gsamaras@gsamaras:/usr/lib/jvm$ ls
java-1.7.0-openjdk-amd64  java-7-openjdk-amd64
gsamaras@gsamaras:/usr/lib/jvm$ $JAVA_HOME
bash: /usr/lib/jvm/java-8-oracle: No such file or directory

gsamaras@gsamaras:~$ which java
/usr/bin/java
gsamaras@gsamaras:~$ ls -alh /usr/bin/java
lrwxrwxrwx 1 root root 22 Feb 10 00:54 /usr/bin/java -> /etc/alternatives/java
gsamaras@gsamaras:~$ ls -alh /etc/alternatives/java
lrwxrwxrwx 1 root root 46 Feb 10 00:54 /etc/alternatives/java -> /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java

export JAVA_HOME=/usr/lib/jvm/java-7-openjdk-amd64
export PATH=${JAVA_HOME}/bin:${PATH}
export HADOOP_CLASSPATH=${JAVA_HOME}/lib/tools.jar

gsamaras@gsamaras:~/spark-1.6.0-bin-hadoop2.6/conf$ ls
docker.properties.template  metrics.properties.template   spark-env.sh.template
fairscheduler.xml.template  slaves.template

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

sudo update-java-alternatives -s java-8-oracle

gsamaras@gsamaras:/usr/lib/jvm$ $JAVA_HOME
bash: /usr/lib/jvm/java-8-oracle: No such file or directory