Throw exception with meaningful error description in case of missing state definition. (#15)

Throwing exception with meaningful error description in case of missing state definition.

Author: Piotr Chmielowski

src/main/kotlin/com/tinder/StateMachine.kt

@@ -51,8 +51,7 @@ class StateMachine<STATE : Any, EVENT : Any, SIDE_EFFECT : Any> private construc
     private fun STATE.getDefinition() = graph.stateDefinitions
         .filter { it.key.matches(this) }
         .map { it.value }
-        .firstOrNull()
-        .let { checkNotNull(it) }
+        .firstOrNull() ?: error("Missing definition for state ${this.javaClass.simpleName}!")
 
     private fun STATE.notifyOnEnter(cause: EVENT) {
         getDefinition().onEnterListeners.forEach { it(this, cause) }

src/test/kotlin/com/tinder/StateMachineTest.kt

@@ -698,6 +698,27 @@ internal class StateMachineTest {
             }
         }
 
+        class WithMissingStateDefinition {
+
+            private val stateMachine = StateMachine.create<String, Int, Nothing> {
+                initialState(STATE_A)
+                state(STATE_A) {
+                    on(EVENT_1) {
+                        transitionTo(STATE_B)
+                    }
+                }
+                // Missing STATE_B definition.
+            }
+
+            @Test
+            fun transition_givenMissingDestinationStateDefinition_shouldThrowIllegalStateExceptionWithStateName() {
+                // Then
+                assertThatIllegalStateException()
+                    .isThrownBy { stateMachine.transition(EVENT_1) }
+                    .withMessage("Missing definition for state ${STATE_B.javaClass.simpleName}!")
+            }
+        }
+
         private companion object {
             private const val STATE_A = "a"
             private const val STATE_B = "b"