What is the difference between creating a locale for en-US and en_US?

As of Java8, initializing the locale should be done using the language tag en-US. Locale.forLanguageTag("en-US").toString(); returns the output: en_US

Where as Locale.forLanguageTag("en_US") does not create the required locale. It will default to the system locale. Locale.forLanguageTag("en_US").toString() returns null

"en-US" is an IETF language tag. While Java'a Locale class was clearly based on IETF language tags, it uses underscores in place of hyphens when separating language codes from country codes (and also variants), so calling toString() on the equivalent Locale will give you en_US.

As of Java 7 you can use Locale.forLanguageTag(String) and toLanguageTag() to convert between language tags and Locale objects.

When converting strings to Locale objects it's a good idea to normalize by splitting components on hyphens and underscores, lowercasing the first component (the language code) and upper-casing the second component (the country code).

Or you could use Locale us = Locale.forLanguageTag("en-US") and us.toLanguageTag(), and that will do the conversion for you without having to create your own error-prone implementation.

"en" is the language code specified by ISO 639. while US is country code specified by 3166.
In Java, the Locale object recognizes the language as languageCode_countryCode (e.g. en_US) and not as languageCode-countryCode.