Cloud Firestore integration in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get an instance of Firestore.

Android Kotlin

val db = FirebaseFirestore.[1]()

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

AgetInstance

Binitialize

Cconnect

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'initialize()' instead of 'getInstance()'.

Trying to create Firestore with 'connect()'.

The correct method to get a Firestore instance is getInstance().

2fill in blank

medium

Complete the code to add a new document to the 'users' collection.

Android Kotlin

db.collection("users").[1](userData)

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

Aget

Bupdate

Cadd

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'get()' which only reads data.

Using 'update()' which requires an existing document.

Use add() to add a new document with generated ID to a collection.

3fill in blank

hard

Fix the error in the code to listen for realtime updates on 'messages' collection.

Android Kotlin

db.collection("messages").addSnapshotListener { snapshots, [1] ->
    if (error != null) {
        return@addSnapshotListener
    }
    // process snapshots
}

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

Athrowable

Bexception

Cfailure

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'exception' or 'error' which are not the parameter names here.

Using 'failure' which is not defined.

The listener callback uses throwable to check for problems.

4fill in blank

hard

Fill both blanks to update the 'age' field of a document with ID 'userId' in 'users' collection.

Android Kotlin

db.collection("users").document(userId).[1](mapOf("age" to [2]))

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

Aupdate

Bset

C25

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set()' which replaces the whole document.

Using a wrong age value.

Use update() to change specific fields. The age value here is 25.

5fill in blank

hard

Fill all three blanks to query 'products' collection for items with price greater than 100 and order by price ascending.

Android Kotlin

db.collection("products").whereGreaterThan("[1]", [2]).orderBy("[3]")

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

Aprice

B100

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'name' instead of 'price' for filtering or ordering.

Using a wrong number for comparison.

We filter by 'price' greater than 100 and order by 'price'.

Practice

1. What is the primary purpose of Cloud Firestore in an Android app?

easy

A. To compile Kotlin code

B. To design the app's user interface

C. To store and sync app data in the cloud

D. To manage app permissions

Solution

1. Step 1: Understand Cloud Firestore's role

   Cloud Firestore is a cloud database service that stores and syncs app data.

2. Step 2: Differentiate from other app functions

   UI design, code compilation, and permission management are handled by other tools, not Firestore.

3. Final Answer:

   To store and sync app data in the cloud -> Option C

4. Quick Check:

   Cloud Firestore = Data storage and sync [OK]

Hint: Remember Firestore is a cloud database, not UI or code tool [OK]

Common Mistakes:

• Confusing Firestore with UI design tools
• Thinking Firestore compiles code
• Assuming Firestore manages app permissions

2. Which Kotlin code correctly initializes a Firestore instance?

easy

A. val db = FirebaseFirestore.getInstance()

B. val db = Firestore.getInstance()

C. val db = Firebase.getFirestore()

D. val db = FirestoreClient.get()

Solution

1. Step 1: Recall Firestore initialization syntax

   The official way to get Firestore instance in Kotlin is FirebaseFirestore.getInstance().

2. Step 2: Check other options for correctness

   Other options use incorrect class or method names not in Firestore API.

3. Final Answer:

   val db = FirebaseFirestore.getInstance() -> Option A

4. Quick Check:

   Correct Firestore init = val db = FirebaseFirestore.getInstance() [OK]

Hint: Use FirebaseFirestore.getInstance() to start Firestore [OK]

Common Mistakes:

• Using Firestore.getInstance() which does not exist
• Confusing FirebaseFirestore with Firebase class
• Using FirestoreClient which is not part of Android SDK

3. Given this code snippet, what will be printed if the document exists with field "name" = "Alice"?

val db = FirebaseFirestore.getInstance()
db.collection("users").document("user1").get()
  .addOnSuccessListener { document ->
    if (document != null && document.exists()) {
      println(document.getString("name"))
    } else {
      println("No such document")
    }
  }

medium

A. Alice

B. No such document

C. null

D. Error at runtime

Solution

1. Step 1: Understand the get() and success listener

   The get() fetches the document asynchronously; if it exists, the listener runs the success block.

2. Step 2: Check document existence and print field

   Since document exists and has "name" = "Alice", println(document.getString("name")) prints "Alice".

3. Final Answer:

   Alice -> Option A

4. Quick Check:

   Document exists with name = Alice [OK]

Hint: If document exists, getString returns the field value [OK]

Common Mistakes:

• Assuming document is null when it exists
• Expecting 'No such document' incorrectly
• Confusing getString with get() returning document

4. Identify the error in this Firestore write operation:

val db = FirebaseFirestore.getInstance()
db.collection("users").document("user1").set("name", "Bob")

medium

A. document() must be called after set()

B. Collection name cannot be "users"

C. FirebaseFirestore.getInstance() is incorrect

D. set() requires a Map or an object, not two parameters

Solution

1. Step 1: Check set() method signature

   set() expects a single argument: a Map or an object representing fields and values.

2. Step 2: Analyze the provided code

   Passing two parameters "name", "Bob" is invalid and causes a compile error.

3. Final Answer:

   set() requires a Map or an object, not two parameters -> Option D

4. Quick Check:

   set() needs one argument (Map/object) [OK]

Hint: Use a Map for set(), not separate key-value parameters [OK]

Common Mistakes:

• Passing multiple parameters to set() instead of a Map
• Calling document() after set() instead of before
• Thinking collection name restrictions cause error

5. You want to listen in real-time to changes in a Firestore collection "messages" and update your UI accordingly. Which code snippet correctly sets up this listener?

hard

A. db.collection("messages").get().addOnSuccessListener { snapshots -> for (doc in snapshots) { println(doc.getString("text")) } }

B. db.collection("messages").addSnapshotListener { snapshots, e -> if (e != null) { println("Listen failed") } else { for (doc in snapshots!!) { println(doc.getString("text")) } } }

C. db.collection("messages").document().addSnapshotListener { doc, e -> println(doc?.getString("text")) }

D. db.collection("messages").addListener { snapshots -> println(snapshots) }

Solution

1. Step 1: Understand real-time listeners in Firestore

   addSnapshotListener listens for real-time updates on a collection or document.

2. Step 2: Analyze each option

   db.collection("messages").addSnapshotListener { snapshots, e -> if (e != null) { println("Listen failed") } else { for (doc in snapshots!!) { println(doc.getString("text")) } } } correctly uses addSnapshotListener on collection with error check and iterates snapshots. db.collection("messages").get().addOnSuccessListener { snapshots -> for (doc in snapshots) { println(doc.getString("text")) } } uses get() which is one-time fetch. db.collection("messages").document().addSnapshotListener { doc, e -> println(doc?.getString("text")) } listens on a document without specifying ID. db.collection("messages").addListener { snapshots -> println(snapshots) } uses non-existent addListener method.

3. Final Answer:

   db.collection("messages").addSnapshotListener { snapshots, e -> if (e != null) { println("Listen failed") } else { for (doc in snapshots!!) { println(doc.getString("text")) } } } -> Option B

4. Quick Check:

   Real-time listener = addSnapshotListener on collection [OK]

Hint: Use addSnapshotListener on collection for real-time updates [OK]

Common Mistakes:

• Using get() instead of addSnapshotListener for real-time
• Calling addSnapshotListener on document without ID
• Using non-existent addListener method