1. /29@yegor256 1
Built-in Fake Objects
Yegor Bugayenko

2. /29@yegor256 2
“Mock objects are
simulated objects that
mimic the behavior of
real objects in
controlled ways”

3. /29@yegor256 3
Fake
Objects
Mocking
Frameworks

4. /29@yegor256 4
static String report(User user) {
return String.format(
“Balance of %s is %d USD”,
user.profile().name(),
user.account().balance().usd()
);
}

5. /29@yegor256 5
interface Profile {
String name();
}
interface User {
Profile profile();
Account account();
}
interface Account {
Balance balance();
}
interface Balance {
int usd();
}

6. /29@yegor256 6
@Test
void printsReport() {
User user = mock(User.class);
assertThat(
Foo.report(user),
containsString(“Balance of”)
);
}
Mockito

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@Test
void printsReport() {
Profile profile = mock(Profile.class);
Account account = mock(Account.class);
doReturn(“Jeffrey”).when(account).name();
Balance balance = mock(Balance.class);
doReturn(123).when(balance).usd();
doReturn(balance).when(account).balance();
User user = mock(User.class);
doReturn(profile).when(user).profile();
doReturn(account).when(user).account();
assertThat(
Foo.report(user),
containsString(“Balance of Jeffrey is 123 USD”)
);
}

8. /29@yegor256 8
1. verbosity

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@Test
void printsReport() {
Profile profile = mock(Profile.class);
Account account = mock(Account.class);
doReturn(“Jeffrey”).when(account).name();
Balance balance = mock(Balance.class);
doReturn(123).when(balance).usd();
doReturn(balance).when(account).balance();
User user = mock(User.class);
doReturn(profile).when(user).profile();
doReturn(account).when(user).account();
assertThat(
Foo.report(user),
containsString(“Balance of Jeffrey is 123 USD”)
);
}
String report(User user) {
return String.format(
“Balance of %s is %d USD”,
user.profile().name(),
user.account().balance().usd()
);
}

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2. code duplication

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@Test
void reportIncludesUserName() {
Profile profile = mock(Profile.class);
Account account = mock(Account.class);
doReturn(“Jeffrey”).when(account).name();
Balance balance = mock(Balance.class);
doReturn(balance).when(account).balance();
User user = mock(User.class);
doReturn(profile).when(user).profile();
doReturn(account).when(user).account();
assertThat(
Foo.report(user),
containsString(“Jeffrey”)
);
}
@Test
void reportIncludesBalanceInUSD() {
Profile profile = mock(Profile.class);
Account account = mock(Account.class);
Balance balance = mock(Balance.class);
doReturn(123).when(balance).usd();
doReturn(balance).when(account).balance();
User user = mock(User.class);
doReturn(profile).when(user).profile();
doReturn(account).when(user).account();
assertThat(
Foo.report(user),
containsString(“123 USD”)
);
}

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3. complexity

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example!!

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4. fragility
because of coupling

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@Test
void printsReport() {
Profile profile = mock(Profile.class);
Account account = mock(Account.class);
doReturn(“Jeffrey”).when(account).name();
Balance balance = mock(Balance.class);
doReturn(123).when(balance).usd();
doReturn(balance).when(account).balance();
User user = mock(User.class);
doReturn(profile).when(user).profile();
doReturn(account).when(user).account();
assertThat(
Foo.report(user),
containsString(“Balance of Jeffrey is 123 USD”)
);
}
String report(User user) {
return String.format(
“Balance of %s is %d USD”,
user.profile().name(),
user.account().balance().usd()
);
}

16. /29@yegor256 16
@Test
void printsReport() {
Profile profile = mock(Profile.class);
Account account = mock(Account.class);
doReturn(“Jeffrey”).when(account).name();
Balance balance = mock(Balance.class);
doReturn(123).when(balance).usd();
doReturn(balance).when(account).balance();
User user = mock(User.class);
doReturn(profile).when(user).profile();
doReturn(account).when(user).account();
assertThat(
Foo.report(user),
containsString(“Balance of Jeffrey is 123 USD”)
);
}
String report(User user) {
return String.format(
“Balance of %s is %d USD”,
user.profile().name(),
user.account().usd()
);
}

17. /29@yegor256 17
“Unit tests are your
safety net”

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false positive

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throw-away tests

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a solution?
fake objects!

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interface User {
Profile profile();
Account account();
class Fake {
Fake(String name, int balance) {
// ...
}
// ...
}
}

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@Test
void printsReport() {
User user = new User.Fake(“Jeffrey”, 123);
assertThat(
Foo.report(user),
containsString(“Balance of Jeffrey is 123 USD”)
);
}
verbosity?

23. /29@yegor256 23
@Test
void reportIncludesUserName() {
User user = new User.Fake(“Jeffrey”);
assertThat(
Foo.report(user),
containsString(“Jeffrey”)
);
}
code duplication?
@Test
void reportIncludesBalanceInUSD() {
User user = new User.Fake(123);
assertThat(
Foo.report(user),
containsString(“123 USD”)
);
}

24. /29@yegor256 24
@Test
void reportIncludesUserName() {
User user = new User.Fake(“Jeffrey”);
assertThat(
Foo.report(user),
containsString(“Jeffrey”)
);
}
complexity?
@Test
void reportIncludesBalanceInUSD() {
User user = new User.Fake(123);
assertThat(
Foo.report(user),
containsString(“123 USD”)
);
}

25. /29@yegor256 25
String report(User user) {
return String.format(
“Balance of %s is %d USD”,
user.profile().name(),
user.account().balance().usd()
);
}
fragility?
@Test
void reportIncludesBalanceInUSD() {
User user = new User.Fake(123);
assertThat(
Foo.report(user),
containsString(“123 USD”)
);
}

26. /29@yegor256 26
no throw-away tests
any more

27. /29@yegor256 27
always ship them
together!

28. /29@yegor256 28
interface User {
Profile profile();
Account account();
class Fake {
}
}
class UserTest {
@Test
void hasBalance() {
User user = new User.Fake(123);
assertThat(
user.account().balance().usd(),
not(equalTo(0))
);
}
}

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Fake
Objects
Mocking
Frameworks

30. /29@yegor256 30
Section 2.8
Don’t mock; use fakes