Emacs vs Intellij IDEA

Emacs can be controlled entirely with the keyboard.

Customizations can be made to a wide range of Emacs' functions through a Lisp dialect (Emacs Lisp). A robust list of existing Lisp extensions include the practical (git integration, syntax highlighting, etc) to the utilitarian (calculators, calendars) to the sublime (chess, Eliza).

You can debug, compile, manage files, integrate with version control systems, etc. All through the various plugins that can be installed.

You can use Emacs' command line interface or graphical user interface.

Emacs has extensive help support built-in as well as a tutorial accessed with C-h t.

Licensed under GNU GPL.

With 30+ years of use the Emacs documentation is very thorough. There are also a lot of tutorials and guides written by third parties.

You can pass complicated arguments in the mini buffer.

Fully compliant GNU-emacs is available on many platforms, and they all understand .emacs configuration files.

Emacs can select rectangularly.

With lisp customization, any behavior of Emacs can be changed. Update with pre-release patch can be also applied without recompiling the whole Emacs.

Evil mode emulates vim behaviors within Emacs. It enables Vi users to move inside the Emacs universe.

Evil is an extensible vi layer for Emacs. It provides Vim features like Visual selection and text objects.

Review the "Emacs Rocks" video.

Advanced planning and publication which can start as a simple list.

Dynamic word completion.

Works on Linux, Windows, Macintosh, BSD, and others.

Through its programmability, a very broad range of functionalities can be integrated in emacs, turning it even into a "single point of contact" with the underlying operating system.

You can jump straight from your org-mode files to programming tasks - and back - and build a seamless workflow.

Powerful commands, search, and more with the Helm plugin.

Since version 25 you can run GTK widgets inside Emacs buffers. One of these is the WebKitGTK+, which allows the user to run a full-featured web browser inside Emacs with JavaScript and CSS support among other things.

Uses a fast indexing technique to provide contextual hints (auto-completion, available object members, import suggestions).
On-the-fly code analysis to detect errors and propose refactorization.

There is a free community edition (open source) and an ultimate edition, which you can compare here.

The ultimate edition is available for free for one year for students but must be registered through an .edu e-mail account.

IDEA has a clean, intuitive interface with some customization available (such as the Darcula theme).

IntelliJ IDEA hardly ever crashes or has any issues that plague other Java IDEs like file corruption or slowness.

IntelliJ supports many languages besides Java, some of these are: golang, Scala, Clojure, Groovy, Bash, etc...

IDEA places an emphasis in safe refactoring, offering a variety of features to make this possible for a variety of languages.
These features include safe delete, type migration and replacing method code duplicates.

Many plugins are available for almost any task a developer may need to cover. Plugins are developed by Jetbrains themselves or by 3rd parties through the SDK available for writing them.

A very complete development environment support.

The documentation is exhaustive, easy to navigate, and clearly worded.

With ability to step into a certain part of a large method invocation (Shift+F7), drop frame, executing code snippets, showing method return values, etc.

These simplify the daily work, e.g. copy/cut a whole line without the need to select it.