First you should have Java 7. If you don't have, install it first (I don't know what you are using, Linux, Mac, yum, apt, homebrew, you should find out yourself.)
If you already have Java 7, run:
echo $JAVA_HOME
Output should be something like this:/usr/lib/jvm/java-8-oracle. Near this directory, you should see java-7 directory. After you found it, run
export JAVA_HOME=${java-7-dir}
Change {java-7-dir} with your directory path. Then you can run your command.
This is only a temporary solution. To change it permanently, put the above command to your ~/.bashrc file.
EDIT: If you are using Windows, change environment variable of JAVA_HOME to your Java 7 installation directory path.