The Law of Demeter

My first lightning talk in 2023. Presented to Android developers at weekly Android Huddle

1. The Law of Demeter Fusion Squad - NICV Presenter: Hieu Vu
2. Definition The Law of Demeter (LoD) or principle of least knowledge is a design guideline for developing software, particularly object-oriented programs. - “Wikipedia”
3. Statement Method m of an object O may only invoke the methods of the following kinds of objects: • O itself • m’s parameters • Any objects created/instantiated within m • O’s direct component objects
4. Example class OClass() { private val helper: Object fun someMethod() {} fun methodM(param1: Any, param2: Any) { // Valid helper.doSomething() // Valid val result = param1.revert() // Valid val resultHandler = Handler() resultHandler.resolve(result) // Valid someMethod()
5. Advantages • Dependencies between classes & coupling are reduced • Cleaner tests • Code is easier to be analyzed
6. Violation class CreditCardManager() { fun charge(creditCard: CreditCard): Result { val processor: CreditCardProcessor = CreditCardProcessor() processor.extractCardNumber(creditCard) val info = processor.getCardInfoManager().processPrechargeInfo() (*) return this.updateChargeInfo(info) } fun updateChargeInfo(info: PrechargeInfo): Result { ... } }
7. Testing class CreditCartManagerTest() { // Given this testee already setted up private val testee: CreditCardManager @Test fun givenCreditCard_whenCharge_thenReturnSuccess() { // given val creditCard = CreditCard(balance = 12) // when val result = testee.charge(creditCard = creditCard) // then assertResultSuccess(result = result) } }
8. Testing Exception in thread "main" java.lang.NullPointerException at FileKt.main (File.kt:3) at FileKt.main (File.kt:-1) at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (:-2) When build: When run test:
9. Refactor Code class CreditCardManager() { fun charge(creditCard: CreditCard, cardInfoManager: CardInfoManager): Result { val processor: CreditCartProcessor = CreditCardProcessor() processor.extractCardNumber(creditCard) val info = cardInfoManager.processPrechargeInfo() return this.updateChargeInfo(info) } fun updateChargeInfo(info: PrechargeInfo): Result { ... } }
10. Refactor Test @Test fun givenCreditCard_whenCharge_thenReturnSuccess() { // given val creditCard = CreditCard(balance = 12) val cardInfoManager = CardInfoManager(dependencies = easyToMockDependencies) // when val result = testee.charge(creditCard = creditCard, cardInfoManager = cardInfoManager) // then assertResultSuccess(result = result) }
11. References The Law of Demeter in the area of microservices Breaking the Law of Demeter - Google Testing blog The Beautiful Law of Demeter
12. DISCUSSIONS
13. THANK YOU ! hieuwu hieuwu hieuwu99