1. Download Oracle JDK8 for ARM on your RaspberryPi and get the resulting file to your ‘home’ folder on Raspbian:
http://www.oracle.com/technetwork/java/javase/downloads/jdk8-arm-downloads-2187472.html

2. UnTAR the file jdk-8u6-linux-arm-vfp-hflt.tar.gz:

# tar xvzf jdk-8u6-linux-arm-vfp-hflt.tar.gz -C /opt

3. Set default java and javac to the new installed JDK8.

# update-alternatives --install /usr/bin/java java /opt/jdk1.8.0_06/bin/java 1
# update-alternatives --install /usr/bin/javac javac /opt/jdk1.8.0_06/bin/javac 1
# update-alternatives --set java /opt/jdk1.8.0_06/bin/java
# update-alternatives --config javac
# update-alternatives --config java
# update-alternatives --display java

4. Now,  check that version 1.8 is really installed

# java -version

This order should return something similar to:

java version "1.8.0_06"
Java(TM) SE Runtime Environment (build 1.8.0_06-b23)
Java HotSpot(TM) Client VM (build 25.6-b23, mixed mode)

5. Export JAVA_HOME path variable:

# export JAVA_HOME="/opt/jdk1.8.0_06"
# export PATH=$PATH:$JAVA_HOME/bin

6. Now, it is time to download Apache Cassandra:
# wget http://mirror.reverse.net/pub/apache/cassandra/2.1.2/apache-cassandra-2.1.2-bin.tar.gz

7. UnTAR and move the main directory to /opt:

# tar -xvsf apache-cassandra-2.1.2-bin.tar.gz
# mv apache-cassandra-2.1.2 /opt/cassandra

8. Download and replace snappy-java:

# cd /opt/cassandra/lib
# mv snappy-java-1.0.5.2.jar snappy-java-1.0.5.2.jarX
# wget https://oss.sonatype.org/content/repositories/snapshots/org/xerial/snappy/snappy-java/1.1.1.7-SNAPSHOT/snappy-java-1.1.1.7-SNAPSHOT.jar

And now, launch Cassandra…
/opt/cassandra/bin/cassandra -f

You will know if cassandra is running succesfully if you are able to see the line:

 Listening for thrift clients...