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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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If a new method or constructor cannot change resolution in existing clients, then the change is a binary-preserving source transformation. In binary-preserving source compatibility, existing clients will yield equivalent class files if recompiled. The difference between behaviorally equivalent and compilation preserving source compatibility that is not behaviorally equivalent depends on the implementation of the methods in question. If a new method changes resolution, if the different class file that results has similar enough behavior, the change may still be acceptable, while changing resolution in such a way that does not preserve semantics is likely problematic. Changing a library in such a way that current clients no longer compile is seldom appropriate.

Binary Compatibility

JLSv3 JLS §13.2 – What Binary Compatibility Is and Is Not
A change to a type is binary compatible with (equivalently, does not break binary compatibility with) preexisting binaries if preexisting binaries that previously linked without error will continue to link without error.

The JLS defines binary compatibility strictly according to linkage; if P links with L₁ and continues to link with L₂, the change made in L₂ is binary compatible. The runtime behavior after linking is not included in binary compatibility:

JLSv3 JLS §13.4.22 – Method and Constructor Body
Changes to the body of a method or constructor do not break [binary] compatibility with pre-existing binaries.

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Besides Java SE APIs, as a platform the JDK exposes many other kinds of exported interfaces. These interfaces should generally be evolved analogously to behavioral compatibility in Java SE APIs, avoiding gratuitously breaking clients of the interface.

Managing Compatibility

Original Preface to JLS
Except for timing dependencies or other non-determinisms and given sufficient time and sufficient memory space, a program written in the Java programming language should compute the same result on all machines and in all implementations.

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