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Whether or not a program is valid can also be affected by language changes. Usually previously invalid program are made valid, as when generics were added, but sometimes existing programs are rendered invalid, as when keywords were added (strictfp, assert, and enum). The version number of the resulting class file is also an external compatibility issue of sorts since it restricts which platform versions the code can be run on. Also, the compilation strategies and environment can vary when different platform versions are used to produce different class file versions, meaning compiler bug fixes and differences in compiler-internal contracts can affect the contents of the resulting class files.
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