File access and storage in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the difference between internal and external storage in Android?

Internal storage is private to the app and not accessible by other apps. External storage is shared and can be accessed by other apps and the user. External storage may be removable like an SD card.

beginner

How do you write a text file to internal storage in Kotlin?

Use openFileOutput(filename, Context.MODE_PRIVATE) to get a FileOutputStream, then write bytes or strings to it. This saves the file privately for your app.

intermediate

What permission is required to write to external storage on Android 10 and below?

You need the WRITE_EXTERNAL_STORAGE permission declared in the manifest and requested at runtime for Android 6.0 to 10.

intermediate

What is the recommended way to access shared media files on Android 11 and above?

Use the MediaStore API with scoped storage. Direct file path access is restricted, so apps should use content URIs and system APIs to read/write media.

beginner

How can you check if external storage is available for read and write?

Check Environment.getExternalStorageState() and compare it to Environment.MEDIA_MOUNTED for read/write access.

Which method writes a file to internal storage in Android Kotlin?

A getExternalFilesDir()

B FileOutputStream(Environment.getExternalStorageDirectory())

C openFileOutput(filename, Context.MODE_PRIVATE)

D MediaStore.insert()

openFileOutput() opens a private file in internal storage for writing.

What permission is needed to write files to external storage on Android 9?

A INTERNET

B READ_CONTACTS

C ACCESS_FINE_LOCATION

D WRITE_EXTERNAL_STORAGE

WRITE_EXTERNAL_STORAGE permission is required to write to external storage on Android 9.

How do you check if external storage is writable?

A Environment.getExternalStorageState() == Environment.MEDIA_MOUNTED

B File.canWrite()

C Context.checkSelfPermission()

D MediaStore.isWritable()

Checking if external storage state equals MEDIA_MOUNTED means it is writable.

Which API should you use to access shared media files on Android 11+?

A File API with direct paths

B MediaStore API

C openFileOutput()

D SharedPreferences

MediaStore API is recommended for accessing shared media files with scoped storage.

Where are files saved when using openFileOutput()?

A App's internal storage directory

B Downloads folder

C External storage root

D Cache directory

openFileOutput() saves files in the app's internal storage directory, private to the app.

Explain how to save and read a text file in internal storage using Kotlin in Android.

Think about how you open a file stream to write and read files privately inside your app.

Describe the changes in Android storage permissions and access from Android 10 to Android 11.

Focus on how Android limits file access for privacy and what APIs replace old methods.

Practice

1. Which method is used in Android Kotlin to write data to a private file within the app's internal storage?

easy

A. writeExternalStorage

B. openFileInput

C. readFile

D. openFileOutput

Solution

1. Step 1: Understand file writing methods

   openFileOutput is the method designed to write data to a private file in internal storage.

2. Step 2: Differentiate from reading methods

   openFileInput is used for reading, not writing files.

3. Final Answer:

   openFileOutput -> Option D

4. Quick Check:

   Write file = openFileOutput [OK]

Hint: Writing files uses openFileOutput, reading uses openFileInput [OK]

Common Mistakes:

• Confusing openFileInput with writing method
• Using external storage methods for internal files
• Assuming readFile is a valid Android method

2. Which of the following is the correct Kotlin syntax to open a file named "data.txt" for reading in an Android app?

easy

A. val input = openFileOutput("data.txt", Context.MODE_PRIVATE)

B. val input = openFileInput("data.txt")

C. val input = openFile("data.txt", Context.MODE_PRIVATE)

D. val input = readFile("data.txt")

Solution

1. Step 1: Identify the correct method for reading files

   openFileInput is the correct method to open a file for reading in Android Kotlin.

2. Step 2: Check syntax correctness

   openFileInput takes only the filename as a parameter, no mode needed.

3. Final Answer:

   val input = openFileInput("data.txt") -> Option B

4. Quick Check:

   Read file = openFileInput(filename) [OK]

Hint: openFileInput(filename) opens file for reading [OK]

Common Mistakes:

• Using openFileOutput for reading
• Adding mode parameter to openFileInput
• Using non-existent readFile method

3. What will be the output of the following Kotlin code snippet in an Android app?

val filename = "note.txt"
val fileContents = "Hello World"
openFileOutput(filename, Context.MODE_PRIVATE).use {
  it.write(fileContents.toByteArray())
}
val input = openFileInput(filename).bufferedReader().use { it.readText() }
println(input)

medium

A. note.txt

B. null

C. Hello World

D. FileNotFoundException

Solution

1. Step 1: Write text to file

   The code writes "Hello World" to "note.txt" using openFileOutput.

2. Step 2: Read text from the same file

   openFileInput reads the file content back as a string.

3. Final Answer:

   Hello World -> Option C

4. Quick Check:

   Write then read file = "Hello World" [OK]

Hint: Write then read file returns the original string [OK]

Common Mistakes:

• Expecting filename instead of content
• Assuming file is empty or null
• Not closing streams causing errors

4. Consider this Kotlin code snippet in an Android app:

val input = openFileInput("config.txt")
val text = input.bufferedReader().readText()
input.close()

What is the main issue with this code?

medium

A. Not using 'use' block to safely close the stream

B. The input stream is closed before reading text

C. bufferedReader() cannot be called on input streams

D. openFileInput should be called with Context.MODE_PRIVATE

Solution

1. Step 1: Check resource management

   The code manually closes the input stream but risks exceptions if readText() fails.

2. Step 2: Use 'use' block for safety

   Using 'use' automatically closes the stream safely even if errors occur.

3. Final Answer:

   Not using 'use' block to safely close the stream -> Option A

4. Quick Check:

   Safe stream closing = use block [OK]

Hint: Always use 'use' to auto-close streams safely [OK]

Common Mistakes:

• Adding mode parameter to openFileInput
• Closing stream before reading
• Believing bufferedReader() is invalid here

5. You want to save user preferences as a JSON string in a private file named "prefs.json" and then read it back safely. Which Kotlin code snippet correctly writes and reads this file in an Android app?

hard

A. openFileOutput("prefs.json", Context.MODE_PRIVATE).use { it.write(jsonString.toByteArray()) }
val json = openFileInput("prefs.json").bufferedReader().use { it.readText() }

B. openFileInput("prefs.json").use { it.write(jsonString.toByteArray()) }
val json = openFileOutput("prefs.json", Context.MODE_PRIVATE).bufferedReader().use { it.readText() }

C. val file = File("prefs.json")
file.writeText(jsonString)
val json = file.readText()

D. val fos = openFileOutput("prefs.json")
fos.write(jsonString.toByteArray())
fos.close()
val fis = openFileInput("prefs.json")
val json = fis.bufferedReader().readText()
fis.close()

Solution

1. Step 1: Write file using openFileOutput with 'use'

   openFileOutput("prefs.json", Context.MODE_PRIVATE).use { it.write(jsonString.toByteArray()) } val json = openFileInput("prefs.json").bufferedReader().use { it.readText() } correctly writes the JSON string to "prefs.json" using openFileOutput and safely closes the stream with 'use'.

2. Step 2: Read file using openFileInput with 'use'

   It then reads the file back safely using openFileInput and 'use' to auto-close the stream.

3. Final Answer:

   openFileOutput("prefs.json", Context.MODE_PRIVATE).use { it.write(jsonString.toByteArray()) } val json = openFileInput("prefs.json").bufferedReader().use { it.readText() } -> Option A

4. Quick Check:

   Write/read private file safely = openFileOutput/input + use [OK]

Hint: Use openFileOutput/input with 'use' to safely write/read files [OK]

Common Mistakes:

• Mixing up openFileInput and openFileOutput
• Using File class for internal storage without context
• Not closing streams properly causing leaks