Edited 07.12.2018:

I think dependency for StringUtils you are using is not proper.

Please add below dependency to gradle and Sync project and import your class.

implementation 'org.apache.commons:commons-lang3:3.6'

This on is using deprecated task (but should still works):

compile 'org.apache.commons:commons-lang3:3.5'

Edit:

As OoDeLally mentioned in a comment,

Above mentioned version is deprecated, Please use below dependency:

implementation 'org.apache.commons:commons-text:1.9'

Edit 2:

deprecated as for July 2019. Use stackoverflow.com/a/55567755/1541141 instead

Thanks @OoDeLally!

Pick the ones you need and add them to build.gradle's dependencies block:

I recommend checking the link to make sure you are getting the latest version. Feel free to suggest edits.

collections: (link)

implementation 'org.apache.commons:commons-collections4:4.4'

lang: (link)

implementation 'org.apache.commons:commons-lang3:3.9'

compress: (link)

implementation 'org.apache.commons:commons-compress:1.19'

net: (link)

implementation 'commons-net:commons-net:3.6'

io: (link)

implementation 'commons-io:commons-io:2.6'

exec (link)

implementation 'org.apache.commons:commons-exec:1.3'

codec (link)

implementation 'commons-codec:commons-codec:1.13'

http (link)

implementation 'org.apache.httpcomponents:httpclient:4.5.10'

validator (link)

implementation 'commons-validator:commons-validator:1.6'

math (link)

implementation 'org.apache.commons:commons-math3:3.6.1'

See all of the Apache libraries here

These are the steps to add a local library as dependency (.jar):

• put the jar file into "libs" folder
• add the following line to your build.gradle, inside dependencies:

compile fileTree(dir: 'libs', include: ['*.jar'])

like:

dependencies {
     compile fileTree(dir: 'libs', include: ['*.jar'])
}