mock objects from concept to code

April 12, 2023 © Agile Institute 2012 1

Mock Objects: From Concept to Code

Rob Myersfor

Agile Development Practices East

07 Nov 2012

Café

difficult to set up

A database.A complex object graph.


Investor

AccountFund

Stock

StockStock

Account Fund

Stock

Stock

Stock

expensive to test against*

The production database.Your personal credit card.A “password changed” confirmation

sent to the CEO of a large investment firm.


* Each of these “Career-Limiting Moves” was actually witnessed by me

during the “dot-COM” era.

chaotic

The database.A randomizer.The weather.



The database.The network.The filesystem.Interplanetary radio signals?!

…slow…


Fast!

Reliable!

from Martin Fowler et al

• Fakes provide reasonable facsimiles of the real thing, but are not quite robust enough for production (e.g., an in-memory database).

• Stubs provide scripted responses to calls. They may also record information about calls made during the test.

• Mocks are programmed by tests, creating expectations for potentially the entire specification of a call, including arguments.


classicists v. mockists

Hand-Crafted Mocks (“Stubs”)+ Simple & familiar.+ Black-box.+ Can do something clever (e.g., throw exception for

a special argument), or hold state (e.g., collect a buffered result for an assertion).

- Can get too complex if reused everywhere.

Dynamic Mocking Tools (“Mocks”)+ Great for interactions/protocols.+ Can be used in numerous variations or scenarios.- White-box: Can be too explicit about

implementation.April 12, 2023 © Agile Institute 2012 10


never an absolutist


Eart

hb

ou

nd

Invest

men

ts


Create a StockQuote for the current price of a LunEx stock.

Quote total should reflect our 2% commission on all LunEx transactions, plus the flat LunEx communication surcharge of $10.

Be sure we invoke LunEx once per quote, as they charge $0.05 per invocation!

Write StockQuote class


StockQuote

+ double total()

LunExServices

+ double currentPrice( String symbol )

DATABASE

Testing with a Mock


StockQuote

+ double total()

LunExServices


StockQuoteTests

+ totalWorksRight()

MockLunExServices


defines

arranges


Create a StockQuote for the current price of a LunEx stock.

Quote total should reflect our 2% commission on all LunEx transactions, plus the flat LunEx communication surcharge of $10.

Be sure we invoke LunEx once per quote, as they charge $0.05 per invocation!

Write StockQuote class


a test

@Testpublic void totalCalculatesCorrectly() {

// givenLunExServices myService = Mockito.mock(LunExServices.class);Mockito.when(myService.currentPrice("HE3"))

.thenReturn(12.00);

StockQuote quote = new StockQuote("HE3", 100, myService);

// whendouble total = quote.total();// thenAssert.assertEquals(1234.00, total, 0.00001);

}


just enough to compile & fail

package earthboundInvestments;

import lunEx.LunExServices;

public class StockQuote {public StockQuote(String symbol,

int numberOfShares,LunExServices service) {

}

public double total() {return 0;

}}


“Obvious Implementation”

public class StockQuote {private final double total;public StockQuote(String symbol,

int numberOfShares,LunExServices service) {

total = service.currentPrice(symbol) * numberOfShares

* 1.02 + 10.00;}

public double total() {return total;

}}


what else?@Testpublic void totalCalculatesCorrectly() {

// givenLunExServices myService = Mockito.mock(LunExServices.class);Mockito.when(myService.currentPrice("HE3"))

.thenReturn(12.00);


// whendouble total = quote.total();// thenAssert.assertEquals(1234.00, total, 0.00001);Mockito.verify(myService, Mockito.times(1))

.currentPrice("HE3");}


test using hand-crafted mock

@Testpublic void totalCalculatesCorrectly_usingHandCraftedMock() {

// givenMockLunExServices myService = new

MockLunExServices(12.00);


// whendouble total = quote.total();

// thenAssert.assertEquals(1234.00, total, 0.00001);

Assert.assertTrue(myService.wasCalledOnceAndOnlyOnce());}


hand-crafted mock

public class MockLunExServices extends LunExServices {private double valueToReturn;private int callCount = 0;

public MockLunExServices(double valueToReturn) {this.valueToReturn = valueToReturn;

}@Overridepublic double currentPrice(String symbol) {

callCount++;return valueToReturn;

}

public boolean wasCalledOnceAndOnlyOnce() {return callCount == 1;

}}


Summary:

1. Agile: Good!2. “Agilist’s Dilemma”: Very bad.3. TDD: Great!4. Slow, non-deterministic tests: Bad.5. Mocks: Good.6. “Never an Absolutist!”


[email protected]

http://PowersOfTwo.agileInstitute.com/

@agilecoach

mock objects from concept to code

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quote total

public double total

return total

total thenassert

stockquote quote

myservice whendouble

lunex stock