Answers for "The statement System.exit(0); is found at the end of every working Java program."

install composer ubuntu

sudo apt update
sudo apt install php-cli unzip
cd ~
curl -sS https://getcomposer.org/installer -o composer-setup.php

HASH=`curl -sS https://composer.github.io/installer.sig`
echo $HASH
php -r "if (hash_file('SHA384', 'composer-setup.php') === '$HASH') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer
composer

apt search mbstring
sudo apt install php-mbstring

ubuntu install composer

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"

install composer 2 ubuntu

Teniendo composer instaldo, correr 
composer self-update --2

composer install ubuntu

sudo apt-get updatemixed

composer install ubuntu

sudo mv composer.phar /usr/local/bin/composermixed

how to exit a java program

System.exit(0);