Why Kotlin over Java - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a Kotlin variable with type inference.

Kotlin

val name = [1]

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

A"John"

Bvar

CString

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'var' instead of a value

Using type name instead of value

Using 'val' again instead of a value

In Kotlin, you can declare a variable with val and assign a value directly. Here, the value should be a string literal like "John".

2fill in blank

medium

Complete the code to create a Kotlin function that returns the length of a string.

Kotlin

fun lengthOfString(str: String): Int = str.[1]

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

Asize

Bcount

Clength()

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using length() with parentheses (Java style)

Using size which is for collections

Using count which requires a predicate

In Kotlin, strings have a length property (unlike Java's length() method) that returns the number of characters.

3fill in blank

hard

Fix the error in the Kotlin code to safely access the length of a nullable string.

Kotlin

val length = str[1]length ?: 0

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

A?.

B?

C!!

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!!' which throws an exception if null

Using '?' alone which is incomplete

Using ':' which is for Elvis operator

The safe call operator ?. allows accessing properties of a nullable object without throwing an error if it's null.

4fill in blank

hard

Fill both blanks to create a Kotlin data class with two properties.

Kotlin

data class User(val name: [1], val age: [2])

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

AString

BInt

CDouble

DBoolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using Double for age

Using Boolean for name

Mixing types incorrectly

A Kotlin data class holds data. Here, name is a String and age is an Int.

5fill in blank

hard

Fill all three blanks to create a Kotlin map comprehension filtering entries with values greater than 10.

Kotlin

val filtered = mapOf("a" to 5, "b" to 15, "c" to 20).filter { it.[1] [2] [3] }

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

Avalue

B>

C10

Dkey

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'key' instead of 'value'

Using '<' instead of '>'

Using a wrong number

This code filters the map entries where the value is greater than 10.

Practice

1. Why do many developers prefer Kotlin over Java for Android development?

easy

A. Kotlin does not support interoperability with Java

B. Kotlin reduces boilerplate code and improves safety

C. Kotlin requires more lines of code than Java

D. Kotlin is slower to compile than Java

Solution

1. Step 1: Understand Kotlin's code simplicity

   Kotlin reduces the amount of code needed compared to Java, making it easier to write and read.

2. Step 2: Recognize Kotlin's safety features

   Kotlin has built-in null safety and other features that help prevent common errors.

3. Final Answer:

   Kotlin reduces boilerplate code and improves safety -> Option B

4. Quick Check:

   Kotlin is easier and safer than Java [OK]

Hint: Remember Kotlin cuts code and adds safety [OK]

Common Mistakes:

• Thinking Kotlin requires more code
• Believing Kotlin can't work with Java
• Assuming Kotlin is slower to compile

2. Which of the following is a correct Kotlin syntax feature that improves safety compared to Java?

easy

A. Using var x: Int = null to allow null values

B. Declaring variables without specifying type always

C. Using val name: String? to declare a nullable variable

D. Using void as a return type

Solution

1. Step 1: Identify Kotlin's null safety syntax

   Kotlin uses ? after a type to mark it nullable, e.g., String?.

2. Step 2: Check other options for correctness

   Using var x: Int = null to allow null values is wrong because Int cannot be assigned null without ?. Declaring variables without specifying type always is incorrect because Kotlin requires type inference or explicit types. Using void as a return type is Java syntax, not Kotlin.

3. Final Answer:

   Using val name: String? to declare a nullable variable -> Option C

4. Quick Check:

   Nullable types use ? in Kotlin [OK]

Hint: Nullable types end with ? in Kotlin [OK]

Common Mistakes:

• Assigning null to non-nullable types
• Confusing Kotlin and Java syntax
• Using void instead of Unit in Kotlin

3. What will be the output of this Kotlin code snippet?

val list = listOf(1, 2, 3)
val doubled = list.map { it * 2 }
println(doubled)

medium

A. Compilation error

B. [1, 2, 3]

C. [3, 6, 9]

D. [2, 4, 6]

Solution

1. Step 1: Understand the map function

   The map function applies the lambda { it * 2 } to each element, doubling each number.

2. Step 2: Calculate the resulting list

   Original list is [1, 2, 3], doubling gives [2, 4, 6].

3. Final Answer:

   [2, 4, 6] -> Option D

4. Quick Check:

   map doubles each element [OK]

Hint: map transforms each element in the list [OK]

Common Mistakes:

• Confusing map with filter
• Expecting original list unchanged
• Thinking map returns a single value

4. Identify the error in this Kotlin code snippet:

fun main() {
  val name: String = null
  println(name)
}

medium

A. Cannot assign null to a non-nullable type

B. Missing semicolon after println

C. Function main must return Int

D. Variable name must be declared with var

Solution

1. Step 1: Check variable declaration

   The variable name is declared as non-nullable String, but assigned null.

2. Step 2: Understand Kotlin null safety

   Kotlin does not allow assigning null to non-nullable types. To allow null, use String?.

3. Final Answer:

   Cannot assign null to a non-nullable type -> Option A

4. Quick Check:

   Non-nullable types cannot hold null [OK]

Hint: Non-nullable types reject null assignments [OK]

Common Mistakes:

• Thinking semicolon is required
• Assuming main must return Int
• Confusing var and val for nullability

5. You have a Java project and want to start using Kotlin without rewriting everything. What is the best reason to choose Kotlin here?

hard

A. Kotlin can interoperate with Java code seamlessly

B. Kotlin requires all Java code to be converted first

C. Kotlin does not support Java libraries

D. Kotlin needs a separate runtime incompatible with Java

Solution

1. Step 1: Understand Kotlin-Java interoperability

   Kotlin is designed to work smoothly with existing Java code and libraries without conversion.

2. Step 2: Evaluate other options

   Kotlin requires all Java code to be converted first is false because conversion is not mandatory. Kotlin does not support Java libraries is false because Kotlin supports Java libraries. Kotlin needs a separate runtime incompatible with Java is false because Kotlin runs on the JVM, compatible with Java.

3. Final Answer:

   Kotlin can interoperate with Java code seamlessly -> Option A

4. Quick Check:

   Kotlin and Java work together easily [OK]

Hint: Kotlin works with Java code without rewriting [OK]

Common Mistakes:

• Thinking Kotlin requires full Java rewrite
• Believing Kotlin can't use Java libraries
• Assuming Kotlin needs a separate incompatible runtime