You probably have Java 10 (not proper version).
You should install Java 8 (Ubuntu solution):
sudo apt install openjdk-8-jdk
It won't automatically change your default java compiler. Your should make it by your own. Change default java compiler:
sudo update-alternatives --config java
Type 0 or 1 or 2 ... option. Probably last entry.
Now:
./logstash -f ../your-logstash-file.conf
