MathNerdUnitTests.java

public void testCacheUpdate() {
    
   //Arrange
    CalculationCache calcCache = mock(CalculationCache.class);

    when(calcCache.contains()).thenReturn(false);

    Calculator calculator = mock(Calculator.class);

    MathNerd mathNerd = new MathNerd(calcCache, calculator);

    Calculation calculation = new Calculation("e^2000");

    //Act
    mathNerd.doIntenseCalculationInBackground(calculation, null);

    //Assert should use calculator to perform calcluation because cache was empty
    verify(calculator).doIntenseCalculationInBackground(any());
}