When comparing Emacs vs Eclipse, the Slant community recommends Emacs for most people. In the question“What are the best IDEs for web development?” Emacs is ranked 8th while Eclipse is ranked 15th. The most important reason people chose Emacs is:
Emacs can be controlled entirely with the keyboard. While true, I often find the mouse and menus handy for those lesser-used commands. An aide-memoir.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Keyboard-focused, mouse-free editing
Emacs can be controlled entirely with the keyboard. While true, I often find the mouse and menus handy for those lesser-used commands. An aide-memoir.
Pro Total customizability
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).
Pro It's also an IDE
You can debug, compile, manage files, integrate with version control systems, etc. All through the various plugins that can be installed.
Pro Free
Licensed under GNU GPL.
Pro Self documenting
Emacs has extensive help support built-in as well as a tutorial accessed with C-h t.
Pro Works in terminal or as a GUI application
You can use Emacs' command line interface or graphical user interface.
Pro Great documentation
With 30+ years of use the Emacs documentation is very thorough. There are also a lot of tutorials and guides written by third parties.
Pro Ubiquity
Fully compliant GNU-emacs is available on many platforms, and they all understand .emacs configuration files.
Pro Lisp customizations
With lisp customization, any behavior of Emacs can be changed. Update with pre-release patch can be also applied without recompiling the whole Emacs.
Pro dabbrev-expand (Alt-/)
Dynamic word completion.
Pro Provides org-mode
Advanced planning and publication which can start as a simple list.
Pro Mini buffer
You can pass complicated arguments in the mini buffer.
Pro Enormous range of functionalities (way beyond simple "text editing")
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.
Pro Rectangular cut and paste
Emacs can select rectangularly.

Pro Vi keybindings through Evil mode
Evil mode emulates vim behaviors within Emacs. It enables Vi users to move inside the Emacs universe.
Pro Has been widely used for a long time
The first verion of Emacs was written in 1974 and GNU Emacs in 1984.
Pro Visual selection and text objects with Evil
Evil is an extensible vi layer for Emacs. It provides Vim features like Visual selection and text objects.
Pro Support multi-line editing, multiple frame, powerful paren, crazy jumping style
Review the "Emacs Rocks" video.
Pro Cross-platform
Works on Linux, Windows, Macintosh, BSD, and others.
Pro Integrates planning in your development process
You can jump straight from your org-mode files to programming tasks - and back - and build a seamless workflow.
Pro Helm plugin adds even more power to Emacs
Powerful commands, search, and more with the Helm plugin.
Pro Versatile
Emacs is great for everything.
Pro GTK+ widgets support
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.
Pro Excelent tutorial to get you started
The tutorial you are presented with at startup shows you exactly what you need to get started and teaches you how to use the built-in help yourself later.
Pro Interactive Shells
Emacs has a number of shell variants: ansi-term, shell, and eshell.
Pro Gnus
Managing several large mailing lists has never been easier using Gnus. The threading commands and the various ways of scoring articles means that I never miss important messages/authors, etc. A joy to use.
Pro eshell is cross platform
You can use the underlying operating system shell as a terminal emulation in an Emacs buffer. Don't like the default shell for your configuration? You can change it to your liking.
Pro Excellent Lisp editing support
Built-in packages make editing Lisp source code feel natural.
Pro use-package and org-mode
Missing some neural package that predicts actions
Maybe in the next release ...
Pro Large selection of plugins
Eclipse has a large and active community, which has resulted in a wide variety of plugins.
Pro Fast compiler
Eclipse uses a custom compiler (which can also be used outside of Eclipse), which is often faster than the normal Java Compiler, especially for incremental compilation.
Pro Free and open source
Eclipse is an open source project and free to use.
Pro Highly customizable
Thanks to the large variety of plugins and various configuration options, Eclipse is very customizable.
Pro Multiple languages - one IDE
Pro Good refactoring tools
Pro Great debugger
Shows threads, concurrency locks, and conditional breakpoints.
Pro Good font rendering
Because Eclipse is based on SWT, it uses the native font rendering and thus looks better than other IDEs on some Linux systems, where the Java font rendering is not optimal.
Pro Gives good perspectives on your project
The concept of perspectives is outstanding. It puts right tools at your fingertips, keeping the tools you currently don't need out from the workbench. For example, in VCS perspective it's all about versions and branches. In debug perspective it's all about state.
In java ee project it can show http endpoints in a very accessible manner.
Pro Faster than any other Java IDE
Fast, suitable for big projects, customizable, supports UML, many programming languages, plugins, and widgets vs NetBeans and JDeveloper. Support for Workspaces and Perspectives. Long term tested, free of charge, vs IntelliJ IDEA.
Pro Dark theme improved!
Cons
Con Learning curve is long
While it's better than it used to be, with most functions being possible through the menu, Emacs is still quite a bit different from your standard editor. You'll need to learn new keyboard shortcuts.
Con Sometimes the extensibility can distract you from your actual work
If I ever want to lose half a day, I'll start by tweaking my .spacemacs config file.
Con Keyboard combinations can be confusing for new users
For example, for navigation it uses the b, n, p, l keys. Which for some people may seem strange in the begging. However they can be changed easily.
Con Documentation is not beginner-friendly
Although lots of good built-in documentation _exists_, I have after four years of Emacs as my primary editor not figured out how to actually make use of it, and rely completely on Google / StackOverflow for help.
Con Hard customization
For customization, you need to learn Lisp
Con A lot of jokes in this serious software
Con Using Emacs on a new machine without your .emacs file
Con UI can be confusing
There's an overly abundant presence of menus, this forces you to constantly click around the different menu structures; foreign ideas, like Views and Perspectives; strange menu choices, like configure settings located in Windows menu->Preferences.
Con Plugins can be unstable
Though there are plenty of plugins to choose from, they aren't always reliable. Some aren't maintained, bug fixes can be slow, and you may need to download plugins from multiple sources.
Con Lack of plugins with good user interfaces
Many Eclipse plugins are extremely confusing, with UIs that are even less consistent than Eclipse itself.
Con Tends to be slow and lags a lot
Con Poor language support via plugins
Eclipse supports other languages with a huge amount of plugins. Many languages have their own distribution, but multi-language is hard to exist in one project. Like Scala, there is no official support from Eclipse for this language. If Eclipse gets an update, languages such as these will not.
Con Newer versions are getting less stable
Eclipse 4 Neon randomly hangs. For example, during installing new software.
https://bugs.eclipse.org/bugs/show_bug.cgi?id=513218
Con Some old bugs don't get fixed
E.g. change a parameter name to the same as a field - the field will not be prefixed with "this." like in IDEA.
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