The global AdoptOpenJDK initiative started by the London Java Community to improve developer participation in OpenJDK has released the first set of coverage numbers for OpenJDK.

Oracle have always generated coverage numbers via the jcov tool for their build of Java, but have never released coverage numbers.

Discussions between AdoptOpenJDK members and Oracle's Quality team indicate that the AdoptOpenJDK numbers are not significantly different from the internal Oracle numbers, but Rory O'Donnell cautioned that "comparing our internal numbers with yours is like comparing apples with pears".

The coverage numbers can be found here.

On the whole, the coverage numbers look reasonable, although some care is advised when reading them. For example, one of the worst performing packages according to the coverage metrics is the brand-new semi-functional support in java.util.function, with several of the members scoring 0% coverage.

However, on inspecting the line level coverage, it is immediately apparent that these strange results are actually being caused by Java 8 functional interfaces with very small amounts of obviously-correct code in default methods, and are not significant.

By contrast, one of the best-scoring group of packages is another new Java 8 feature - the new date and time support contained in java.time and its subpackages.