Two ways to install JDK in Ubuntu

1: Installed through the PPA (source) mode.

2: Download the installation package through the official website.

1th is recommended here because the JDK upgrade can be easily obtained by Apt-get upgrade method. install using the ppa/source method Add PPA

sudo add-apt-repository ppa:webupd8team/java

sudo apt-get update

Install Oracle-java-installer

Jdk7:
sudo apt-get install oracle-java7-installer
jdk8:
sudo apt-get install Oracle-java8-installer

The installer will prompt you to agree to Oracle's terms of service and select OK

Then select Yes.

If you are slow to download the installer because of a firewall or some other reason, you can interrupt the operation. then download the corresponding JDK tar.gz package, put in:

   /var/cache/oracle-jdk7-installer             (jdk7)

   /var/cache/oracle-jdk8-installer              (JDK8)

Below, and then install the installer once. Installer will default to set the system default JDK using the tar.gz package you download

JDk7:
sudo update-java-alternatives-s java-7-oracle
JDK8:
sudo update-java-alternatives-s Java-8-oracle

If the installation of the JDK7, and installed the JDK8, to achieve the switch between the two, you can:
jdk8 switch to jdk7
sudo update-java-alternatives-s Java-7-oracle

jdk7 Switch to jdk8
sudo update-java-alternatives-s java-8-oracle

Download the JDK compression package directly (the JDK7 is only introduced here, the JDK8 principle is exactly the same)

  Divided into the following 5 steps

     1. Download JDK

     2. Extract, put to the specified directory

     3. Configure environment variable

     4. Set system default JDK

  5. Test jdk
  '

# # website Download jdk
     Address: http://www.oracle.com/technetwork/articles/javase/index-jsp-138363.html
Select the corresponding. gz package Download

# # decompression, Put in the specified directory (for example, jdk-7u60-linux-x64.gz)

To create a directory:

sudo MKDIR/USR/LIB/JVM
To compress to this directory:

sudo tar-zxvf jdk-7u60-linux-x64.gz-c/USR/LIB/JVM
"' Modify environment variables:

The sudo vim ~/.bashrc
file appends the following:

#set Oracle JDK Environment
Export java_home=/usr/lib/jvm/jdk1.7.0_60  # # Here to note the directory to be replaced by their own extracted JDK directory
export jre_home=${java_home}/jre  
export classpath=.:${java_home}/lib:${jre_ Home}/lib  
export path=${java_home}/bin: $PATH  
make environment variable effective immediately

 source ~/.BASHRC
 '
# Set system default JDK version

sudo update-alternatives–install/usr/bin/java Java/usr/lib/jvm/jdk1.7.0_60/bin/java 300
sudo update-alternatives–install/usr/bin/javac Javac/usr/lib/jvm/jdk1.7.0_60/bin/javac 300
sudo update-alternatives–install/usr/bin/jar jar/usr/lib/jvm/jdk1.7.0_60/bin/jar 300
sudo update-alternatives–install/usr/bin/javah javah/usr/lib/jvm/jdk1.7.0_60/bin/javah 300
sudo update-alternatives–install/usr/bin/javap JAVAP/USR/LIB/JVM/JDK1.7.0_60/BIN/JAVAP 300
Then execute:
sudo update-alternatives–config java
If you install the JDK for the first time, you will have the following tips

There is only one alternative in link Group Java (Providing/usr/bin/java):
/usr/lib/jvm/jdk1.7.0_60/bin/java

No, select the appropriate JDK
"'