When I type following command 

```
ramesh@ramesh-H61M-S2P-B3:~$ sudo apt-get install openjdk-6-jdk

```

getting error as,

```
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package openjdk-6-jdk is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
apt

E: Package 'openjdk-6-jdk' has no installation candidate

```

How to resolve it?