Flow exception handling in Kotlin - Step-by-Step Execution

Concept Flow - Flow exception handling

Start Flow

↓

Emit values

↓

Exception occurs?

No→Collect values

Yes↓

Catch exception

↓

Handle exception

↓

Continue or Stop Flow

↓

End

This flow shows how values are emitted, exceptions are caught and handled, then the flow continues or stops.

Execution Sample

Kotlin

flow {
  emit(1)
  emit(2)
  throw RuntimeException("Error!")
  emit(3)
}.catch { e ->
  emit(-1)
}.collect { value ->
  println(value)
}

This code emits values in a flow, throws an exception, catches it to emit -1, and collects values to print.

Execution Table

Step	Action	Evaluation	Result	Output
1	Emit 1	No exception	Value 1 emitted	1
2	Emit 2	No exception	Value 2 emitted	2
3	Throw RuntimeException	Exception thrown	Exception caught by catch
4	Emit -1 in catch	Handling exception	Value -1 emitted	-1
5	Emit 3 after exception	Not reached	No emission
6	Collect values	All emitted values collected	Printed 1, 2, -1	1 2 -1

💡 Exception thrown at step 3, caught and handled; flow does not emit 3 after exception.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
emittedValue	none	1	2	exception	-1	-1
exceptionCaught	false	false	false	true	true	true

Key Moments - 3 Insights

Why is the value 3 never emitted after the exception?

Because the exception interrupts the flow at step 3, so the code after the throw is not executed (see execution_table step 5).

How does the catch block emit a value after an exception?

The catch block handles the exception and can emit new values, like -1 at step 4, continuing the flow safely.

Does the flow stop completely after an exception?

Not necessarily; if the exception is caught and handled, the flow can continue emitting values or complete gracefully.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 2?

A 3

B -1

C 2

D Exception

💡 Hint

Check the 'Output' column for step 2 in the execution_table.

At which step does the exception get caught?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' and 'Result' columns around steps 3 and 4.

If the catch block did not emit -1, what would be the collected output?

A 1, 2

B 1, 2, -1

C 1, 2, 3

D No output

💡 Hint

Refer to the variable_tracker and execution_table showing emitted values and catch behavior.

Concept Snapshot

Kotlin Flow exception handling:
- Use .catch { e -> } to handle exceptions in a flow.
- Exceptions stop normal emission; catch can emit fallback values.
- Flow continues after catch if handled.
- Use collect { } to receive emitted values.
- Exception interrupts flow unless caught.

Full Transcript

This visual execution shows how Kotlin Flow handles exceptions. The flow emits values 1 and 2 normally. At step 3, a RuntimeException is thrown, interrupting the flow. The catch block catches this exception and emits -1 as a fallback value. The flow does not emit the value 3 after the exception because the throw stops normal execution. The collected output prints 1, 2, and -1. Variables track emitted values and whether an exception was caught. Key moments clarify why emission stops after an exception and how catch allows continuation. The quiz tests understanding of output at each step and the role of catch. The snapshot summarizes the syntax and behavior of flow exception handling in Kotlin.