Preconditions (require, check, error) in Kotlin - Step-by-Step Execution

Concept Flow - Preconditions (require, check, error)

Start Function

↓

Check require(condition)

↓

Throw IllegalArgumentException

↓

Check check(condition)

↓

Throw IllegalStateException

↓

Call error(message)

↓

Throw IllegalStateException

↓

Continue Execution

The function starts by checking conditions with require and check. If conditions fail, exceptions are thrown. error always throws an exception immediately.

Execution Sample

Kotlin

fun validateAge(age: Int) {
    require(age >= 0) { "Age must be non-negative" }
    check(age < 150) { "Age must be realistic" }
    if (age == 100) error("Age cannot be 100")
}

This function checks if age is valid using require, check, and error to stop execution with messages if conditions fail.

Execution Table

Step	Condition Checked	Condition Result	Action Taken	Exception Thrown
1	require(age >= 0)	True	Continue	None
2	check(age < 150)	True	Continue	None
3	if (age == 100)	True	error called	IllegalStateException
4	Function ends	-	Execution completes	None

💡 Execution stops normally because all conditions passed and error was not called.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
age	Input value	Input value	Input value	Input value	Input value

Key Moments - 3 Insights

Why does require throw IllegalArgumentException but check throws IllegalStateException?

require is for checking arguments passed to a function and throws IllegalArgumentException if false (see Step 1). check is for checking internal state and throws IllegalStateException if false (see Step 2).

What happens when error() is called?

error() immediately throws an IllegalStateException with the given message and stops execution (would happen at Step 3 if condition true).

If require condition fails, does the rest of the function run?

No, if require fails (condition false), it throws an exception immediately and stops further execution (see Step 1 false branch).

Visual Quiz - 3 Questions

Look at the execution table, what exception is thrown if require condition fails?

A NullPointerException

B IllegalStateException

C IllegalArgumentException

D No exception

💡 Hint

Check Step 1 in the execution_table where require is evaluated.

At which step does check condition get evaluated?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Condition Checked' column in the execution_table.

If age is 100, what happens at Step 3?

A error() is called and throws IllegalStateException

B No action, continue execution

C require fails and throws IllegalArgumentException

D check fails and throws IllegalStateException

💡 Hint

See the condition and action at Step 3 in the execution_table.

Concept Snapshot

Preconditions in Kotlin:
- require(condition) checks arguments, throws IllegalArgumentException if false
- check(condition) checks internal state, throws IllegalStateException if false
- error(message) always throws IllegalStateException immediately
Use these to stop execution early with clear error messages.

Full Transcript

This visual execution shows how Kotlin preconditions work. The function starts and first uses require to check if the argument meets a condition. If require fails, it throws IllegalArgumentException and stops. If require passes, check tests internal state and throws IllegalStateException if it fails. The error function always throws an exception immediately when called. The execution table traces these steps with conditions and actions. Variable tracking shows the input value remains unchanged. Key moments clarify the difference between require and check exceptions and what error does. The quiz tests understanding of which exceptions are thrown and when. This helps beginners see how preconditions protect code by stopping execution early with helpful messages.