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.

You have two options ahead of you:

1. You can uninstall JDK-16.0.2, then install OpenJDK 11 and have just that implementation on your system
2. You can install OpenJDK 11 and use update-alternatives to specify which version you want to be treated as default

This answer will focus on the second option, though you can modify it to work for the first.

0. Open a Terminal (if one is not already open) or SSH into the server you're installing Java onto
1. Install the default Java runtime. For Ubuntu 20.04, this is version 11.0.11:

   sudo apt install default-jre
   

   If you want to be absolutely certain of the version, you can also specify the version:

   sudo apt install openjdk-11-jre-headless
   

2. Install the default Java Development Kit:

   sudo apt install default-jdk
   

3. Confirm the versions:

   java -version
   

   This should give you something like:

   openjdk version "11.0.11"
   OpenJDK Runtime Environment (build 11.0.11+9-post-Ubuntu-3ubuntu1)
   OpenJDK 64-Bit Server VM (build 11.0.11+9-post-Ubuntu-3ubuntu1, mixed mode, sharing)
   

   Also check the compiler:

   javac -version
   

   Which will give you something like:

   javac 11.0.11
   

4. Set version 11 as the default for the system:

   sudo update-alternatives --config java
   

   This will give you an output similar to:

   There are 2 choices for the alternative java (providing /usr/bin/java).
   
     Selection    Path                                         Priority   Status
     --------------------------------------------------------------------------------
     0            /usr/lib/jvm/java-16-openjdk-amd64/bin/java   1111      auto mode
     1            /usr/lib/jvm/java-16-openjdk-amd64/bin/java   1111      manual mode
   * 2            /usr/lib/jvm/java-11-openjdk-amd64/bin/java   1091      manual mode
   
   Press <enter> to keep the current choice[*], or type selection number:
   

   You can also do this for the compiler with:

   sudo update-alternatives --config javac
   

5. Confirm your JAVA_HOME variable is correct:

   sudo vi /etc/environment
   

   ^{Note: Feel free to use any text editor that you prefer. The use of vi here is more a force of habit than an endorsement.} Locate the JAVA_HOME variable and ensure it's set correctly:

   JAVA_HOME="/usr/lib/jvm/java-11-openjdk-amd64"
   

   Reload your sources:

   source /etc/environment
   

   Verify the variable is set:

   echo $JAVA_HOME
   

   You should see:

   /usr/lib/jvm/java-11-openjdk-amd64
   

That's all there is to it 👍🏻

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.

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.

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

1. Create new custom variable that point to the preferred version

   

2. Set %JAVA_HOME%/bin as first entry to Path Variable

   

3. Profit!

   Check with java -version

Why not just remove the Oracle Java path entry and not worry about your JAVA_HOME placement in the Path?

You might suggest to simply remove the Oracle Java path that was prepended to your Windows PATH by the installer. Yes, I thought of that too, but the installer also copies the java.exe, javaw.exe, and javaws.exe files to C:\Windows\System32 directory, which is also in the Windows PATH variable. So rather than chase down all the possible directories that the installer put Java, and to avoid deleting the System32 files that actually may be necessary to some process somewhere, I decided that simply prepending my preferred Java Home directory to the path was best option.

Source: How to set custom Java path after installing JDK 8 – Douglas C. Ayers

First, there is no need to "downgrade"; multiple simultaneous versions are supported. You can install Java 6, something like

sudo apt-get install openjdk-6-jdk

Then you can do something like (depending on platform)

export INSTALL4J_JAVA_HOME="/usr/lib/jvm/java-6-openjdk-amd64"