IntellJ IDEA is a commercial Java IDE (integrated development environment) that also offers a free and open source (FOSS) community version.
IntellJ is sometimes referred to as an intelligent IDE. It indexes projects in their entirety, analyses their content and builds a syntax tree. That makes it possible for the IDE to understand any area where a programmer starts in and know what types of things can be done at that point.
IntellJ provides the foundation for a number of prominent applications, including Google's Android Studio. Other popular Java IDEs include Eclipse and NetBeans.