First install JDK 6 sudo apt-Get install sun-java6-jdk and then set the default Java program sudo Update-alternatives-confi

Install JDK 6 first
Sudo apt-Get install sun-java6-jdk

 

Then you need to set the default JavaProgram
Sudo Update-alternatives -- config Java
Enter the corresponding options as prompted, and set them to JDK 6.

The following settings
Sudo Vim/etc/environment

Add the following two lines:
Classpath =/usr/lib/JVM/Java-6-sun/lib
Java_home =/usr/lib/JVM/Java-6-sun
If classpath and java_home are already set, change them to the above format and press ZZ to save and exit.

Next install eclipse
Sudo apt-Get install eclipse

Note that eclipse does not care about the update-alternative

Therefore, you should modify the eclipse configuration. First, set the SUN-JDK-6 to the system default:

Sudo Update-Java-alternatives-s Java-6-sun

Then edit the JVM configuration file:
Sudo Vim/etc/JVM

Add
/Usr/lib/JVM/Java-6-sun
(If not, create it by yourself)

Sudo Vim/etc/Eclipse/java_home

Also add at the top of the file
/Usr/lib/JVM/Java-6-sun

Start eclipse, select help-> about eclipse SDK, and select configuration details. You can see that jdk6 settings have taken effect.