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Built-in Fake Objects

Yegor Bugayenko

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“Mock objects are simulated objects that mimic the behavior of

real objects in controlled ways”

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FakeObjects

Mocking Frameworks

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

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interface Profile { String name(); }

interface User { Profile profile(); Account account(); }

interface Account { Balance balance(); }

interface Balance { int usd(); }

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

Mockito

/29@yegor256 7@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”) ); }

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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 coupl

ing

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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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@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() ); }

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“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?

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@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”) ); }

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@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”) ); }

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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”) ); }

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no throw-away tests any more

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always ship themtogether!

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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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FakeObjects

Mocking Frameworks

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Section 2.8

Don’t mock; use fakes