Top 15 Must‑Know Java Utility Classes and Their Most Useful Methods

In Java, utility classes define a set of methods that perform common functions.

This article presents the most frequently used Java utility classes and their most popular methods, ranked by popularity based on data from 50,000 randomly selected open‑source Java projects on GitHub.

By browsing the list you can discover existing popular functionalities and avoid re‑implementing them; method names usually indicate their purpose, and you can also see how other developers use them in open‑source projects.

org.apache.commons.io.IOUtils

closeQuietly()
toString()
copy()
toByteArray()
write()
toInputStream()
readLines()
copyLarge()
lineIterator()
readFully()

org.apache.commons.io.FileUtils

deleteDirectory()
readFileToString()
deleteQuietly()
copyFile()
writeStringToFile()
forceMkdir()
write()
listFiles()
copyDirectory()
forceDelete()

org.apache.commons.lang.StringUtils

isBlank()
isNotBlank()
isEmpty()
isNotEmpty()
equals()
join()
split()
EMPTY
trimToNull()
replace()

org.apache.http.util.EntityUtils

toString()
consume()
toByteArray()
consumeQuietly()
getContentCharSet()

org.apache.commons.lang3.StringUtils

isBlank()
isNotBlank()
isEmpty()
isNotEmpty()
join()
equals()
split()
EMPTY
replace()
capitalize()

org.apache.commons.io.FilenameUtils

getExtension()
getBaseName()
getName()
concat()
removeExtension()
normalize()
wildcardMatch()
separatorsToUnix()
getFullPath()
isExtension()

org.springframework.util.StringUtils

hasText()
hasLength()
isEmpty()
commaDelimitedListToStringArray()
collectionToDelimitedString()
replace()
delimitedListToStringArray()
uncapitalize()
collectionToCommaDelimitedString()
tokenizeToStringArray()

org.apache.commons.lang.ArrayUtils

contains()
addAll()
clone()
isEmpty()
add()
EMPTY_BYTE_ARRAY
subarray()
indexOf()
isEquals()
toObject()

org.apache.commons.lang.StringEscapeUtils

escapeHtml()
unescapeHtml()
escapeXml()
escapeSql()
unescapeJava()
escapeJava()
escapeJavaScript()
unescapeXml()
unescapeJavaScript()

org.apache.http.client.utils.URLEncodedUtils

format()
parse()

org.apache.commons.codec.digest.DigestUtils

md5Hex()
shaHex()
sha256Hex()
sha1Hex()
sha()
md5()
sha512Hex()
sha1()

org.apache.commons.collections.CollectionUtils

isEmpty()
isNotEmpty()
select()
transform()
filter()
find()
collect()
forAllDo()
addAll()
isEqualCollection()

org.apache.commons.lang3.ArrayUtils

contains()
isEmpty()
isNotEmpty()
add()
clone()
addAll()
subarray()
indexOf()
EMPTY_OBJECT_ARRAY
EMPTY_STRING_ARRAY()

org.apache.commons.beanutils.PropertyUtils

getProperty()
setProperty()
getPropertyDescriptors()
isReadable()
copyProperties()
getPropertyDescriptor()
getSimpleProperty()
isWriteable()
setSimpleProperty()
getPropertyType()

org.apache.commons.lang3.StringEscapeUtils

unescapeHtml4()
escapeHtml4()
escapeXml()
unescapeXml()
escapeJava()
escapeEcmaScript()
unescapeJava()
escapeJson()
escapeXml10()

org.apache.commons.beanutils.BeanUtils

copyProperties()
getProperty()
setProperty()
describe()
populate()
copyProperty()
cloneBean()