Java on Linux doesn't need to be installed as root. You can install as many different Java versions you want on Linux, either in separate user accounts or in a single account.

I do it all the time (switching from one Java version to another) to test on various versions of the JVM.

Changing your Java version can be as simple as this:

Copy... $  which  java
/home/b/jdk1.5.0_22/bin/java

... $  export  PATH=/home/b/jdk1.6.0_25/bin:$PATH

... $  which  java
/home/b/jdk1.6.0_25/bin/java

To fetch an old version, go to the "Oracle Java Archive" page (Google if link becomes broken):

http://www.oracle.com/technetwork/java/archive-139210.html

Then pick your poison. I download the .bin, chmod +x it and then I extract the Java version I want from the .tgz.

Then I simply set the PATH and I'm usually good to go.

I run my IDE (IntelliJ IDEA) using one Java version, I typically compile using another JDK and I test on several JVMs.

All this from the same user account.

So it's not as if you had to install "one" Java version on a Linux system...

Now, concretely, if I were you, I'd simply remove all traces from Java while being root, and then I'd download the old version I need from the Oracle Java Archive.

Install the version you need ,

# cd /<place you want java installed>/
# wget --no-cookies --no-check-certificate --header "Cookie: gpw_e24=http%3A%2F%2Fwww.oracle.com%2F; oraclelicense=accept-securebackup-cookie" "http://download.oracle.com/otn-pub/java/jdk/<version>/jdk-<version>-linux-x64.tar.gz"

# tar xzf jdk-<version>-linux-x64.tar.gz

Install Java with alternatives,

# cd /<place java install>/jdk1.6.0_17/
# alternatives --install /usr/bin/java java /opt/jdk1.6.0_17/bin/java 2
# alternatives --config java

Then select the version want

There are 2 programs which provide 'java'.

Selection    Command
-----------------------------------------------
* + 1           /opt/jdk1.6.0_38/bin/java
2           /opt/jdk1.6.0_17/bin/java

Enter to keep the current selection[+], or type selection number: 

Enter 2

1. You have to install the openjdk-8-jre:

    sudo apt-get install openjdk-8-jre
   

2. Next switch to the jre-8 version:

    $ sudo update-alternatives --config java
    There are 2 choices for the alternative java (providing /usr/bin/java).
   
    Selection    Path                                            Priority   Status
   
    * 0            /usr/lib/jvm/java-9-openjdk-amd64/bin/java       1500      auto mode
      1            /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java   1500      manual mode
      2            /usr/lib/jvm/java-9-openjdk-amd64/bin/java       1500      manual mode
   

3. Add JAVA_HOME variable:

    export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64
   

4. check java version:

    $ java -version
    openjdk version "1.8.0_151"
    OpenJDK Runtime Environment (build 1.8.0_151-8u151-b12-0ubuntu0.17.10.2-b12)
    OpenJDK 64-Bit Server VM (build 25.151-b12, mixed mode)
   

Same procedure for Oracle java.

Download

Go to official java download page, scroll down to the bottom of the page until you see Previous Releases. Click Download

Choose Java SE 7 on the next page and you will see web-page where you can choose specific Java version. If you unsure what to choose, choose Development Kit - it will cover all your needs

Now you will get this page. And you see something like screenshot below. Choose tar.gz depending on your architecture, I highlighted two of them that fits Ubuntu:

Oracle may ask you to sign up before download

Installation

Now you have file jdk*.tar.gz. Here is great manual about installing java. I took out the instruction and changed it a little bit.

• Uncompress .tar.gz

  tar -xvf jdk*

The JDK package is extracted into jdk.xxx directory. Check carefully this folder name, because we will use it the next step. I'm assuming that path to this new extracted folder is path_to_jdk_folder and folder name is folder_name, you should manually change it before executing commands below.

Usually it is:

path_to_jdk_folder - /home/c0rp/Downloads/jdk1.7.0

folder_name - jdk1.7.0

• Installing

Run sudo update-alternatives --list java to check if you have configured java already.

$ sudo update-alternatives --list java
/usr/lib/jvm/jdk1.6.0_45_x586/bin/java
/usr/lib/jvm/jdk1.7.0_21/bin/java
/usr/lib/jvm/jdk1.8.0/bin/java

If it is saying update-alternatives: error: no alternatives for java run this set of commands. Don't forget replace path_to_jdk_folder and folder_name:

sudo mkdir /usr/lib/jvm
sudo mv path_to_jdk_folder /usr/lib/jvm/folder_name
sudo update-alternatives --install "/usr/bin/java" "java" "/usr/lib/jvm/folder_name/bin/java" 1
sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/lib/jvm/folder_name/bin/javac" 1
sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/lib/jvm/folder_name/bin/javaws" 1
sudo chmod a+x /usr/bin/java
sudo chmod a+x /usr/bin/javac
sudo chmod a+x /usr/bin/javaws

If there is no errors and your java list is not empty, run this set of commands. Don't forget replace path_to_jdk_folder and folder_name:

LNUM=$[$(update-alternatives --list java | wc -l) + 1]
sudo mv path_to_jdk_folder /usr/lib/jvm/folder_name
sudo update-alternatives --install "/usr/bin/java" "java" "/usr/lib/jvm/folder_name/bin/java" "$LNUM"
sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/lib/jvm/folder_name/bin/javac" "$LNUM"
sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/lib/jvm/folder_name/bin/javaws" "$LNUM"

• Run

  sudo update-alternatives --config java
  

You will see output similar to the one below - choose the number of new jdk1.7.0 (folder_name) - for example 2 in this list (unless you have have never installed Java in your computer in which case a sentence saying There is nothing to configure will appear):

    $ sudo update-alternatives --config java
    There are 3 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                            Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      auto mode
  1            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      manual mode
  2            /usr/lib/jvm/jdk1.7.0/bin/java                   1         manual mode
* 3            /usr/lib/jvm/jdk1.8.0/bin/java                   1         manual mode

Press enter to keep the current choice[*], or type selection number: 2
update-alternatives: using /usr/lib/jvm/jdk1.8.0/bin/java to provide /usr/bin/java (java) in manual mode

Repeat the above for:

sudo update-alternatives --config javac
sudo update-alternatives --config javaws

Check installation

java -version

When you downgrade, you'll probably want to keep it that way (having the update manager ignore the newer version), this list thread and the APT-HowTo seem to go through what's needed to do that.

Specifically check out How to keep a mixed system:

# apt-get install package=version

And How to keep specific versions of packages installed (complex), adding this to /etc/apt/preferences:

Package: <package>
Pin: <pin definition>
Pin-Priority: <pin's priority>

Update: To clarify some things: 1. Ubuntu 10.04 does not use Sun's JRE by default. Infact you need to enable repositories to install it.

As stated in my comments, just follow the above pattern:

apt-get install default-jre=1.6.0_17 

Or for Sun's

apt-get install sun-java6-jre=1.6.0_17 

But I doubt that will work, considering that the latest version of Sun's JRE is listed as 6.20.