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Downgrade open jdk 8 to 7 in ubuntu 14.04

I accidentally upgraded the whole system in ubuntu 14.04.
I am trying to deploy a war file which requires JDK7.
I tried to install JDK7 and use it as default
root:floyd~# update-alternatives --config java
There is only one alternative in link group java (providing /usr/bin/java): /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java
Nothing to configure.
but it is not installing as well. It gives following error while trying to install JDK 7.
root#floyd:~# apt-get install openjdk-7-jdk
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package openjdk-7-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
N: Ignoring file '50unattended-upgrades.ucf-dist' in directory '/etc/apt/apt.conf.d/' as it has an invalid filename extension
E: Package 'openjdk-7-jdk' has no installation candidate
I guess I have two options.
Install JDK7 and use it as default.
Downgrade JDK8 to JDK7.
I think downgrading can be a good option but I am not able to do any of them.

Installing JRE/JDK in default configuration
OpenJDK 7 is installed as default in Ubuntu 12.10 and later.
Installing Java whereby apt-get is easy:
sudo apt-get update
java -version
If after execution we have something like: "The program java can be found in the following packages", it means that Java isn't installed. So, we need to to:
sudo apt-get install default-jre
As result, Java Runtime Environment (JRE) will be installed. When we want to install Java Development Kit (JDK), which is needed for compilling Java-app (for example, Apache Ant, Apache Maven, Eclipse and IntelliJ IDEA) we need to do:
sudo apt-get install default-jdk
Now, Java is installed.
Installing OpenJDK 7 (optionally)
sudo apt-get install openjdk-7-jre
After execution Java Runtime Environment (JRE) will be installed. For Java Development Kit (JDK), we execute:
sudo apt-get install openjdk-7-jdk
Installing Oracle JDK (optionally)
Oracle JDK is an official JDK but, now, Oracle doesn't support it like default for installation in Ubuntu.
We can install it by apt-get but before we need to execute next commands:
sudo apt-get install python-software-properties
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
Later, we have to choose needed version and execute:
For Oracle JDK 6
sudo apt-get install oracle-java6-installer
For Oracle JDK 7
sudo apt-get install oracle-java7-installer
For Oracle JDK 8
sudo apt-get install oracle-java8-installer
For Oracle JDK 9
sudo apt-get install oracle-java9-installer
Managing of Java (optionally)
When we have some version Java installed we can choose one of them as default:
sudo update-alternatives --config java
As result we will see something like this:
There are 2 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/lib/jvm/java-7-oracle/jre/bin/java 1062 auto mode
1 /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java 1061 manual mode
2 /usr/lib/jvm/java-7-oracle/jre/bin/java 1062 manual mode
Press enter to keep the current choice[*], or type selection number:
We can see it on the screen:
The same action we can do to choose the compiler (javac):
sudo update-alternatives --config javac
This command can be used to choose other java componenst, for example, like: keytool, javadoc and jarsigner.
Installing "JAVA_HOME"
To be installed **JAVA_HOME**:
sudo update-alternatives --config java
Result like:
There are 2 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/lib/jvm/java-7-oracle/jre/bin/java 1062 auto mode
1 /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java 1061 manual mode
2 /usr/lib/jvm/java-7-oracle/jre/bin/java 1062 manual mode
Press enter to keep the current choice[*], or type selection number:
As we can see the paths for java will be:
/usr/lib/jvm/java-7-oracle
/usr/lib/jvm/java-6-openjdk-amd64
/usr/lib/jvm/java-7-oracle
Now, we need to copy one of the ways and paste it into:
/etc/environment:
sudo nano /etc/environment
In the file we are going to add a path(where YOUR_PATH - is path for desired version of java, for example: "/usr/lib/jvm/java-7-oracle"):
JAVA_HOME="/usr/lib/jvm/java-7-oracle"
On the screen:
Now, we need to reboot this file:
source /etc/environment
To check it we can:
echo $JAVA_HOME
On the screen:
When we have, now, input way("/usr/lib/jvm/java-7-oracle" in our example) it will be means we do it sucessfully. In other way we need to be more attentive and check all these steps once.

How to remove old version of Java and install new version

I have a Linux box on which Java 1.7 is installed:
#java -version
java version "1.7.0_09-icedtea"
OpenJDK Runtime Environment (rhel-2.3.4.1.el6_3-x86_64)
OpenJDK 64-Bit Server VM (build 23.2-b09, mixed mode)
But for some new tool I need to install the jdk-8u45-linux-x64.tar.gz
So how can I remove old one and install the new one?

To remove OpenJDK (the one you've already installed)
sudo apt-get purge openjdk-\*
Make a new directory for your new JDK
sudo mkdir -p /usr/local/java
Copy the file to the directory (you should be in that file path)
sudo cp -r jdk-8u45-linux-x64.tar.gz /usr/local/java/
Extract the file
sudo tar xvzf jdk-8u45-linux-x64.tar.gz
You should add this to your PATH now. To do that:
a. Open /etc/profile : sudo gedit /etc/profile
b. Scroll down (the end) and add the path where your jdk was installed
JAVA_HOME=/usr/local/java/jdk1.8.0_45
PATH=$PATH:$HOME/bin:$JAVA_HOME/bin
export JAVA_HOME
export PATH
Save and exit
Inform your Linux system where your Oracle Java JDK/JRE is located.
a. Notify the system that Oracle Java JRE is available for use
sudo update-alternatives --install "/usr/bin/java" "java" "/usr/local/java/jdk1.8.0_45/bin/java" 1
b. Notify the system that Oracle Java JDK is available for use
sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/local/java/jdk1.8.0_45/bin/javac" 1
c. Notify the system that Oracle Java Web start is available for use
sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/local/java/jdk1.8.0_20/bin/javaws" 1
Inform your Linux system that Oracle Java JDK/JRE must be the default Java.
a. Set the java runtime environment for the system
sudo update-alternatives --set java /usr/local/java/jdk1.8.0_45/bin/java
b. Set the javac compiler for the system
sudo update-alternatives --set javac /usr/local/java/jdk1.8.0_45/bin/javac
c. Set Java Web start for the system
sudo update-alternatives --set javaws /usr/local/java/jdk1.8.0_20/bin/javaws
Reload your system wide PATH
source /etc/profile
Check the new version and you're done!
java -version

Just unpack the new Java version, for example into /opt. Then do
export JAVA_HOME=/opt/jdk1.8.0_45
export PATH=$JAVA_HOME/bin:$PATH
Put these exports into the startup files for your shell and you should be set. It is not necessary to uninstall the Java 7 installation.

On Centos/Redhat
First do : rpm -qa|grep jdk
And then : rpm -e <the version which you dont want>

I had java 7 installed on my ubuntu machine and wanted java 8. For that i followed the following steps
purge the older with sudo apt-get purge openjdk-\*
Add repository sudo add-apt-repository ppa:openjdk-r/ppa
Update the packages sudo apt-get update
Install java 8 with sudo apt-get install openjdk-8-jdk
To check list of java versions installed dosudo update-java-alternatives --list
Select default java for java sudo update-alternatives --config java
Select default java for javac sudo update-alternatives --config javac
check with java -version and javac -version to check if update version is installed
I referred following link Install OpenJDK8 on Unbuntu Trusty
Hope this helps!

I went to this article, the final option did it for me:
A final option for you to consider is yum remove libpcap, see how many packages it wants to remove (yum won't actually remove anything until you hit the Y button and press enter), since you may be able to remove these packages and use yum, if it turns out you couldn't after all, you can roll back with yum history undo command (I don't remember in what version of yum this was implemented in yum, so check first if this sub command is available to you). Also, make sure that you got your repositories pointing to the repos which has these files available.
After that I could remove my old jdk with:
sudo rpm -e java-1.8.0-openjdk-1.8.0.352.b08-2.el7_9

Installing Java 7 on Ubuntu

Note: This question was asked before Oracle made the OpenJDK the free version of the Oracle JDK, and the historic answers reflect that. As of 2022 you should not use Java 7 unless you must for projects which cannot run on OpenJDK 8.
To install java I have always used the classic way from the terminal.
I would like to install java manually.
I placed the folder of the JDK on the desk and I set environment variables (PATH, CLASSPATH and JAVA_HOME).
From the terminal, if I type java -version I get printed
foralobo#ubuntu-vincy:~$ java -version
java version "1.7.0_21"
Java(TM) SE Runtime Environment (build 1.7.0_21-b11)
Java HotSpot(TM) 64-Bit Server VM (build 23.21-b01, mixed mode)
But when I try to install eclipse or netbeans, the system warns by saying that there is no java installed on the machine.
What is missing to compleatare manual installation?
(Ubuntu 13.04)

This answer used to describe how to install Oracle Java 7. This no longer works since Oracle end-of-lifed Java 7 and put the binary downloads for versions with security patches behind a paywall. Also, OpenJDK has grown up and is a more viable alternative nowadays.
In Ubuntu 16.04 and higher, Java 7 is no longer available. Usually you're best off installing Java 8 (or 9) instead.
sudo apt-get install openjdk-8-jre
or, f you also want the compiler, get the jdk:
sudo apt-get install openjdk-8-jdk
In Trusty, the easiest way to install Java 7 currently is to install OpenJDK package:
sudo apt-get install openjdk-7-jre
or, for the jdk:
sudo apt-get install openjdk-7-jdk
If you are specifically looking for Java 7 on a version of Ubuntu that no longer supports it, see https://askubuntu.com/questions/761127/how-do-i-install-openjdk-7-on-ubuntu-16-04-or-higher .

sudo apt-get update
sudo apt-get install openjdk-7-jdk
and if you already have other JDK versions installed
sudo update-alternatives --config java
then select the Java 7 version.

In addition to flup's answer you might also want to run the following to set JAVA_HOME and PATH:
sudo apt-get install oracle-java7-set-default
More information at:
http://www.ubuntuupdates.org/package/webupd8_java/precise/main/base/oracle-java7-set-default

Note: According to comments this text was copied from a GIST without attribution.
Download java jdk<version>-linux-x64.tar.gz file from https://www.oracle.com/technetwork/java/javase/downloads/index.html.
Extract this file where you want. like: /home/java(Folder name created by user in home directory).
Now open terminal.
Set path JAVA_HOME=path of your jdk folder(open jdk folder then right click on any folder, go to properties then copy the path using select all)
and paste here.
Like: JAVA_HOME=/home/xxxx/java/JDK1.8.0_201
Let Ubuntu know where our JDK/JRE is located.
sudo update-alternatives --install /usr/bin/java java /home/xxxx/java/jdk1.8.0_201/bin/java 20000
sudo update-alternatives --install /usr/bin/javac javac /home/xxxx/java/jdk1.8.0_201/bin/javac 20000
sudo update-alternatives --install /usr/bin/javaws javaws /home/xxxx/java/jdk1.8.0_201/bin/javaws 20000
Tell Ubuntu that our installation i.e., jdk1.8.0_05 must be the default Java.
sudo update-alternatives --set java /home/xxxx/sipTest/jdk1.8.0_201/bin/java
sudo update-alternatives --set javac /home/xxxx/java/sipTest/jdk1.8.0_201/bin/javac
sudo update-alternatives --set javaws /home/xxxxx/sipTest/jdk1.8.0_201/bin/javaws
Now try:
$ 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-6-oracle1/bin/java 1047 auto mode
1 /usr/bin/gij-4.6 1046 manual mode
2 /usr/lib/jvm/java-6-oracle1/bin/java 1047 manual mode
3 /usr/lib/jvm/jdk1.7.0_75/bin/java 1 manual mode
Press enter to keep the current choice [*], or type selection number: 3
update-alternatives: using /usr/lib/jvm/jdk1.7.0_75/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

I think you should consider Java installation procedure carefully. Following is the detailed process which covers almost all possible failures.
Installing Java with apt-get is easy. First, update the package index:
sudo apt-get update
Then, check if Java is not already installed:
java -version
If it returns "The program java can be found in the following packages", Java hasn't been installed yet, so execute the following command:
sudo apt-get install default-jre
You are fine till now as I assume.
This will install the Java Runtime Environment (JRE). If you instead need the Java Development Kit (JDK), which is usually needed to compile Java applications (for example Apache Ant, Apache Maven, Eclipse and IntelliJ IDEA execute the following command:
sudo apt-get install default-jdk
That is everything that is needed to install Java.
Installing OpenJDK 7:
To install OpenJDK 7, execute the following command:
sudo apt-get install openjdk-7-jre
This will install the Java Runtime Environment (JRE). If you instead need the Java Development Kit (JDK), execute the following command:
sudo apt-get install openjdk-7-jdk
Installing Oracle JDK:
The Oracle JDK is the official JDK; however, it is no longer provided by Oracle as a default installation for Ubuntu.
You can still install it using apt-get. To install any version, first execute the following commands:
sudo apt-get install python-software-properties
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
Then, depending on the version you want to install, execute one of the following commands:
Oracle JDK 7:
sudo apt-get install oracle-java7-installer
Oracle JDK 8:
sudo apt-get install oracle-java8-installer

PPA method no longer works.
While Oracle Java 6 and 7 are not supported for quite a while, they were still available for download on Oracle's website until recently.
However, the binaries were removed about 10 days ago (?), so the Oracle Java (JDK) 6 and 7 installers available in the WebUpd8 Oracle Java PPA no longer work.
Oracle Java 6 and 7 are now only available for those with an Oracle Support account (which is not free), so I can't support this for the PPA packages.
Source : http://www.webupd8.org/2017/06/why-oracle-java-7-and-6-installers-no.html
Dated : June 2017
Oracle's download page says
Updates for Java SE 7 released after April 2015, and updates for Java SE 6 released after April 2013 are only available to Oracle Customers through My Oracle Support (requires support login).
Java SE Advanced offers users commercial features, access to critical bug fixes, security fixes, and general maintenance".
I had to download it from Oracle archives -
http://www.oracle.com/technetwork/java/javase/downloads/java-archive-downloads-javase7-521261.html
You need an account for this though.

Oracle Java 1.7.0 from .deb packages
wget https://raw.github.com/flexiondotorg/oab-java6/master/oab-java.sh
chmod +x oab-java.sh
sudo ./oab-java.sh -7
sudo apt-get update
sudo sudo apt-get install oracle-java7-jdk oracle-java7-fonts oracle-java7-source
sudo apt-get dist-upgrade
Workaround for 1.7.0_51
There is an Issue 123 currently in OAB and a pull request
Here is the patched vesion:
wget https://raw.github.com/ladios/oab-java6/master/oab-java.sh
chmod +x oab-java.sh
sudo ./oab-java.sh -7
sudo apt-get update
sudo sudo apt-get install oracle-java7-jdk oracle-java7-fonts oracle-java7-source
sudo apt-get dist-upgrade

flup's answer is the best but it did not work for me completely. I had to do the following as well to get it working:
export JAVA_HOME=/usr/lib/jvm/java-7-oracle/jre/
chmod 777 on the folder
./gradlew build - Building Hibernate

Oracle as well as modern versions of Ubuntu have moved to newer versions of Java. The default for Ubuntu 20.04 is OpenJDK 11 which is good enough for most purposes.
If you really need it for running legacy programs, OpenJDK 8 is also available for Ubuntu 20.04 from the official repositories.
If you really need exactly Java 7, the best bet as of 2020 is to download a Zulu distribution. The easiest to install if you have root privileges is the .DEB version, otherwise download the .ZIP one.
https://www.azul.com/downloads/?version=java-7-lts&os=ubuntu&architecture=x86-64-bit&package=jdk#download-openjdk

Open Applicaction -> Accessories -> Terminal
Type commandline as below...
sudo apt-get install openjdk-7-jdk
Type commandline as below...
apt-cache search jdk
(Note: openjdk-7-jdk is symbolically used here. You can choose the JDK version as per your requirement.)
For "JAVA_HOME" (Environment Variable) type command as shown below, in "Terminal" using your installation path...
export JAVA_HOME=/usr/lib/jvm/java-7-openjdk
(Note: "/usr/lib/jvm/java-7-openjdk" is symbolically used here just for demostration. You should use your path as per your installation.)
For "PATH" (Environment Variable) type command as shown below, in "Terminal" using your installation path...
export PATH=$PATH:/usr/lib/jvm/java-7-openjdk/bin
(Note: "/usr/lib/jvm/java-7-openjdk" is symbolically used here just for demostration. You should use your path as per your installation.)
Check for "open jdk" installation, just type command in "Terminal" as shown below
javac -version

How to install the JDK on Ubuntu Linux

Note: This is an old question and the answers reflect the world as it was then. Modern Ubuntu distributions have OpenJDK available which can be installed with
sudo apt install default-jdk
I am trying to install the Java Development Kit (JDK) on Ubuntu Linux distribution, but I am unable to install it.
What are the steps to install it on Ubuntu?

Referring to Ask Ubuntu question How to set JAVA_HOME for OpenJDK?,
How to install Open JDK (Java developement kit) in Ubuntu (Linux)?
Open Terminal from Application Dash or press Ctrl+Alt+T
Update repository:
sudo add-apt-repository ppa:openjdk-r/ppa # only Ubuntu 17.4 and earlier
sudo apt update
Optional: To search available distributions of openjdk, use the following command:
apt search openjdk
Install the appropriate version with the following command:
sudo apt install openjdk-8-jdk
sudo apt install openjdk-8-source #this is optional, the jdk source code
For JAVA_HOME (Environment Variable) type command as shown below, in "Terminal" using your installation path...
export JAVA_HOME=/usr/lib/jvm/java-8-openjdk
(Note: /usr/lib/jvm/java-8-openjdk is symbolically used here just for demostration. You should use your path as per your installation.)
For PATH (Environment Variable) type command as shown below, in Terminal:
export PATH=$PATH:$JAVA_HOME/bin
To check your installation:
java -version

The following used to work before the Oracle Java license changes in early 2019.
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java7-installer
The PPA is discontinued, until the author finds a workaround for the license issues.

You can install Oracle's JDK 1.7 fairly easily too; as an example this is how to install JDK 1.7.0_13;
Download the JDK from Oracle's site. The download to the newest version is always linked from http://java.oracle.com.
As root, do;
cd /usr/local
tar xzf <the file you just downloaded>
As your normal user, add or change these two lines in your ~/.profile to point to the installation;
export JAVA_HOME=/usr/local/jdk1.7.0_13
export PATH=$PATH:$JAVA_HOME/bin
If it's an update, you may also want to remove the old java installation directory in /usr/local.
Log out and in again (or do . ~/.profile), and everything should just work.
The downside with Oracle's JDK is that it won't update with the rest of your system like OpenJDK will, so I'd mostly consider it if you're running programs that require it.

In case you have already downloaded the ZIP file follow these steps.
Run the following command to unzip your file.
tar -xvf ~/Downloads/jdk-7u3-linux-i586.tar.gz
sudo mkdir -p /usr/lib/jvm/jdk1.7.0
sudo mv jdk1.7.0_03/* /usr/lib/jvm/jdk1.7.0/
sudo update-alternatives --install "/usr/bin/java" "java" "/usr/lib/jvm/jdk1.7.0/bin/java" 1
sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/lib/jvm/jdk1.7.0/bin/javac" 1
sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/lib/jvm/jdk1.7.0/bin/javaws" 1
After installation is complete, set environment variables as follows.
Edit the system path in file /etc/profile:
sudo gedit /etc/profile
Add the following lines at the end.
JAVA_HOME=/usr/lib/jvm/jdk1.7.0
PATH=$PATH:$HOME/bin:$JAVA_HOME/bin
export JAVA_HOME
export PATH
Source: http://javaandme.com/

Note: WebUpd8 team's PPA has been discontinued with effective from April 16, 2019. Thus this PPA doesn't have any Java files. More information can be found on PPA's page on Launchpad. Hence the below method no longer works and exists because of historical reasons.
If you want to install the latest JDK 1.8, use the webupd8team PPA.
Add the repository in your system:
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
You can now install Oracle Java 8 using the following command:
sudo apt-get install oracle-java8-installer
This ppa repository also provides a package to set environment variables automatically. Just type:
sudo apt-get install oracle-java8-set-default

OpenJDK is OK for the most cases, but Oracle JDK can be required for some bank client applications (my case) - I can't use OpenJDK.
I'm surprised that I don't see any answer with the default method (repository without external PPAs) in Ubuntu 12.10+ for Oracle's JDK - I will try to describe it.
Install JavaPackage:
sudo apt-get install java-package
Download Oracle JDK from Oracle downloads page
Make a Debian package from the downloaded .tar.gz archive:
make-jpkg jdk-YOUR_VERSION-linux-PLATFORM.tar.gz
This command will produce a .deb package.
Install the package in your favourite way (for example, sudo dpkg -i oracle-java8-jdk_8u40_amd64.deb)
It's the officially supported way from Debian developers for installing Oracle JDK, and I suppose it's very simple.

Note: WebUpd8 team's PPA has been discontinued with effective from April 16, 2019. Thus this PPA doesn't have any Java files. More information can be found on PPA's page on Launchpad. Hence the below method no longer works and exists because of historical reasons.
Installing Java 8 on Ubuntu
First you need to add webupd8team Java PPA repository in your system and install Oracle Java 8 using following set of commands.
$ sudo add-apt-repository ppa:webupd8team/java
$ sudo apt-get update
$ sudo apt-get install oracle-java8-installer
Verify Installed Java Version
After successfully installing Oracle Java using the above steps, verify the installed version using the following command.
$ java -version
java version "1.8.0_77"
Java(TM) SE Runtime Environment (build 1.8.0_77-b03)
Java HotSpot(TM) 64-Bit Server VM (build 25.77-b03, mixed mode)
###Configuring the Java environment
In Webupd8 ppa repository also providing a package to set environment variables, Install this package using following command.
$ sudo apt-get install oracle-java8-set-default
Reference

Simply run:
sudo apt-get install default-jdk

Wiki from Ubuntu website:
For Ubuntu 10.04 LTS, the sun-java6 packages have been dropped from the Multiverse section of the Ubuntu archive. It is recommended that you use openjdk-6 instead.
If you can not switch from the proprietary Sun JDK/JRE to OpenJDK, you can install sun-java6 packages from the Canonical Partner Repository. You can configure your system to use this repository via command-line:
sudo add-apt-repository "deb http://archive.canonical.com/ lucid partner"
sudo apt-get update
sudo apt-get install sun-java6-jre sun-java6-plugin
sudo update-alternatives --config java
For Ubuntu 10.10, the sun-java6 packages have been dropped from the Multiverse section of the Ubuntu archive. It is recommended that you use openjdk-6 instead.
If you can not switch from the proprietary Sun JDK/JRE to OpenJDK, you can install sun-java6 packages from the Canonical Partner Repository. You can configure your system to use this repository via command-line:
sudo add-apt-repository "deb http://archive.canonical.com/ maverick partner"
sudo apt-get update
sudo apt-get install sun-java6-jre sun-java6-plugin
sudo update-alternatives --config java

Installed in ubuntu 18.04
My workaround was,
$ sudo apt update
Install OpenJDK 8:
$ sudo apt install openjdk-8-jdk
Verify the Java installation by running the following command which will print the Java version:
$ java -version
The output should look like this:
Output:
openjdk version "1.8.0_191"
OpenJDK Runtime Environment (build 1.8.0_191-8u191-b12-2ubuntu0.18.04.1-b12)
OpenJDK 64-Bit Server VM (build 25.191-b12, mixed mode)

Using a PPA (Obsolete)
Note: WebUpd8 team's PPA has been discontinued with effective from April 16, 2019. Thus this PPA doesn't have any Java files. More information can be found on PPA's page on Launchpad. Hence the below method no longer works and exists because of historical reasons.
You can use WebUpd8 PPA (this will download the required files from Oracle and install JDK 8):
sudo apt-add-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java8-installer
Are PPA's safe to add to my system and what are some “red flags” to watch out for?
Also ensure your JAVA_HOME variable has been set to:
/usr/lib/jvm/java-8-oracle
Manual install
The tar.gz provided by Oracle don't have an actual installation process. You just extract those files to a location you want and add them to your path. So the process is the following:
Download a .tar.gz from Oracle (here I will be using jdk-8u20-linux-x64.tar.gz);
Extract it to somewhere;
Move the extracted folder to /usr/lib/jvm. This is not required but it is the place where Java runtime software is installed
sudo mv /path/to/jdk1.8.0_20 /usr/lib/jvm/oracle_jdk8
Create a file /etc/profile.d/oraclejdk.sh with the following content (adapt the paths to reflect the path where you stored your JDK):
export J2SDKDIR=/usr/lib/jvm/oracle_jdk8
export J2REDIR=/usr/lib/jvm/oracle_jdk8/jre
export PATH=$PATH:/usr/lib/jvm/oracle_jdk8/bin:/usr/lib/jvm/oracle_jdk8/db/bin:/usr/lib/jvm/oracle_jdk8/jre/bin
export JAVA_HOME=/usr/lib/jvm/oracle_jdk8
export DERBY_HOME=/usr/lib/jvm/oracle_jdk8/db
Done! Those paths will only be recognized after you logout or restart, so if you want to use them right away run source /etc/profile.d/oraclejdk.sh.

In Ubuntu 18.04, We can install java like a normal package without using an external repository
Just run this command
sudo apt install openjdk-8-jdk

You can use the sudo apt-get install default-jdk terminal command to install the default JDK version.
Before installing Java, type the sudo apt-get update terminal command and then type the install terminal command. You can get more information from here.

Have a look at OpenJDK. It is the standard JVM implementation on Linux.

The best is to install default Java until a specific Java version is not required.
Before this, execute java -version to check if Java is not already installed.
sudo apt-get update
sudo apt-get install default-jre
sudo apt-get install default-jdk
That is everything that is needed to install Java.

I recommend JavaPackage.
It's very simple. You just need to follow the instructions to create a .deb package from the Oracle tar.gz file.

You can install via apt-get:
sudo add-apt-repository ppa:linuxuprising/java
sudo apt-get update
sudo apt-get install oracle-java11-installer
After, do not forget to check the version:
java -version

sun-java6-jdk is a virtual package provided by oracle-java8-installer or oracle-java7-installer or oracle-java7-installer.
sudo apt-get install oracle-java8-installer
will give you sun-java6-jdk.

I have successfully installed JDK 10 on Ubuntu 18.04 LTS following this video.
I am copying the excerpt from the description of the video.
Just open the terminal and give these commands :
For Java Installation (PPA)
sudo add-apt-repository ppa:linuxuprising/java
sudo apt-get update
sudo apt-get install oracle-java10-installer
For setting up environment variables (make java10 default)
sudo apt-get install oracle-java10-set-default
The same procedure can be followed on Ubuntu 16.04, Linux Mint, Debian and other related Linux systems to install JDK 10.

You can use SDKMan,
curl -s "https://get.sdkman.io" | bash
source "~/.sdkman/bin/sdkman-init.sh"
sdk install java

If you want to install Oracle JDK, you can use this automated script that does all the work for you.
There are detailed instructions how to use it on the author's blog.

Installing Oracle's Java JDK requires you to accept the Oracle license before the installation begins. This is only required once. If for some reason you need the installation to be automated, you can run the following commands to install without user interaction, useful for an automatic script for example.
sudo add-apt-repository -y ppa:webupd8team/java
sudo apt-get update
echo debconf shared/accepted-oracle-license-v1-1 select true | sudo debconf-set-selections
echo debconf shared/accepted-oracle-license-v1-1 seen true | sudo debconf-set-selections
sudo apt-get -y install oracle-java8-installer
java -version

Try to use SDKMAN! package manager - https://sdkman.io/install
Now it's the easiest way to install many JVM-based SDKs, tools and frameworks on any Linux distribution.
Upon installation, run the following command to see all available Java distributions:
sdk list java
Select the distribution that you need and install it:
sdk install java <version>

Install the appropriate version of OpenJDK
JAVA_VERSION=8
sudo add-apt-repository -y ppa:openjdk-r/ppa
sudo apt-get update
sudo apt-get -qq install -y openjdk-$JAVA_VERSION-jdk
Set Environment Variables in /etc/profile.d/jdk.sh
JAVA_HOME=/usr/lib/jvm/java-$JAVA_VERSION-openjdk-amd64
echo "export JAVA_HOME=$JAVA_HOME" | sudo tee -a /etc/profile.d/jdk.sh
echo "export J2SDKDIR=$JAVA_HOME" | sudo tee -a /etc/profile.d/jdk.sh
echo "export J2REDIR=$JAVA_HOME/jre" | sudo tee -a /etc/profile.d/jdk.sh
echo "export PATH=$PATH:$JAVA_HOME/bin:$J2REDIR/bin" | sudo tee -a /etc/profile.d/jdk.sh
Check your installation
/bin/bash /etc/profile.d/jdk.sh
java -version
echo $JAVA_HOME
echo $J2REDIR
echo $PATH

Step 01: Download JDK
Download the JDK of required version from oracle official website :
[Oracle website to download JDK]: https://www.oracle.com/in/java/technologies/javase-downloads.html
e.i - jdk-11.0.12_linux-x64_bin.tar.gz
Step 02: Installation:
a) Extract the downloaded files using following command
sudo -tar -vzxf jdk-11.0.12_linux-x64_bin.tar.gz
b) Create a directory of name java inside usr/lib directory
mkdir java
c) Move the extracted jdk-11.0.12 directory inside the new created directory java
sudo mv jdk-11.0.12 /usr/lib/java
Step 03:
a) Execute following commands one by one
sudo update-alternative -install /usr/lib/java java /usr/lib/java/jdk-11.0.12/bin/java 1
sudo update-alternative -install /usr/lib/javac javac /usr/lib/java/jdk-11.0.12/bin/javac 1
Step 04: installation varification :
Verify the installation by using following commands
java --version
javac --version

You can use oraji. It can install/uninstall both JDK or JRE from oracle java (.tar.gz).
To install run sudo oraji '/path/to/the/jdk_or_jre_archive'
To uninstall run oraji -u and confirm the version number.

I had the same problem and none of the comments worked for me. Finally, I noticed that I disabled my updates. When I reactivate it, so sudo apt-get update worked correctly and the issue was solved.
(update in system settings> software and updates>updates tab
here I ticked two first option of important update and recommended updates).

Try this in case you do not want to install OpenJDK: JDK Source Installer for Ubuntu

In Ubuntu1604 I faced "No installation candidate error". Following below steps helped me install.
-sudo apt-get update
-sudo apt-get upgrade
-apt-get install software-properties-common
-sudo add-apt-repository ppa:webupd8team/java
-apt-get update
-sudo apt install oracle-java8-installer

Over Debian you can try
apt-get install default-jdk

Java not visible when using oracle self extracting installer for ubuntu

I want to install the oracle jdk 6 in ubuntu for all users using the self extracting installer at oracles website (jdk-6u37-linux-x64.bin)
This is what i do step by step:
1- Download the jdk-6u37-linux-x64.bin
2- Create a folder in the unix system resources and move the downloaded file there
sudo mkdir -p /usr/local/java
sudo mv /home/sfrj/Downloads/jdk-6u37-linux-x64.bin /usr/local/java
3- Make the file executable
sudo chmod 700 jdk-6u37-linux-x64.bin
4. Execute the installer
sudo ./jdk-6u37-linux-x64.bin
5. Remove the installer(Don't need it anymore)
sudo rm jdk-6u37-linux-x64.bin
6. Create a symbolic link
sudo ln -s jdk1.6.0_37 /usr/local/java/latest
7. Edit the file /etc/environment
JAVA_HOME="/usr/local/java/jdk1.6.0_37"
JRE_HOME="/usr/local/java/jdk1.6.0_37/jre"
PATH="/usr/local/java/jdk1.6.0_37/bin:\/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"
8. Reload the environment file
source /etc/environment
So far so good, after all that and without closing the terminal I type: java -version
and I see this:
java version "1.6.0_37" Java(TM) SE Runtime Environment (build
1.6.0_37-b06) Java HotSpot(TM) 64-Bit Server VM (build 20.12-b01, mixed mode)
The problem is, when close the terminal, open it again and type the command java -version again, I see this:
The program 'java' can be found in the following packages:
* default-jre
* gcj-4.6-jre-headless
* gcj-4.7-jre-headless
* openjdk-7-jre-headless
* openjdk-6-jre-headless
Try: sudo apt-get install <selected package>
I am confused:
-Why is this?
-Did I install the jdk correctly?
-What is missing?
I want to install it this way, without using apt-get, so please don't answer to me use apt-get install...

-Why is this?
You are not setting your path in a way which is reloaded automatically.
-Did I install the jdk correctly?
There is two ways of installing it. For the self unpacking version this is correct.
-What is missing?
Your environment is not set correctly.

This is my online guide. There are a few differences, and it works for me.
JDK Installation - Ubuntu

Debian style linux distros have the alternative mechanism for this kind of problems. They link /usr/bin/java to /etc/java which in turn is linked to the correct binary.
sudo update-alternatives --install /usr/bin/java java /usr/local/java/jdk1.6.0_37/bin/java 1
sudo update-alternatives --install /usr/bin/javac javac /usr/local/java/jdk1.6.0_37/bin/javac 1
sudo update-alternatives --install /usr/bin/javaws java /usr/local/java/jdk1.6.0_37/bin/javaws" 1
sudo update-alternatives --install /usr/bin/jar jar /usr/local/java/jdk1.6.0_37/bin/jar 1
sudo update-alternatives --config java
sudo update-alternatives --config javac
sudo update-alternatives --config jar
sudo update-alternatives --config javaws
You may want to repeat this for the other commands like wsimport. Have a look at https://help.ubuntu.com/community/Java. There are also other options listed like using a PPA, but if you strictly don't want to use apt-get, this is not an option.

These scripts will help you install sun's jdk on Ubuntu. Works great.