I want to start a project in eclipse, but when I try to start eclipse, it doesn't open. My instructor said I'd have to set the environment variables. What is the importance of environment variables like
So that Eclipse will know where Java is.
JAVA_HOME is not used by Java itself. Some third-party programs (for example Apache Tomcat) expect one of these environment variables to be set to the installation directory of the JDK or JRE. If you are not using software that requires them, you do not need to set JAVA_HOME and JRE_HOME.
CLASSPATH is an environment variable which contains a list of directories and / or JAR files, which Java will look through when it searches for Java classes to load. You do not normally need to set the CLASSPATH environment variable. Instead of using this environment variable, you can use the -cp or -classpath option on the command line when using the javac and java commands.
PATH is an environment variable used by the operating system (Windows, Mac OS X, Linux) where it will look for native executable programs to run. You should add the bin subdirectory of your JDK installation directory to the PATH, so that you can use the javac and java commands and other JDK tools in a command prompt window. The JDK installation instructions explain how to set PATH.