How to migrate from JRE 1.6 to JDK 1.6?
Java JRE and JDKS 1.6 and lower are supplied from Apple and 1.7 or 8 are from Oracle.

The latest Apple one is here I think.
This installs the JDK and the JRE.

the JDK is under /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/ but /usr/bin contains java, javac etc so will be on the path