If you try to open standard Java class from JDK in JDeveloper 12c, you will notice that it does not open Source.
This happens as there is no JDK 7 source included in default installation. You can see this by using
Tools -
Manage Libraries menu. Select
Java SE Definitions tab, you will notice there is no
Source Path.
You can first download JDK 7 Source from
http://sourceforge.net/projects/jdk7src/files/jdk7u9/2012-10-27/. Download
src-jdk.zip file and place it in
oracle_common/jdk folder as shown below.
Now go back to JDeveloper. Go to
Java SE Definitions under
Tools -
Manage Libraries. Select 1.7.* definition under
User (not under
Extension), and add
src-jdk.zip to
Source Path. JDK definition under Extension is Read Only, but when you restart JDeveloper after this change, Source Path will be updated for Extension JDK as well.
Now restart JDeveloper. You will now see source for JDK 7 java classes.