You wanna look at a JVM class while you are coding and you cannot. Here is the solution.
First of all, download your related JDK source code files unless you already have it. In general, source code is included in the JavaSE bundle and located under the root directory of your installation (src.zip mostly).
1. Try to “Open Decleration (F3)” to any JVM class (i.e String class). You will take “Source Not Found” message and below it you will see Attach Source… button. Press it, select the source code file (External File…) and press OK.
2. Go to Project > Properties > Java Build Path > Libraries and expand JRE System Library [your jre version] then, rt.jar. Select Source attachment, click Edit…. Select the source code file (External File…) and press OK.
3. Go to Window > Preferences > Java > Installed JRES and click Edit… for your desired JRE. Expand rt.jar, select Source attachment and click Source Attachment…. Select the source code file (External File…) and press OK.
You can use the above techniques for other libraries that you need to navigate/discover if their source codes are in your hand.
And you’re OK now. Happy coding. 
P.S: Menu structure is taken from Eclipse 3.4.0 version.
reference from:
http://www.cavdar.net/2008/07/14/3-ways-of-jdk-source-code-attachment-in-eclipse/