Gradle error: package com.example.Example does not exist

I am in Android Studio, trying to do a Unit Test. I follow the instructions in Vogella tutorial, all ok, except when I import and refer to a class subject of the test. For example, a test like this works:

 assertTrue("test", true);

but not this:

assertTrue("test", myMethod.returnTrue());

where the myMethod is in a class that I have imported (with a classic import like:)

import com.example.Example.myMethod;

my build.gradle contains:

dependencies {
...

testCompile 'junit:junit:4.12'
testCompile 'org.mockito:mockito-core:1.9.5'
}

how to resolve?