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Freenet requires at least Java 7, but it is recommended that you install the most recent version of Java available for your operating system.
There are also multiple Java implementations for most operating systems. Theoretically, Freenet should work with all of them, but very little testing has been done for anything except Oracle Java and OpenJDK.
On Windows the most supported and tested Java implementation is Oracle's JRE or JDK, which is bundled with the Freenet installer for Windows.
You can also install newer versions of Java separately from Oracle's website.
Linux on x86/x64
Most Linux distributions provide OpenJDK packages that can be installed and updated using the system package manager (apt/dnf/yum/emerge/pacman), which should work well with Freenet. See the Installing on POSIX page for instructions for your specific Linux distribution.
Alternatively you can install the Oracle JRE or JDK, but this is not necessary, provides no real benefit, and makes Java updates more complicated.
Linux on ARM
As of the end of 2017, running Freenet on ARM generally requires the Oracle embedded JRE for Java 8, which you can obtain here.
This is because OpenJDK 7/8, while available on most ARM Linux distributions, does not contain an optimized JIT compiler for ARM. In practice this will cause Freenet to be extremely slow, and for some low performance ARM hardware like the Raspberry Pi B+ Freenet may be completely unusable without an optimized JIT.
Note: Oracle has discontinued the embedded JRE, Java 8 will be the last embedded JRE provided. However, Oracle has also donated their optimized JIT compiler for ARM to the OpenJDK project, so in the near future it should be possible to use OpenJDK 9 on ARM as well.
There have been OpenJDK builds for macOS produced by a variety of 3rd parties, but they are not maintained by anyone so updates are not guaranteed.