$ yum search jdk

...[snip]....

jdk.x86_64                               2000:1.6.0_01-fcs      installed
Matched from:
jdk
The Java Platform Standard Edition Development Kit (JDK) includes both
the runtime environment (Java virtual machine, the Java platform classes
and supporting files) and development tools (compilers, debuggers,
tool libraries and other tools).

The JDK is a development environment for building applications, applets
and components that can be deployed with the Java Platform Standard
Edition Runtime Environment.

...[snip]...

$ yum install jdk

...[snip]...

No Match for argument: jdk
Nothing to do

$ yum install jdk.x86_64

...[snip]...

No Match for argument: jdk
Nothing to do