You can learn Java, understand each and every method, constructor with the help of some files that you may already have seen on docs.oracle.com website. Here is an example of the javax.swing.JFrame class. You can create it your own javadoc(just similar to what you've seen in the above link) in your local computer so you don't need to search it on the internet. This is one of the good ways for learning Java from home.
Copy this zip file to any location that you want (where you want those static html javadoc files to be). For example, i've copied it to E:\
Now, extract all the files of the src.zip file using your favorite extractor, say 7 Zip
For an entire package (not including sub packages). For just a single file,
Copy src.zip
src.zip file contains (.java) source files which you need. So, copy the src.zip folder which can be found in the jdk1.x.x folder (after the installation of Java). The Java folder can be found in the Program Files folder. A sample structure might look like this.
C:\Program Files\Java\jdk1.7.0\src.zip
Copy this zip file to any location that you want (where you want those static html javadoc files to be). For example, i've copied it to E:\
Now, extract all the files of the src.zip file using your favorite extractor, say 7 Zip
Open Command Prompt
Open command prompt and then go to the folder where you have extracted those files [.java files] in command prompt. For example, if you have extracted those files in E:\src, Go to it from command prompt and then type
javadoc javax.swing
For an entire package (not including sub packages). For just a single file,
javadoc javax.swing.JFrame
The resultant html files will be named as same as the name of the class and will be in the same directory as that of the .java file. For example, when you type the above command (javadoc javax.swing.JFrame) you will get JFrame.html in the directory javax\swing
For a more better understand, i've done it practically in the form of a video. Click like and share it if you like it.