How to mock an object which is inside the method of serviceimpl class

code looks something like this

public GetPortfoliosResponse retrievePortfolios(
      RequestIdentityInfo requestIdentityInfo,
      GetPortfoliosRequest getPortfoliosRequest) {
 PortfolioReferenceParameter portfolioReferenceParameter = new PortfolioReferenceParameter();
//some code
 List<Portfolio> portfolios = portfolioReferenceParameter.getPortfolios();

}

And in my test class I'm doing something like this:

 @Mock
    private PortfolioReferenceParameter portfolioReferenceParameter;
 @Before
    public void setup() {
        MockitoAnnotations.initMocks(this);
        List<Portfolio> portfolios = new ArrayList<Portfolio>();
//code to build the list
        when(portfolioReferenceParameter.getPortfolios()).thenReturn(portfolios);


}

But this doesn't seem to be working. When I make a call to the serviceimpl method it is still taking the actual object of PortfolioReferenceParameter which is locally created in that method. Any ideas how can I mock that object with my object?