Flow operators (map, filter, transform) in Kotlin

Introduction

Flow operators help you change or pick data from a stream step by step. They make working with lists of data easy and clear.

When you want to change each item in a list to a new form.

When you want to keep only some items from a list based on a rule.

When you want to do complex changes or multiple steps on data flowing through.

When you want to process data one piece at a time without waiting for the whole list.

When you want to write clean and readable code for data streams.

Syntax

Kotlin

flowOf(1, 2, 3, 4, 5)
    .map { it * 2 }
    .filter { it > 5 }
    .transform {
        emit(it)
        emit(it + 1)
    }

map changes each item to something new.

filter keeps only items that match a condition.

transform lets you emit zero, one, or many items for each input.

Examples

This multiplies each number by 10.

Kotlin

flowOf(1, 2, 3)
    .map { it * 10 }

This keeps only even numbers.

Kotlin

flowOf(1, 2, 3, 4)
    .filter { it % 2 == 0 }

This emits the original number and then the number times 100.

Kotlin

flowOf(1, 2)
    .transform {
        emit(it)
        emit(it * 100)
    }

Sample Program

This program creates a flow of numbers 1 to 5. It multiplies each by 3, keeps only those greater than 7, then for each number it emits the number and the number plus one. Finally, it prints all emitted numbers.

Kotlin

import kotlinx.coroutines.flow.*
import kotlinx.coroutines.runBlocking

fun main() = runBlocking {
    flowOf(1, 2, 3, 4, 5)
        .map { it * 3 } // multiply each by 3
        .filter { it > 7 } // keep numbers greater than 7
        .transform {
            emit(it) // emit the number
            emit(it + 1) // emit number plus one
        }
        .collect { println(it) } // print each emitted number
}

Important Notes

Use map when you want exactly one output per input.

Use filter to remove unwanted items.

transform is more flexible and can emit many or no items per input.

Summary

map changes each item in the flow.

filter keeps only items that match a condition.

transform lets you emit any number of items for each input.

Practice

1. Which Kotlin Flow operator is used to change each item in the flow to a new value?

easy

A. filter

B. map

C. transform

D. collect

Solution

1. Step 1: Understand the purpose of map

   The map operator changes each item in the flow to a new value by applying a function.

2. Step 2: Compare with other operators

   filter keeps items based on a condition, transform can emit multiple items, and collect gathers items but does not change them.

3. Final Answer:

   map -> Option B

4. Quick Check:

   Change each item = map [OK]

Hint: Map means change each item to a new one [OK]

Common Mistakes:

• Confusing map with filter
• Thinking transform only changes one item
• Using collect to change items

2. Which of the following is the correct syntax to filter even numbers from a Kotlin Flow named numbers?

easy

A. numbers.collect { it % 2 == 0 }

B. numbers.map { it % 2 == 0 }

C. numbers.filter { it % 2 == 0 }

D. numbers.transform { emit(it % 2 == 0) }

Solution

1. Step 1: Identify the filter syntax

   The filter operator takes a lambda returning a Boolean to keep items matching the condition.

2. Step 2: Check each option

   numbers.filter { it % 2 == 0 } correctly uses filter { it % 2 == 0 } to keep even numbers. numbers.map { it % 2 == 0 } uses map which changes items to booleans. numbers.transform { emit(it % 2 == 0) } emits booleans instead of filtering. numbers.collect { it % 2 == 0 } misuses collect.

3. Final Answer:

   numbers.filter { it % 2 == 0 } -> Option C

4. Quick Check:

   Filter even numbers = filter { it % 2 == 0 } [OK]

Hint: Filter keeps items; map changes items [OK]

Common Mistakes:

• Using map instead of filter
• Misusing collect as a filter
• Confusing transform syntax

3. What is the output of this Kotlin Flow code?

flowOf(1, 2, 3, 4)
    .filter { it % 2 == 0 }
    .map { it * 10 }
    .toList()

medium

A. [20, 40]

B. [10, 20]

C. [2, 4]

D. [1, 2, 3, 4]

Solution

1. Step 1: Apply filter to keep even numbers

   The flow starts with [1, 2, 3, 4]. After filtering even numbers, we get [2, 4].

2. Step 2: Apply map to multiply each by 10

   Mapping [2, 4] by multiplying each by 10 results in [20, 40].

3. Final Answer:

   [20, 40] -> Option A

4. Quick Check:

   Filter evens then multiply by 10 = [20, 40] [OK]

Hint: Filter first, then map to change values [OK]

Common Mistakes:

• Mapping before filtering
• Confusing output with original list
• Ignoring order of operators

4. Identify the error in this Kotlin Flow code snippet:

flowOf(1, 2, 3)
    .filter { it > 1 }
    .map { it * 2 }
    .transform { emit(it + 1) }

medium

A. Using transform after map is invalid

B. The filter condition syntax is wrong

C. The emit call inside transform is incorrect

D. The code misses a terminal operator like collect

Solution

1. Step 1: Check the flow operators usage

   The operators filter, map, and transform are used correctly in sequence.

2. Step 2: Identify missing terminal operation

   Flows are cold and need a terminal operator like collect or toList to start processing. This code lacks that.

3. Final Answer:

   The code misses a terminal operator like collect -> Option D

4. Quick Check:

   Flows need terminal operator to run [OK]

Hint: Flows need collect or toList to run [OK]

Common Mistakes:

• Forgetting terminal operator
• Thinking transform can't follow map
• Misreading filter condition syntax

5. Given a Flow of integers, how can you use transform to emit both the original number and its square for each item?

hard

A. flow.transform { emit(it); emit(it * it) }

B. flow.map { listOf(it, it * it) }

C. flow.filter { emit(it); emit(it * it) }

D. flow.collect { emit(it); emit(it * it) }

Solution

1. Step 1: Understand transform usage

   transform allows emitting zero or more items per input, so emitting both original and square is possible.

2. Step 2: Check each option's correctness

   flow.transform { emit(it); emit(it * it) } correctly uses transform with two emit calls. flow.map { listOf(it, it * it) } uses map which emits a list, not separate items. flow.filter { emit(it); emit(it * it) } misuses filter. flow.collect { emit(it); emit(it * it) } misuses collect which is terminal and cannot emit.

3. Final Answer:

   flow.transform { emit(it); emit(it * it) } -> Option A

4. Quick Check:

   Emit multiple items per input = transform [OK]

Hint: Use transform to emit multiple items per input [OK]

Common Mistakes:

• Using map to emit multiple items separately
• Misusing filter or collect for emission
• Not calling emit inside transform