You will need Apple JDK 1.6 to run the JDeveloper installer. Download and install Apple Java 1.6 from: http://support.apple.com/kb/DL1572
After Apple JDK 1.6 was installed, you should check "Known Issues" from Jdev Release Notes and create symbolic link as mentioned above:
cd /Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/
su root
mkdir jre
cd jre
mkdir lib
cd lib
ln -s ../../../Classes/classes.jar rt.jar
And now you could run Jdev installer:
/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/bin/java -jar ~/Downloads/jdevstudio11119install.jar
When installer asks for JDK, point it to Oracle JDK 1.7 location. JDeveloper and integrated WebLogic Server will use Oracle JDK 1.7 after installation. You could find JDK 1.7 in your OS if you'll run:
/usr/libexec/java_home -v 1.7
Now get back to release notes and open jdev.conf from MW_HOME/jdeveloper/jdev/bin. Add this line to jdev.conf:
AddVmOption -Djava.io.tmpdir=/var/tmp
Now you can run Jdev!