Sometimes the method definition is not displayed hovering over the method name. On hovering the desired method, following message is displayed:

"Note: This element neither has attached source nor attached Javadoc and hence no Javadoc could be found. ". The reason for this behavior is that eclipse did not know that where to look for the definition of the desired method.
Solution:

The solution of the above issue is that we have to set the path of javadoc in our eclipse for this jar file. For eg: I have configured log4j in my project and added its jar files too but on hovering the method, definition is not displayed in eclipse.

Javadoc path for log4j: https://logging.apache.org/log4j/1.2/apidocs/

So, first search the javadoc for log4j and then add java doc location path in jar file. Right click project > Build Path > Configure Build Path > Libraries tab > Javadoc location(edit) > Give the path > Save.

Note: Do not give index.html in path.