try using this command in the terminal:

sudo apt install default-jre

then you can try using:

java -version

to check version

Before I start, please note that the JDK also contains the JRE within it.

Method 1:

To install the openJDK JDK and JRE 8 use (replace 8 with the version you want, such as 7 or 6):

sudo apt-get install openjdk-8-jdk

Method 2:

If you instead want to install the official Oracle JDK and JRE and definitely want to install through apt-get then do (you can replace the 8 with other versions such as 9, or 7):

sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java8-installer

Method 3:

Or if you want to install the official Oracle Java version manually:

1. Go here and click the download button:

   

2. Select the Accept License Agreement radio button (once you have read the license agreement).

3. Select the file you want to download, if you are running on a 32-bit machine click jdk-[javaversion]-linux-i586.tar.gz ([javaversion] replaced with the Java version, such as 8u60), if 64-bit then jdk-[javaversion]-linux-x64.tar.gz.

4. Launch Terminal (CTRL + ALT + T), then cd to the directory where you downloaded the file to (probably ~/Downloads), and run tar -xvf /path/to/file.tar.gz to unpack that archive into the directory the file is in, you may then move that unpacked archive into a desired location, such as the /opt directory (storing executable files in your home directory is a security risk).

5. Once you have done the above you may delete the original .tar.gz file, and then open up your ~/.bashrc file in a desired file editor such as gedit or vim, I will be using vim in this example (install it with sudo apt-get install vim if it is not already installed). So run vim ~/.bashrc and then insert two new lines at the bottom of the file which read (that is with /path/to/unpacked/archive replaced with the path to the archive you just unpacked):

   JAVA_HOME=/path/to/unpacked/archive
   export PATH=$PATH:$JAVA_HOME/bin
   

6. Either restart Terminal or run source ~/.bashrc to start using the new .bashrc file.

   Note: With this method you will have to carry on doing this every time there is a new Java version released which it will not notify you about.

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 .

Referring to Ask Ubuntu question How to set JAVA_HOME for OpenJDK?,

How to install Open JDK (Java developement kit) in Ubuntu (Linux)?

1. Open Terminal from Application Dash or press Ctrl+Alt+T

2. Update repository:

   sudo add-apt-repository ppa:openjdk-r/ppa  # only Ubuntu 17.4 and earlier
   sudo apt update
   

3. Optional: To search available distributions of openjdk, use the following command:

   apt search openjdk
   

4. 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
   

5. 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.)

6. For PATH (Environment Variable) type command as shown below, in Terminal:

   export PATH=$PATH:$JAVA_HOME/bin

7. To check your installation:

   java -version