一些常用Java工具包

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>();