Error: Compile Error: Method is not visible:

Error: Compile Error: Method is not visible:

We will get this error when we are accessing private methods/variables in test class.
Example:
Apex Class:
public class PrivateMethodDemo {
    private void privateMethod() {
    }
}
Apex Test Class:
@isTest
private class PrivateMethodDemoTest {
    public static testmethod void validate() {
        PrivateMethodDemo demo = new PrivateMethodDemo ();
        demo.privateMethod();
    }
}
When we are saving the above test class,we will get that error.

Solution: Use @TestVisible annotation before private variables/methods of main class.It won't change any functionality of original class.
Modify main class as below
public class PrivateMethodDemo {
    @TestVisible private void privateMethod() {
    }
}
Now save the test class and run.That's it...