In jdk7u241, a backport of 7068616 was introduced setting "-Xlint:all,-deprecation -Werror" javac compilation arguments for NIO libraries. This exposed an "unchecked method invocation" warning in sun/nio/cs/ext/DoubleByteDecoder.java (along with a couple more of related warnings) which is fatal.

8, 11 and newer releases are not affected by this bug.