Develop Reference

Intellij is showing only part of the expected documentation for builtin classes(javadoc)

I am having a weird problem with intellij. A handful of people in here, had similar issues in the past but none of the proposed solutions worked for me.
So I am trying to view the javadoc for a builtin class(in my example java.io.FileReader) but I am only getting information about the signature, not details about the method as it is usually happening. I have also tried things around quick and external documentation, I have even added an external doc url in the project settings but nothing happened.
In the screenshot you can see the output I am getting. Any help appreciated.

Looks like Eclipse uses a different approach for the documentation. Also I had a non-fully document API and thats, combined, the reason it was not working.
The solution is as follows: File -> Project structure -> SDKs -> Documentation paths -> Click the add button, with the earth on it -> Add an online documentation source.
If you click the other one, you need to space a folder in your filesystem.
Both work. Thank you for your help.

Ctrl + Q
This assumes you have the JDK (with documentation) downloaded and configured with IntelliJ for the relevant object/class/method.
Edit:
Turned out it was a problem with the Users' JDK.
The JDK source files didn't contain any comments (which IntelliJ uses to show JDK Documentation). This was resolved by installing the JDK Documentation and configuring it with IntelliJ.

Java decompiler plugin not working

I have the JD-eclipse-plugin installed. I want to decompile the classes within a jar-file. So I select the jar-file within Project-Explorer and click Decompiler in the Menu. However both decompilers (JAD and JD-core) are greyed out.
I also tried the steps shown in this blog on AVAJAVA Web Tutorials. I guess the blog is a bit outdated, however I can see within Windows-->Preferences-->Java-->Decompiler that Jad is correctly set up. But there is no JD-eclipse found within Preferences-->General-->Editors-->File Associations as you can see in the following image.. Now I right click the jar-file within Project-Explorer, but there is no such "Attach Source File".
Did I miss anything to set up the plugin?
EDIT: I´m using the following setup: Eclipse: Mars.1 Release (4.5.1); JadClipse 4.0.0; JDK 1.8
EDIT2: Forgot to mention that I use Tomcat and want to dive into the libraries from /WebContent/WEB-INF/lib. This is not working, I cannot further collapse the jar-files.

For Eclipse 2018-09 , the problem was related to " class without source
"
I had resolved this issue by going to:
Preferences
General
Editors
File Associations
Class without source
and setting decompliler viewer as default : see the following screen shot:

Obviously I mixed the actual JD-eclipse-plugin an a plugin called JadClipse for Eclipse which is a slightly different plugin from eclipse marketplace. Thus when I followed the installation instructions of JD-eclipse, I could set the options for the screenshot posted within the question. So I set JD-eclipse as default for both *.class-files with and those without source-code.
Second mistake I did was a pretty silly and obvious one, namely that I wanted to browse the jar-files from within WEB-INF-folder of my Tomcat instead of from within Java Resources. Thus I could not dive into the jar-files at all hindering me to get to the class-level and thus to decompile those classes contained within the jar.
I hope this helps anyone outside also having trouble with the plugin for eclipse.

I had the same problem but "class without source" did have decompliler viewer as default. So all I did was set class file viewer as default and then switched back and that fixed the issue.

I had to update all of my plugins. the Procyon decompiler has this symptom when running under java 11 or later.

How do I download and build code of Eclipse IDE for Java?

Is there any manual on this? I can't find any.
What are necessary packages and where is their repo, what is the project structure and build process?
UPD the ultimate goal I'm trying to achieve is to debug Eclipse on Windows

Check these links
http://wiki.eclipse.org/Development_Resources
https://wiki.eclipse.org/Eclipse_Corner#Eclipse_Platform_Technical_Overview
https://wiki.eclipse.org/Platform_UI/How_to_Contribute#Setting_up_your_SDK
http://wiki.eclipse.org/Platform
There is a new tool called oomph Try this its easy to setup development environment. Check this video

Unable to comment so posting as answer
Please check this link should get you started
http://archive.eclipse.org/eclipse/downloads/drops/R-3.1.2-200601181600/srcIncludedBuildInstructions.html
I agree with the comment from #user3159253 . Cannot really understand the need to go through this trouble. Use RPM.

Eclipse: add javadoc

How can I generally add javadoc in eclipse for different packages.
As an example:
I want to add all the javadoc for hibernate in eclipse, but I don't know how.
I've read this article How to add hibernate javadocs in Eclipse? but I don't want to get hibernate tools because I already use STS and I don't understand the other comment.
I'm using eclipse on Ubuntu.

I am using Eclipse Helios x64 in Windows 7 x64.
In your project properties:

Another option is to use maven/m2eclipse to manage your dependencies.
It's way overkill if you have the problem for 1 or 2 libraries, but worth considering if you have a dozen.
Most open source projects publish both the jar and the -src.jar artifacts, and m2eclipse can go and download automatically the source for you.

Run
mvn eclipse:eclipse -DdownloadJavadocs=true -DdownloadSources=true
and refresh your eclipse module. Voila! No need for any eclipse plugins (unless you are already using them).

In Eclipse Kepler (some years after the question, admittedly), there's a Download JavaDoc entry on the Maven submenu. This just downloaded the Spring JavaDoc (and the status message said "Downloading Javadoc and sources", but I haven't checked that). (There's also a Download Sources entry on that submenu.)
Hope this helps.

Right-click on the hibernate jar in the package explorer, then choose "Properties", and fill the "javadoc location" field.

Try viewing the properties of your project to add a JavaDoc Location.

How to add online available javadoc to a downloaded Library in Netbeans?

maybe the answer is easy, but I'm unable to find information about the topic on google.
I've downloaded the MacWidgets Library from http://code.google.com/p/macwidgets/.
On the site you can also find the javadocs and I would like to integrate this into Netbeans to provide me with information about classes / methods while typing like on the normal java api.
Is this somehow possible? Or do I have to download the source, create a project out of it and reference the project instead of the libraries?

I confirmed with the latest NetBeans IDE 7.0 M2 version released on http://www.netbeans.org
They have provided support for URL configuration for the Javadoc.
Steps to use this feature
First create a library for the MacWidgets JAR files using Tools > Libraries dialog box.
For the created library set the URL of the Javadoc using the Javadoc tab and URL button, in this case the URL shall be http://exploding-pixels.com/google_code/javadoc_0.9.5/
Now when any class from this library is referred in the Java code editor NetBeans will show Javadoc in below the context sensitive code completion as well as in the javadoc window if it is open.
This feature is recently added and only available from NetBeans IDE 7.0 M2, release and onwards. This feature is still having some issues and opening Javadoc directly by right clicking the JAR node in the project panel library node does not work yet.
UPDATE (29 Apr 2011)
With the release of latest NetBeans IDE 7.0 the issues related to Javadoc from URL are solved. You can use a HTTP URL to specify Javadoc for third party libraries without any issues now. The issue reported by me about right clicking the JAR node is also fixed by the NetBeans developer team.

Just as an addition, if you are using cut and paste from on-line javadocs, don't forget to remove the index.html as it messes up the url.

Step 1: Go to the /Libraries/.jar
Step 2: Right-click on the particular jar file.
Step 3: 4 options will appear --> select Edit
Step 4: Click on Edit.
Step 5: Browse Javadoc and provide the doc path till API.