I have an apex function that while doing other tasks, also make a web call out.  I am not sure how to write a test for this.  I've found documentation for when I need to test a function that returns a HttpResponse but in my case the function returns a PageReference.

Controller Code

public  PageReference finalize() {
        
        String specialSerial;
        
        Integer i = 0;
        Integer size = products.size();
        test = '';
        List<List<String>> productswithSerial = new List<List<String>>{};
      
            ....

  string serial =   generate(prod2.ProductFamilyID__c, String.valueOf(products[i].Pak_Size__c));

.....
}

 
    public  string generate(decimal productFamily, string pak) {
        HttpRequest req = new HttpRequest();              
        HttpResponse res2 = new HttpResponse(); 
        Http http = new Http();
        req.setEndpoint('xxxxxxxxxxxxxxxx');              
        req.setMethod('GET');                  
        res2 = http.send(req);              
        if(res2.getstatusCode() == 200 && res2.getbody() != null){   
            return res2.getbody();
            //   test = test + '<br/>' + res2.getbody();                  
        }else{
            return 'Invalid';
        }
    }

My Test Code

Test.setMock(HttpCalloutMock.class, new MockHttpResponseGenerator());
        
        // Call method to test.
        // This causes a fake response to be sent
        // from the class that implements HttpCalloutMock. 
        HttpResponse res = tsc.finalize();
        
        // Verify response received contains fake values
        String contentType = res.getHeader('Content-Type');
        System.assert(contentType == 'application/json');
        String actualValue = res.getBody();
        String expectedValue = '{"xxxxxxxxxxxxxxxxxx"}';
        System.assertEquals(actualValue, expectedValue);
        System.assertEquals(200, res.getStatusCode());

I'm getting the error on this line "HttpResponse res = tsc.finalize();" which states "Illegal assignment from System.PageReference to System.HttpResponse" which I understand but don't know how to work-around.