Well, first what version do you want? 7 and 8 are both available on Jessie (assuming you use backports). If you want it for browser support, you'll want 7 (icedtea-8 still MIA).
Code:
apt-get install openjdk-7-jdk
If you just need a runtime environment to run java applets and not develop java programs, you can install jre instead of jdk.
Will get you the runtime environment and the development kit installed, which will more than likely cover whatever you need java for. If you also want the plugin for iceweasel then
Code:
apt-get install icedtea-plugin
