ProGuard and R8 optimization in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable R8 optimization in your Android app's build.gradle file.

Android Kotlin

android {
    buildTypes {
        release {
            minifyEnabled = [1]
        }
    }
}

Drag options to blanks, or click blank then click option'

Afalse

B"true"

Ctrue

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around true like "true" causes a syntax error.

Setting minifyEnabled to false disables code shrinking.

Setting minifyEnabled = true enables R8 to shrink and optimize your app code during release builds.

2fill in blank

medium

Complete the code to specify the ProGuard rules file in your build.gradle for release builds.

Android Kotlin

android {
    buildTypes {
        release {
            proguardFiles getDefaultProguardFile([1]), 'proguard-rules.pro'
        }
    }
}

Drag options to blanks, or click blank then click option'

A"proguard-default.txt"

B"proguard-android-optimize.txt"

C"proguard-rules.pro"

D"proguard-android.txt"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong file name disables optimization.

Omitting quotes causes syntax errors.

The getDefaultProguardFile("proguard-android-optimize.txt") loads the default optimized ProGuard rules for Android.

3fill in blank

hard

Fix the error in the ProGuard rule to keep all classes in the package com.example.app.models.

Android Kotlin

-keep class [1].** { *; }

Drag options to blanks, or click blank then click option'

Acom.example.app.models.**

Bcom.example.app.model

Ccom.example.app.models.*

Dcom.example.app.models

Attempts:

3 left

💡 Hint

Common Mistakes

Adding wildcards inside the package name causes errors.

Misspelling the package name prevents rules from applying.

The correct package name is com.example.app.models. The .** after it keeps all classes recursively.

4fill in blank

hard

Fill both blanks to keep all public void methods in the class com.example.app.utils.NetworkUtils.

Android Kotlin

-keepclassmembers class [1] {
    public [2] *;
}

Drag options to blanks, or click blank then click option'

Acom.example.app.utils.NetworkUtils

Bvoid

Cmethod

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'method' instead of a return type causes syntax errors.

Misspelling the class name prevents the rule from applying.

The class name is com.example.app.utils.NetworkUtils. The return type void matches public methods with no return value.

5fill in blank

hard

Fill both blanks to create a ProGuard rule that keeps all classes annotated with @Keep in the package com.example.app.

Android Kotlin

-keep @[1] class [2].** { *; }

Drag options to blanks, or click blank then click option'

Aandroidx.annotation.Keep

Bcom.example.app

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing annotation and package names.

Using incomplete package names.

The annotation is androidx.annotation.Keep. The package is com.example.app. This rule keeps all classes in that package annotated with @Keep.

Practice

1. What is the main purpose of enabling minifyEnabled true in your Android app's release build?

easy

A. To automatically generate UI layouts

B. To increase the app's debug information for easier troubleshooting

C. To disable ProGuard and R8 optimizations

D. To shrink and optimize the app code by removing unused parts

Solution

1. Step 1: Understand the role of minifyEnabled

   Setting minifyEnabled true activates code shrinking and optimization tools like ProGuard and R8.

2. Step 2: Identify the effect on app code

   These tools remove unused code and optimize the remaining code to reduce app size and improve performance.

3. Final Answer:

   To shrink and optimize the app code by removing unused parts -> Option D

4. Quick Check:

   minifyEnabled true = shrink and optimize [OK]

Hint: minifyEnabled true means code shrinking is ON [OK]

Common Mistakes:

• Thinking it increases debug info instead of reducing code
• Confusing minifyEnabled with disabling optimizations
• Assuming it generates UI layouts automatically

2. Which of the following is the correct way to specify a ProGuard rules file in your build.gradle for release builds?

easy

A. proguardFiles 'proguard-rules.txt'

B. proguardFiles 'proguard-android-optimize.txt', getDefaultProguardFile('proguard-rules.pro')

C. proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'

D. minifyEnabled false

Solution

1. Step 1: Recall the standard syntax for ProGuard files

   The correct syntax uses getDefaultProguardFile for the default file and then adds custom rules.

2. Step 2: Match the correct order and file names

   proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro' correctly calls getDefaultProguardFile('proguard-android-optimize.txt') and then adds 'proguard-rules.pro'.

3. Final Answer:

   proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro' -> Option C

4. Quick Check:

   Correct ProGuard file syntax = proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro' [OK]

Hint: Default file uses getDefaultProguardFile() [OK]

Common Mistakes:

• Swapping file names or forgetting getDefaultProguardFile()
• Setting minifyEnabled false disables ProGuard
• Using wrong file extensions like .txt instead of .pro

3. Given this ProGuard rule:

-keep class com.example.MyClass { *; }

What will happen to MyClass during code shrinking?

medium

A. It will be kept intact with all its members

B. It will be removed if unused

C. Only its public methods will be kept

D. It will be renamed but removed if unused

Solution

1. Step 1: Understand the -keep rule

   The -keep directive tells ProGuard to keep the specified class and its members without removing or renaming them.

2. Step 2: Analyze the rule syntax

   The rule -keep class com.example.MyClass { *; } means keep the entire class and all its members intact.

3. Final Answer:

   It will be kept intact with all its members -> Option A

4. Quick Check:

   -keep class = keep all members [OK]

Hint: -keep means keep class and members intact [OK]

Common Mistakes:

• Thinking -keep removes unused classes
• Assuming only public methods are kept
• Confusing renaming with removal

4. You enabled minifyEnabled true but your app crashes at runtime due to missing classes. What is the most likely cause?

medium

A. You set minifyEnabled false by mistake

B. You forgot to add -keep rules for classes used via reflection

C. You used the wrong ProGuard file name in build.gradle

D. You disabled R8 in your project

Solution

1. Step 1: Identify the cause of missing classes

   When code shrinking removes classes used only by reflection, runtime crashes occur.

2. Step 2: Understand the role of -keep rules

   Adding -keep rules for such classes prevents their removal during shrinking.

3. Final Answer:

   You forgot to add -keep rules for classes used via reflection -> Option B

4. Quick Check:

   Missing classes after shrinking = missing -keep rules [OK]

Hint: Add -keep rules for reflection-used classes [OK]

Common Mistakes:

• Assuming minifyEnabled false causes missing classes
• Confusing file name errors with missing classes
• Thinking disabling R8 causes missing classes

5. You want to keep all classes in the package com.example.models but allow R8 to rename their members to reduce size. Which ProGuard rule should you use?

hard

A. -keepnames class com.example.models.**

B. -keepclassmembers class com.example.models.** { *; }

C. -keep class com.example.models.** { *; }

D. -keepclassmembernames class com.example.models.** { *; }

Solution

1. Step 1: Understand the difference between -keep and -keepnames

   -keep prevents removal and renaming; -keepnames prevents renaming but allows removal.

2. Step 2: Match the requirement

   You want to keep classes but allow member renaming, so -keepnames on classes is correct.

3. Final Answer:

   -keepnames class com.example.models.** -> Option A

4. Quick Check:

   Keep classes but rename members = -keepnames [OK]

Hint: -keepnames keeps class names but allows member renaming [OK]

Common Mistakes:

• Using -keep which prevents renaming
• Confusing -keepclassmembers with -keepnames
• Assuming -keepclassmembernames keeps classes