Check out example codes for "> Task :run FAILED Error: Could not find or load main class Caused by: java.lang.ClassNotFoundException:". It will help you in understanding the concepts better.

Code Example 1

Sounds silly, but if you've copied .bashrc lines from somewhere
check that the quotation marks are like this:
    export HADOOP_OPTS="-Djava.library.path=$HADOOP_HOME/lib/native"
and not like this
    export HADOOP_OPTS=”-Djava.library.path=$HADOOP_HOME/lib/native”

...took me some time

Code Example 2

Check your build.gradle file and update the application block:

application {
    // Define the main class for the application.
  	// insert the entire name of the class
    mainClassName = 'packageName.MainClass'

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