Downloading and Installing JDK

This page shows how to download and install the latest version of the Java Development Kit (JDK). Read the instructions carefully to set the "classpath" mentioned in Step 3. Once JDK has been installed, you can write a simple Java program using an editor such as notepad and run it from a command prompt. Alternatively, Java programs can be written using an Integrated Development Environment (IDE) such as NetBeans, described below.

3. How to Install JDK on Ubuntu

There are several JDK implementations available for Linux, such as Oracle JDK, OpenJDK, Sun JDK, IBM JDK and GNU Java Compiler. We shall choose the Oracle JDK 8. Ubuntu chooses OpenJDK as its default JDK, which is not 100% compatible with Oracle JDK.

Step 0: Check if JDK has already been Installed

Open a Terminal and issue this command:

$ javac -version

If a JDK version number (e.g., "javac x.x.x") appears, JDK has already been installed. You can skip the installation and go to step 2. 

To remove OpenJDK, issue command:

$ sudo apt-get purge openjdk-\*

Step 1: Download and Install JDK 

1. Goto JDK (Java SE) download site @ Under "Java Platform, Standard Edition" ⇒ "Java SE 11.0.{x}" ⇒ Click JDK's "Download" ⇒ Under "Java SE Development Kit 11.0.{x}" ⇒ Check "Accept License Agreement" ⇒ Select "Linux", "tar.gz" package, (e.g., "jdk-11.0.{x}-linux-x64_bin.tar.gz" - 171MB). 
The tarball will be downloaded in directory "~/Downloads", by default. 

2. We shall install JDK under "/usr/local/java" (or Ubuntu's default JDK directory /usr/lib/jvm; or /opt/java). First, create a directory "java" under "/usr/local". Open a Terminal and issue these commands:
    $ cd /usr/local 
    $ sudo mkdir java

    Extract the downloaded package (Check your downloaded filename!)

    $ cd /usr/local/java
    $ sudo tar xzvf ~/Downloads/jdk-11.0.{x}-linux-x64_bin.tar.gz
           // x: extract, z: for unzipping gz, v: verbose, f: filename

    JDK shall be extracted in a folder "/usr/local/java/jdk-11.0.{x}", where {x} is the update number.

    3. Inform the Ubuntu to use this JDK/JRE:

    // Setup the location of java, javac and javaws
    $ sudo update-alternatives --install "/usr/bin/java" "java" "/usr/local/java/jdk-11.0.{x}/bin/java" 1
          // --install symlink name path priority
    $ sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/local/java/jdk-11.0.{x}/bin/javac" 1
    $ sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/local/java/jdk-11.0.{x}/bin/javaws" 1
    // Use this Oracle JDK/JRE as the default
    $ sudo update-alternatives --set java /usr/local/java/jdk-11.0.{x}/bin/java
          // --set name path
    $ sudo update-alternatives --set javac /usr/local/java/jdk-11.0.{x}/bin/javac
    $ sudo update-alternatives --set javaws /usr/local/java/jdk-11.0.{x}/bin/javaws

    The above steps set up symlinks java, javac, javaws at /usr/bin (which is in the PATH), that link to /etc/alternatives and then to JDK bin directory. The "alternatives" system aims to resolve the situation where several programs fulfilling the same function (e.g., different version of JDKs). It sets up symlinks thru /etc/alternatives to refer to the actual programs to be used.

    $ ls -ld /usr/bin/java*
    lrwxrwxrwx 1 root root xx xxx xx xx:xx /usr/bin/java -> /etc/alternatives/java
    lrwxrwxrwx 1 root root xx xxx xx xx:xx /usr/bin/javac -> /etc/alternatives/javac
    lrwxrwxrwx 1 root root xx xxx xx xx:xx /usr/bin/javaws -> /etc/alternatives/javaws
    $ ls -ld /etc/alternatives/java*
    lrwxrwxrwx 1 root root xx xxx xx xx:xx /etc/alternatives/java -> /usr/local/java/jdk-11.0.{x}/bin/java
    lrwxrwxrwx 1 root root xx xxx xx xx:xx /etc/alternatives/javac -> /usr/local/java/jdk-11.0.{x}/bin/javac
    lrwxrwxrwx 1 root root xx xxx xx xx:xx /etc/alternatives/javaws -> /usr/local/java/jdk-11.0.{x}/bin/javaws
Alternatively, you can include the JDK's bin and JRE's bin into the PATH directly.

    4. To verify the JDK installation, issue these commands:

    // Show the Java Compiler (javac) version
    $ javac -version
    javac 11.0.{x}
    // Show the Java Runtime (java) version
    $ java -version
    java version "11.0.{x}"
    // Show the location of javac and java
    $ which javac
    $ which java

    5. [Don't Do this step - It is taken care by "alternative" in Step 3. Keep here to show you how to set PATH.] Add JDK's binary directory ("bin") to the "PATH" by editing "/etc/profile":

    $ cd /etc
    $ gksudo gedit profile   // OR "sudo nano profile" to use the console-based nano editor

    Add these lines at the end of the file "/etc/profile", replace "{x}" with the actual number:

    export JAVA_HOME=/usr/local/java/jdk-11.0.{x}
    export PATH=$JAVA_HOME/bin:$PATH

    Rerun the configuration file by:

    // Refresh
    $ source /etc/profile
    // Check the new settings for JAVA_HOME and PATH
    $ echo $JAVA_HOME
    $ echo $PATH

Step 2: Compile and Run a Hello-world Java Program

  1. File Explorer ⇒ Home ⇒ Create a new folder called "myProject" to keep our works.
  2. Open "Text Editor" (gedit). Enter the following source code and save as "" under the "~/myProject" directory created earlier.
    public class Hello {   // To save as "" under "~/myProject"
       public static void main(String[] args) {
          System.out.println("Hello, world from Ubuntu!");

3. To compile the Hello-world Java program, launch a Terminal and issue these commands:
    // Change directory to where the source code resides
    $ cd ~/myProject
    // List the contents of current directory. Check for ""
    $ ls
    ...... ......
    // Compile "" into "Hello.class"
    $ javac
    // Check for "Hello.class"
    $ ls
    ...... Hello.class ......

4. Run the Hello-world Java program:
    // Run "Hello.class"
    $ java Hello
    Hello, world from Ubuntu!