Photo by Gabriel Sollmann on Unsplash Lessons From a Librarian James Barr - @JBarr21 Zac Sweers - @ZacSweers 

subscribe() 

subscribe(new Consumer() { @Override public void accept(String value) { // Do stuff } }) 

subscribe(new CrashOnErrorConsumer() { @Override public void accept(String value) { // Do stuff } }) 

subscribe(new CrashOnErrorConsumer() { @Override public void accept(String value) { // Do stuff } }) 

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subscribe(new CrashOnErrorConsumer() { @Override public void accept(String value) { // Do stuff } }) 

subscribe(value -> { // Do stuff }) 

subscribe(value -> { // Do stuff }) github.com/uber/rxdogtag 

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subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) subscribe(value -> { // Do stuff }) 

- Anonymous audience member "But how?!" 

Lessons from a Librarian 

"Writing Code That Lasts Forever" - Jesse Wilson - Droidcon NYC 2018 

Writing Code That Lasts Forever 

Helping Everyone Write Code That Lasts Forever 

Codebase Health ✅ Builds quickly ✅ Test coverage ✅ Well documented ❌ Duplicate code ❌ Deprecated APIs ❌ Large modules/classes 

Name Build Time Undocumented Public Methods Module Size Deprecated Method Calls Quality Score (Overall: 75%) app 5m25s 0 10,000 LOC 0 75% feature1 4m20s 0 120,000 LOC 0 50% feature2 20s 0 4,000 LOC 0 100% library1 7s 90 1,000 LOC 0 75% library2 5s 0 2,000 LOC 88 75% Codebase Health 

Name Build Time Undocumented Public Methods Module Size Deprecated Method Calls Quality Score (Overall: 75%) app 5m25s 0 10,000 LOC 0 75% feature1 4m20s 0 120,000 LOC 0 50% feature2 20s 0 4,000 LOC 0 100% library1 7s 90 1,000 LOC 0 75% library2 5s 0 2,000 LOC 88 75% Codebase Health 

How does tech debt happen? 

API Design 

Limit Surface Area 

Limit Surface Area class Hammer { public void hammer(Nail nail) { // ow } } 

Limit Surface Area • Final by default final class Hammer { public void hammer(Nail nail) { // ow } } 

Limit Surface Area • Final by default • Visibility final class Hammer { public void hammer(Nail nail) { // ow } } 

Limit Surface Area • Final by default • Visibility • Factories final class Hammer { public static Hammer create() { // Stuff } private Hammer() { ... } } 

Limit Surface Area final class Hammer { public void hammer(Nail nail) { // ow } } • Final by default • Visibility • Factories 

API Design • Limit surface area • Final by default • Visibility • Factories • Consume interfaces interface Nail { void onHammer(); int length(); } 

API Design • Limit surface area • Final by default • Visibility • Factories • Consume interfaces • Fluent APIs as navigation final class Hammer { public void removeNail(Nail nail) { // } } 

API Design • Limit surface area • Final by default • Visibility • Factories • Consume interfaces • Fluent APIs as navigation final class Hammer { public RemovedNail removeNail(Nail nail) { // } } 

API Design final class Hammer { public RemovedNail removeNail(Nail nail) { // } } hammer.removeNail() .dispose() • Limit surface area • Final by default • Visibility • Factories • Consume interfaces • Fluent APIs as navigation 

API Design final class Hammer { public RemovedNail removeNail(Nail nail) { // } } hammer.removeNail() .dispose() • Limit surface area • Final by default • Visibility • Factories • Consume interfaces • Fluent APIs as navigation 

- you, writing libraries "Does this put the user on the right track?" 

- you, writing libraries "Does this put the user on the right track?" "Is this going to limit me later?" 

API vs Implementation 

API vs Implementation class Hammer { public void hammer(Nail nail) { // ow } } 

API vs Implementation class Hammer { public void hammer(Nail nail) { // ow } } :hammer 

API vs Implementation class Hammer { public void hammer(Nail nail) { // ow } } :hammer libA libA App 

API vs Implementation class Hammer { public void hammer(Nail nail) { // ow } } :hammer libA libA App 

API vs Implementation class Hammer { public void hammer(Nail nail) { // ow } } :hammer libA libA App 

API vs Implementation class Hammer { public void hammer(Nail nail) { // ow } } :hammer libA libA App :hammer-api interface Hammer { void hammer(Nail nail) } 

API vs Implementation class Hammer { public void hammer(Nail nail) { // ow } } :hammer libA libA App :hammer-api interface Hammer { void hammer(Nail nail) } 

API vs Implementation class Hammer { public void hammer(Nail nail) { // ow } } :hammer libA libA App :hammer-api interface Hammer { void hammer(Nail nail) } 

BuildConfig 

BuildConfig public class BuildConfig { public static final String VERSION_NAME = "1.0"; public static final int VERSION_CODE = 1; public static final boolean DEBUG == false; } 

BuildConfig public class BuildConfig { public static final String VERSION_NAME = "1.0"; public static final int VERSION_CODE = 1; public static final boolean DEBUG == false; } public interface AppBuildConfig { String versionName(); int versionCode(); boolean isDebug(); } 

BuildConfig final class RealAppBuildConfig implements AppBuildConfig { // ... } public interface AppBuildConfig { String versionName(); int versionCode(); boolean isDebug(); } :appbuildconfig :app 

BuildConfig final class RealAppBuildConfig implements AppBuildConfig { // ... } public interface AppBuildConfig { String versionName(); int versionCode(); boolean isDebug(); } :appbuildconfig :realappbuildconfig :app 

BuildConfig final class RealAppBuildConfig implements AppBuildConfig { // ... } public interface AppBuildConfig { String versionName(); int versionCode(); boolean isDebug(); } :appbuildconfig :realappbuildconfig :app android { // or libraryVariants applicationVariants { generateBuildConfigProvider { enabled = true } } } 

More Resources • API Design As An Art: youtube.com/watch?v=Ve9HlnJ0wCM • Writing Code That Lasts Forever: youtube.com/watch?v=YZstpc2939s • Effective Java • AppBuildConfig pattern: github.com/ZacSweers/CatchUp/pull/200 

Code Review 

Code Review Teach, don't tell 

Code Review Approve with nits 

Code Review 

Static Analysis 

Static Analysis 

Static Analysis 

Removing Tech Debt 

Replace in Path (⌘⇧R) Find/Replace 

Find/Replace Replace in Path (⌘⇧R) Command line tools > find . -name "*.java" | xargs sed -i '' “s/import foo/import barr/g” 

Kotlin ReplaceWith class Example { fun foo(s: String) {a/* old */ } fun usage() { foo("hello") } } 

Kotlin ReplaceWith class Example { @Deprecated("Use Example.bar") fun foo(s: String) { /* old */ } fun bar(s: String) { /* new */ } fun usage() { foo("hello") } } 

Kotlin ReplaceWith class Example { @Deprecated("Use Example.bar", replaceWith = ReplaceWith(“bar(s)")) fun foo(s: String) { /* old */ } fun bar(s: String) { /* new */ } fun usage() { foo("hello") } } 

Kotlin ReplaceWith class Example { @Deprecated("Use Example.bar", replaceWith = ReplaceWith(“bar(s)")) fun foo(s: String) { /* old */ } fun bar(s: String) { /* new */ } fun usage() { foo("hello") } } 

Kotlin ReplaceWith class Example { @Deprecated("Use Example.bar", replaceWith = ReplaceWith(“bar(s)")) fun foo(s: String) { /* old */ } fun bar(s: String) { /* new */ } fun usage() { bar("hello") } } 

import android.util.Pair fun doSomething() { val pair = Pair.create(“a”, “b”) } Android Lint SuggestedFix 

import androidx.core.util.Pair fun doSomething() { val pair = Pair.create(“a”, “b”) } Android Lint SuggestedFix 

Structural Find & Replace void main( Example ex, Other other) { ex.foo() ex.foo(“hi") other.foo() } 

Structural Find & Replace void main( Example ex, Other other) { ex.foo() ex.foo(“hi") other.foo() } void main( Example ex, Other other) { ex.foo() ex.foo(“hi") other.foo() } 

Structural Find & Replace void main( Example ex, Other other) { ex.foo() ex.foo(“hi") other.foo() } $Instance$ type=com.example.Example .foo($Parameter$ [0,∞]) $Instance$ type=com.example.Example .bar($Parameter$ [0,∞]) 

Structural Find & Replace void main( Example ex, Other other) { ex.bar() ex.bar(“hi") other.foo() } $Instance$ type=com.example.Example .foo($Parameter$ [0,∞]) $Instance$ type=com.example.Example .bar($Parameter$ [0,∞]) youtrack.jetbrains.com/issue/KT-10176 

IntelliJ Plugin • Add your own refactor options • Leverage IntelliJ’s tools • Good for automating multiple uses of refactors https://cutt.ly/find-matches-action 

Kotlin Script File(args[0]).walkTopDown() .filter { it.extension in setOf("java", "kt") } .filter { "/test/" !in it.absolutePath } .forEach {} 

Kotlin Script File(args[0]).walkTopDown() .filter { it.extension in setOf("java", "kt") } .filter { "/test/" !in it.absolutePath } .forEach { file: File -> val fileLines = file.readLines().toMutableList() // modify lines of file file.writeText(fileLines.joinToString("\n")) } 

Kotlin Script File(args[0]).walkTopDown() .filter { it.extension in setOf("java", "kt") } .filter { "/test/" !in it.absolutePath } .forEach { file: File -> val fileLines = file.readLines().toMutableList() // modify lines of file file.writeText(fileLines.joinToString("\n")) } $ brew install holgerbrandl/tap/kscript $ kscript MigrateToAndroidX.kts ~/project-root 16,866 files changed, 31,454 insertions(+), 31,415 deletions(-) 

De-risking Change 

Shadow Jobs 

Shadow Jobs Android Gradle Plugin 4 beta/4.1 alpha Kotlin 1.4-M1 Gradle 6.4-rc -SNAPSHOT Androidx alphas JDK 8-14 Shadow Jobs 

Offensive Programming 

Offensive Programming abstract class Model { abstract List modelList(); } 

Offensive Programming abstract class Model { abstract ImmutableList modelList(); } 

Offensive Programming abstract class Model { abstract List modelList(); } ImmutableList 

b7cb262 - Migrate lib1 d43aa98 - Migrate lib2 a1dc8d1 - Migrate lib3 d843d8f - Migrate lib4 vs a13ab55 - Migrate all libs Safety Without Feature Flags 

Photo by Webaroo on Unsplash 

Photo by Pankaj Patel on Unsplash 

Photo by Margarida CSilva on Unsplash 

Photo by Hannes Wolf on Unsplash 

Wrap Up 

Photo by Gabriel Sollmann on Unsplash Lessons From a Librarian James Barr - @JBarr21 Zac Sweers - @ZacSweers