect(s1, s2);

 // Example  of multiple values in a Map
 ListMultimap<String, Validator> validators = new  ArrayListMultimap<String, Validator>();
 validators.put("save" ,  new  RequiredValidator());
 validators.put("save" ,  new  StringValidator());
 validators.put("delete" ,  new  NumberValidator());

 validators.get("save" );  // { RequiredValidator, StringValidator }
 validators.get("foo" );   // empty List (not null)
 validators.values();    // { RequiredValidator, StringValidator, NumberValidator }

java.util.concurrent  

不是每个人都需要这么重的java.util.concurrent，但是很好用:  

 // a map that may be modified (by the same or different thread) while being iterated
 Map<String, Something> repository = new  ConcurrentHashMap<String, Something>();

 // same with lists. This one is only available with Java 6
 List<Something> list = new  CopyOnWriteArrayList<Something>();