http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html

$ sudo mkdir /usr/lib/jvm
$ sudo tar zxvf jdk-7u21-linux-i586.tar.gz -C /usr/lib/jvm
$ cd /usr/lib/jvm
$ sudo mv jdk1.7.0_21 java

$ sudo vim ~/.bashrc

export JAVA_HOME=/usr/lib/jvm/java 
export JRE_HOME=${JAVA_HOME}/jre  
export CLASSPATH=.:${JAVA_HOME}/lib:${JRE_HOME}/lib  
export PATH=${JAVA_HOME}/bin:$PATH

sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/java/bin/java 300  
sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/java/bin/javac 300  
sudo update-alternatives --install /usr/bin/jar jar /usr/lib/jvm/java/bin/jar 300   
sudo update-alternatives --install /usr/bin/javah javah /usr/lib/jvm/java/bin/javah 300   
sudo update-alternatives --install /usr/bin/javap javap /usr/lib/jvm/java/bin/javap 300

sudo update-alternatives --config java

There is only one alternative in link group java (providing /usr/bin/java): /usr/lib/jvm/java/bin/java
无需配置。

$ java -version
java version "1.7.0_21"
Java(TM) SE Runtime Environment (build 1.7.0_21-b11)
Java HotSpot(TM) Server VM (build 23.21-b01, mixed mode)