When you install MPLAB X IDE (based on the NetBeans platform), Java Runtime Environment (JRE) 8 is installed. This JRE is used solely with MPLAB X IDE and is not registered on your computer for general use.
For Windows XP, JRE 7 will need to be installed. The MPLAB X IDE installer will search for JRE 7 on your system and if not found it will recommend sites for download.
Mac OS X 10.8 (Mountain Lion) was the first version to support JRE8. Therefore Mac OS X 10.8 or greater is required for MPLAB X IDE.