When you install Java in your system first of all you need to set the environment variables which are path and class path.
PATH− The path environment variable is used to specify the set of directories which contains executional programs.
When you try to execute a program from command line, the operating system searches for the specified program in the current directly, if available, executes it.
In case the programs are not available in the current directory, operating system verifies in the set of directories specified in the ‘PATH’ environment variable.
You need to set path for compiler (javac.exe) and JVM(java.exe),which exists in the bin folder of the JDK.
CLASSPATH − The class path environment variable is used to specify the location of the classes and packages.
When we try to import classes and packages other that those that are available with Java Standard Library.
JVM verifies the current directly for them, if not available it verifies the set of directories specified in the ‘CLASSPATH’ environment variable.
The rt.jar file in the lib folder of JRE contains all the basic packages for Java therefore, you need to set classpath for this folder.
To set path −
SET PATH=C:\Program Files\Java\jdk_version\bin
To set classpath −
SET CLASSPATH= C:\Program Files\Java\jdk1.8.0_101\lib