As Alex pointed out, Eclipse uses its own compiler which currently doesn’t support Java 7 and, as noted in the Project Plan For Eclipse Project, version Helios, support for Java 7 is deferred and decoupled from the 3.6 release:
(
deferred) Add support for Java SE 7 features. The next feature
release of Java SE is version 7, which
will probably be available in the
second half of 2010. While the
contents of this release are still
under discussion, that release is
expected to contain extensions to the
Java language, including annotations
on types (JSR-308), modularity support
(JSR-294), and other minor language
changes (Coin project). Eclipse Java
tooling will include initial support
for compiling, editing, and launching
applications for Java 7 for those
parts which have publicly available
specifications (only JSR-308 at this
point). [JDT Core, JDT UI] (288548)NOTE: In order to align our
schedule with the delayed official
Java 7 appearance and due to lack of
publicly available specifications
(including lack of a Java 7 JSR), we
have decided to move the development
work to a separate branch and decouple
it from the 3.6 release. In that
branch we will continue to develop the
Java 7 features as they become
publicly accessible. We will deliver
separate updates for the official
builds in order to provide early
access to Java 7 features.