There was a time when your choices for Java IDEs were Eclipse, NetBeans, or IntelliJ IDEA. That has changed somewhat. Among other innovations, Visual Studio Code now has good support for editing, running, and debugging Java code through a set of Java-specific extensions.
Aside from the whole idea of being lightweight and starting quickly, Visual Studio Code has IntelliSense code completion for variables, methods, and imported modules; graphical debugging; linting, multi-cursor editing, parameter hints, and other powerful editing features; snazzy code navigation and refactoring; and built-in source code control including Git support. Much of this was adapted from Visual Studio technology.
Extensions to Visual Studio Code can use the Language Server Protocol, which defines the protocol used between an editor or IDE and a language server that provides language features like auto complete, go to definition, find all references, etc. A Language Server is meant to provide the language-specific smarts and communicate with development tools over a protocol that enables inter-process communication.
In addition, extensions can use the Debug Adapter Protocol (DAP), which defines the abstract protocol used between a development tool (e.g. IDE or editor) and a debugger. The Debug Adapter Protocol makes it possible to implement a generic debugger for a development tool that can communicate with different debuggers via Debug Adapters.