在Java中，有很多比较实用的类库，他们通常都定义了一系列具有常见功能的方法。本文总结了最常用的Java中的实用类以及他们的最常用的方法。无论是类和类中方法都是按照流行程度来排序的。
本文中列出来的类及方法都是经过大量实践的常用类库及方法，我们可以直接拿过来用。当然，这些方法实现的功能我们自己都能实现，但是既然已经有很成熟的方法可以供我们使用了，那么就无需自己定义了。很多类和方法通过他们的名字其实可以理解出具体是做什么的。每个方法都有一个链接，可以查看他们在开源代码中具体是如何使用的。
以下列表是通过分析50K的开源项目得出来的。
1. org.apache.commons.io.IOUtils
closeQuietly ( )
toString ( )
copy ( )
toByteArray ( )
write ( )
toInputStream ( )
readLines ( )
copyLarge ( )
lineIterator ( )
readFully ( )
2. org.apache.commons.io.FileUtils
deleteDirectory ( )
readFileToString ( )
deleteQuietly ( )
copyFile ( )
writeStringToFile ( )
forceMkdir ( )
write ( )
listFiles ( )
copyDirectory ( )
forceDelete ( )
3. org.apache.commons.lang.StringUtils
isBlank ( )
isNotBlank ( )
isEmpty ( )
isNotEmpty ( )
equals ( )
join ( )
split ( )
EMPTY
trimToNull ( )
replace ( )
4. org.apache.http.util.EntityUtils
toString ( )
consume ( )
toByteArray ( )
consumeQuietly ( )
getContentCharSet ( )
5. org.apache.commons.lang3.StringUtils
isBlank ( )
isNotBlank ( )
isEmpty ( )
isNotEmpty ( )
join ( )
equals ( )
split ( )
EMPTY
replace ( )
capitalize ( )
6. org.apache.commons.io.FilenameUtils
getExtension ( )
getBaseName ( )
getName ( )
concat ( )
removeExtension ( )
normalize ( )
wildcardMatch ( )
separatorsToUnix ( )
getFullPath ( )
isExtension ( )
7. org.springframework.util.StringUtils
hasText ( )
hasLength ( )
isEmpty ( )
commaDelimitedListToStringArray ( )
collectionToDelimitedString ( )
replace ( )
delimitedListToStringArray ( )
uncapitalize ( )
collectionToCommaDelimitedString ( )
tokenizeToStringArray ( )
8. org.apache.commons.lang.ArrayUtils
contains ( )
addAll ( )
clone ( )
isEmpty ( )
add ( )
EMPTY_BYTE_ARRAY
subarray ( )
indexOf ( )
isEquals ( )
toObject ( )
9. org.apache.commons.lang.StringEscapeUtils
escapeHtml ( )
unescapeHtml ( )
escapeXml ( )
escapeSql ( )
unescapeJava ( )
escapeJava ( )
escapeJavaScript ( )
unescapeXml ( )
unescapeJavaScript ( )
10. org.apache.http.client.utils.URLEncodedUtils
format ( )
parse ( )
11. org.apache.commons.codec.digest.DigestUtils
md5Hex ( )
shaHex ( )
sha256Hex ( )
sha1Hex ( )
sha ( )
md5 ( )
sha512Hex ( )
sha1 ( )
12. org.apache.commons.collections.CollectionUtils
isEmpty ( )
isNotEmpty ( )
select ( )
transform ( )
filter ( )
find ( )
collect ( )
forAllDo ( )
addAll ( )
isEqualCollection ( )
13. org.apache.commons.lang3.ArrayUtils
contains ( )
isEmpty ( )
isNotEmpty ( )
add ( )
clone ( )
addAll ( )
subarray ( )
indexOf ( )
EMPTY_OBJECT_ARRAY
EMPTY_STRING_ARRAY
14. org.apache.commons.beanutils.PropertyUtils
getProperty ( )
setProperty ( )
getPropertyDescriptors ( )
isReadable ( )
copyProperties ( )
getPropertyDescriptor ( )
getSimpleProperty ( )
isWriteable ( )
setSimpleProperty ( )
getPropertyType ( )
15. org.apache.commons.lang3.StringEscapeUtils
unescapeHtml4 ( )
escapeHtml4 ( )
escapeXml ( )
unescapeXml ( )
escapeJava ( )
escapeEcmaScript ( )
unescapeJava ( )
escapeJson ( )
escapeXml10 ( )
16. org.apache.commons.beanutils.BeanUtils
copyProperties ( )
getProperty ( )
setProperty ( )
describe ( )
populate ( )
copyProperty ( )
cloneBean ( )
