Problem scenario
You want to use Maven to build a Java program from source code. The code is in at least two different .java files. What do you do?
Solution
Prerequisites
This assumes that you have installed Maven. If you need assistance, see this posting.
Procedures
1. Run this command: mkdir -p src/main/java/contint
2.
Problem scenario
You ran a Hadoop command but you receive this error:
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.
What do you do?
Solution
Run hadoop help. This error can happen when you have an incorrect flag.
Problem scenario
You want to use the rdf4j (Resource Description Framework for Java). You want to install the rdf4j server and workbench. You are using Debian 9 Linux. What should you do?
Solution
Prerequisites
i. We recommend having a 30 GB of hard disk free.
ii. We recommend having 5.75 GB of RAM (it can be a combination of RAM and swap space).
Problem scenario
You want determine what version of Java is installed (if any). What do you do?
Solution
Open PowerShell and run this command:
(Get-WmiObject Win32_Product | Where {$_.Name -match “Java”}).Version
Alternative way without PowerShell:
1. Go to Control Panel -Uninstall a Program
2. See if Java is installed
Problem scenario
Your Java program returns this message: “java.sql.SQLException: No database selected”
What should you do?
Possible Solution #1
Your SQL statement was appropriate for a database but not for the circumstance of connecting to a SQL instance. Try show databases; instead of the SQL you were running.
Possible Solution #2
Did you connect to the SQL instance you thought you connected to?
Problem scenario
You have a MySQL database. Rather than use a SQL front-end application, you want to write a Java program that will run a SQL statement against the database. What should you do?
Solution
1. Install the Java compiler if it is not already installed. If you need assistance, see this posting.
2. Run this command:
curl -Lk https://www.javatpoint.com/src/jdbc/mysql-connector.jar /tmp/mysql-connector.jar
3.
Problem scenario
You are using Debian or Ubuntu Linux. You want to compile Java programs. What do you do?
Solution
Run these commands:
sudo apt-get -y update
sudo apt-get -y install default-jdk
Problem scenario
Your Java program returns this message: “com.mysql.jdbc.exceptions.MySQLSyntaxErrorException: Unknown database foobar”
What should you do?
Possible solution #1
Are you using Amazon Aurora? AWS has a MySQL PaaS offering. You may have a section of the Java code that looks like this:
Connection con=DriverManager.getConnection(“jdbc:mysql://foobar-us-west-2b.abcdefghijk.us-west-2.rds.amazonaws.com:3306/foobar”)
Remove the last “foobar” from the end. The connection string should look like this for Aurora databases:
jdbc:mysql://foobar-us-west-2b.abcdefghijk.us-west-2.rds.amazonaws.com:3306/foobar
Recompile the program.
Problem scenario
You are trying to run Apache Zookeeper or some other application. You enounter the problem Error: Config file not found: /usr/lib/jvm/java-9-openjdk-amd64/conf/management/management.properties. What should you do?
Possible solution
This is a workaround. Run these commands:
sudo mkdir -p /usr/lib/jvm/java-9-openjdk-amd64/conf/management/
sudo touch /usr/lib/jvm/java-9-openjdk-amd64/conf/management/management.properties
Problem scenario
In Linux you are trying to run a Java program that connects to a MySQL database. The program compiles, but it does not run. You get this error: “java.lang.ClassNotFoundException: com.mysql.jdbc.Driver” What should you do?
Solution
1. Set the CLASSPATH variable in two steps. First set it to where the .class file is for the Java file you are running. If the .class file of the program you are running is in /home/jdoe/javaprograms run this command:
export CLASSPATH=/home/jdoe/javaprograms
2.