Do not use default locale but specify one to avoid bugs

> Be wary of the default locale
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>Note that there are many convenience methods that automatically use the default locale, but using them may lead to subtle bugs.
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>The default locale is appropriate for tasks that involve presenting data to the user. In this case, you want to use the user's date/time formats, number formats, rules for conversion to lowercase, and so on. In this case, it's safe to use the convenience methods.
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>The default locale is not appropriate for machine-readable output. The best choice there is usually Locale.US – this locale is guaranteed to be available on all devices, and the fact that it has no surprising special cases and is frequently used (especially for computer-computer communication) means that it tends to be the most efficient choice too.
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>A common mistake is to implicitly use the default locale when producing output meant to be machine-readable. This tends to work on the developer's test devices (especially because so many developers use en_US), but fails when run on a device whose user is in a more complex locale. 

Source: http://developer.android.com/reference/java/util/Locale.html