Kotlin Code Smell 25 - State as Properties

When an object changes its state, the best solution is to modify the attribute, right?

Problems

• Mutability

• Attribute Polluting

• Setters

Solutions

1. Use immutable objects

2. The model states as mathematical set inclusion.

3. Separate the state from the object as it is accidental.

Examples

• State diagrams

Sample Code

Wrong

sealed class OrderState
data object OrderStatePending : OrderState()
data object OrderStateConfirmed: OrderState()

class Order(private val items: List<Int>) {

    private var state: OrderState = OrderStatePending

    fun changeState(newState: OrderState) {
        state = newState
    }
}

Right

sealed class OrderState
data object OrderStatePending : OrderState()
data object OrderStateConfirmed: OrderState()

data class Order(val items: List<Int>, val state: OrderState)

val items = listOf(1, 2, 3)
val pendingOrder = Order(items, OrderStatePending)
val confirmedOrder = pendingOrder.copy(state = OrderStateConfirmed)

Exceptions

• Over Design

• Performance issues (if a serious benchmark supports it).

Conclusion

This technique is elegant but can lead to overdesign. For example, changing a visual component's color should be a counterexample of this smell.

We should be aware and very cautious as with any other smell.

These are hints and not rigid rules.

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Credits

• Code Smell 35 - State as Properties by Maxi Contieri