Many IDEs provide support for generating HTML from Javadocs automatically; some build tools (Maven and Gradle, for example) also have plugins that can handle the HTML creation.
However, these tools are not required to generate the Javadoc HTML; this can be done using the command line javadoc tool.
The most basic usage of the tool is:
javadoc JavaFile.java
Which will generate HTML from the Javadoc comments in JavaFile.java.
A more practical use of the command line tool, which will recursively read all java files in [source-directory], create documentation for [package.name] and all sub-packages, and place the generated HTML in the [docs-directory] is:
javadoc -d [docs-directory] -subpackages -sourcepath [source-directory] [package.name]