In Android development, a filter is an object that constrains data using a filtering pattern. In normal use, Android developers extend the base Android SDK Filter class, overriding the class methods and adding implementations according to their own data filtering requirements.
In practice, filters are often used to implement functionality for searching through or constraining data sets within Android apps. The Android SDK contains numerous in-built classes that extend the basic Filter class with common functionality, such as InputFilter. A typical use case for an InputFilter subclass would be for limiting the types of characters that can be entered into text fields in an app, such as a login field where acceptable user names may only contain alphanumeric characters.