installation

What do you need? jdk or jre?
openjdk-1.6 and openjdk-1.7 are available via the default repositories of a centos/redhat 6.
Install one with e.g.

Code:

yum install java-1.7.0-openjdk

If there is a requirement to use the sun/oracle jdk, you need to download it manually for licensing reasons.
Check the certification overview and install guide before you install the rpm with yum.

Java SE Downloads
Oracle JDK 7 and JRE 7 Certified System Configurations
Linux Platform Install (JDK)