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Commitdea7ecf

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ignite1771ohbus
andauthored
iluwatar#1317 Special Case Pattern (iluwatar#1624)
*iluwatar#1317 Add Special Case PatternTo focus on pattern itself, I implement DB andmaintenance lock by the singleton instance.*iluwatar#1317 Add special cases unit testsAssert the logger output(ref:https://stackoverflow.com/a/52229629)*iluwatar#1317 Add README.mdAdd Special Case Pattern README*iluwatar#1317 Format: add a new line to end of fileCo-authored-by: Subhrodip Mohanta <subhrodipmohanta@gmail.com>
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‎pom.xml

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<module>factory</module>
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<module>separated-interface</module>
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<module>data-transfer-object-enum-impl</module>
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<module>special-case</module>
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</modules>
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<repositories>

‎special-case/README.md

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---
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layout:pattern
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title:Special Case
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folder:special-case
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permalink:/patterns/special-case/
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categories:Behavioral
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tags:
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-Extensibility
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---
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##Intent
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Define some special cases, and encapsulates them into subclasses that provide different special behaviors.
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##Explanation
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Real world example
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>In an e-commerce system, presentation layer expects application layer to produce certain view model.
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>We have a successful scenario, in which receipt view model contains actual data from the purchase,
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>and a couple of failure scenarios.
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In plain words
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>Special Case pattern allows returning non-null real objects that perform special behaviors.
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In[Patterns of Enterprise Application Architecture](https://martinfowler.com/books/eaa.html) says
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the difference from Null Object Pattern
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>If you’ll pardon the unresistable pun, I see Null Object as special case of Special Case.
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**Programmatic Example**
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To focus on the pattern itself, we implement DB and maintenance lock of the e-commerce system by the singleton instance.
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```java
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publicclassDb {
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privatestaticDb instance;
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privateMap<String,User> userName2User;
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privateMap<User,Account> user2Account;
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privateMap<String,Product> itemName2Product;
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publicstaticDbgetInstance() {
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if (instance==null) {
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synchronized (Db.class) {
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if (instance==null) {
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instance=newDb();
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instance.userName2User=newHashMap<>();
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instance.user2Account=newHashMap<>();
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instance.itemName2Product=newHashMap<>();
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}
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}
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}
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return instance;
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}
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publicvoidseedUser(StringuserName,Doubleamount) {
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User user=newUser(userName);
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instance.userName2User.put(userName, user);
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Account account=newAccount(amount);
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instance.user2Account.put(user, account);
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}
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publicvoidseedItem(StringitemName,Doubleprice) {
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Product item=newProduct(price);
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itemName2Product.put(itemName, item);
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}
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publicUserfindUserByUserName(StringuserName) {
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if (!userName2User.containsKey(userName)) {
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returnnull;
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}
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return userName2User.get(userName);
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}
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publicAccountfindAccountByUser(Useruser) {
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if (!user2Account.containsKey(user)) {
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returnnull;
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}
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return user2Account.get(user);
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}
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publicProductfindProductByItemName(StringitemName) {
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if (!itemName2Product.containsKey(itemName)) {
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returnnull;
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}
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return itemName2Product.get(itemName);
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}
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publicclassUser {
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privateString userName;
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publicUser(StringuserName) {
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this.userName= userName;
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}
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publicStringgetUserName() {
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return userName;
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}
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publicReceiptDtopurchase(Productitem) {
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returnnewReceiptDto(item.getPrice());
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}
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}
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publicclassAccount {
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privateDouble amount;
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publicAccount(Doubleamount) {
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this.amount= amount;
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}
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publicMoneyTransactionwithdraw(Doubleprice) {
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if (price> amount) {
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returnnull;
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}
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returnnewMoneyTransaction(amount, price);
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}
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publicDoublegetAmount() {
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return amount;
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}
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}
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publicclassProduct {
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privateDouble price;
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publicProduct(Doubleprice) {
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this.price= price;
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}
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publicDoublegetPrice() {
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return price;
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}
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}
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}
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publicclassMaintenanceLock {
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privatestaticfinalLoggerLOGGER=LoggerFactory.getLogger(MaintenanceLock.class);
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privatestaticMaintenanceLock instance;
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privateboolean lock=true;
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publicstaticMaintenanceLockgetInstance() {
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if (instance==null) {
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synchronized (MaintenanceLock.class) {
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if (instance==null) {
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instance=newMaintenanceLock();
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}
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}
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}
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return instance;
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}
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publicbooleanisLock() {
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return lock;
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}
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publicvoidsetLock(booleanlock) {
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this.lock= lock;
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LOGGER.info("Maintenance lock is set to:"+ lock);
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}
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}
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```
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Let's first introduce presentation layer, the receipt view model interface and its implementation of successful scenario.
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```java
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publicinterfaceReceiptViewModel {
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voidshow();
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}
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publicclassReceiptDtoimplementsReceiptViewModel {
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privatestaticfinalLoggerLOGGER=LoggerFactory.getLogger(ReceiptDto.class);
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privateDouble price;
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publicReceiptDto(Doubleprice) {
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this.price= price;
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}
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publicDoublegetPrice() {
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return price;
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}
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@Override
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publicvoidshow() {
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LOGGER.info("Receipt:"+ price+" paid");
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}
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}
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```
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And here are the implementations of failure scenarios, which are the special cases.
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```java
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publicclassDownForMaintenanceimplementsReceiptViewModel {
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privatestaticfinalLoggerLOGGER=LoggerFactory.getLogger(DownForMaintenance.class);
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@Override
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publicvoidshow() {
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LOGGER.info("Down for maintenance");
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}
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}
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publicclassInvalidUserimplementsReceiptViewModel {
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privatestaticfinalLoggerLOGGER=LoggerFactory.getLogger(InvalidUser.class);
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privatefinalString userName;
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publicInvalidUser(StringuserName) {
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this.userName= userName;
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}
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@Override
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publicvoidshow() {
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LOGGER.info("Invalid user:"+ userName);
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}
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}
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publicclassOutOfStockimplementsReceiptViewModel {
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privatestaticfinalLoggerLOGGER=LoggerFactory.getLogger(OutOfStock.class);
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privateString userName;
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privateString itemName;
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publicOutOfStock(StringuserName,StringitemName) {
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this.userName= userName;
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this.itemName= itemName;
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}
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@Override
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publicvoidshow() {
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LOGGER.info("Out of stock:"+ itemName+" for user ="+ userName+" to buy");
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}
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}
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publicclassInsufficientFundsimplementsReceiptViewModel {
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privatestaticfinalLoggerLOGGER=LoggerFactory.getLogger(InsufficientFunds.class);
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privateString userName;
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privateDouble amount;
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privateString itemName;
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publicInsufficientFunds(StringuserName,Doubleamount,StringitemName) {
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this.userName= userName;
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this.amount= amount;
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this.itemName= itemName;
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}
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@Override
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publicvoidshow() {
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LOGGER.info("Insufficient funds:"+ amount+" of user:"+ userName
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+" for buying item:"+ itemName);
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}
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}
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```
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Second, here's the application layer, the application services implementation and the domain services implementation.
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```java
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publicclassApplicationServicesImplimplementsApplicationServices {
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privateDomainServicesImpl domain=newDomainServicesImpl();
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@Override
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publicReceiptViewModelloggedInUserPurchase(StringuserName,StringitemName) {
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if (isDownForMaintenance()) {
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returnnewDownForMaintenance();
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}
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returnthis.domain.purchase(userName, itemName);
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}
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privatebooleanisDownForMaintenance() {
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returnMaintenanceLock.getInstance().isLock();
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}
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}
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publicclassDomainServicesImplimplementsDomainServices {
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publicReceiptViewModelpurchase(StringuserName,StringitemName) {
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Db.User user=Db.getInstance().findUserByUserName(userName);
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if (user==null) {
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returnnewInvalidUser(userName);
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}
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Db.Account account=Db.getInstance().findAccountByUser(user);
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return purchase(user, account, itemName);
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}
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privateReceiptViewModelpurchase(Db.Useruser,Db.Accountaccount,StringitemName) {
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Db.Product item=Db.getInstance().findProductByItemName(itemName);
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if (item==null) {
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returnnewOutOfStock(user.getUserName(), itemName);
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}
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ReceiptDto receipt= user.purchase(item);
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MoneyTransaction transaction= account.withdraw(receipt.getPrice());
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if (transaction==null) {
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returnnewInsufficientFunds(user.getUserName(), account.getAmount(), itemName);
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}
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return receipt;
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}
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}
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```
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Finally, the client send requests the application services to get the presentation view.
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```java
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// DB seeding
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LOGGER.info("Db seeding:"+"1 user: {\"ignite1771\", amount = 1000.0},"
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+"2 products: {\"computer\": price = 800.0,\"car\": price = 20000.0}");
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Db.getInstance().seedUser("ignite1771",1000.0);
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Db.getInstance().seedItem("computer",800.0);
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Db.getInstance().seedItem("car",20000.0);
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var applicationServices=newApplicationServicesImpl();
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ReceiptViewModel receipt;
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LOGGER.info("[REQUEST] User:"+"abc123"+" buy product:"+"tv");
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receipt= applicationServices.loggedInUserPurchase("abc123","tv");
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receipt.show();
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MaintenanceLock.getInstance().setLock(false);
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LOGGER.info("[REQUEST] User:"+"abc123"+" buy product:"+"tv");
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receipt= applicationServices.loggedInUserPurchase("abc123","tv");
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receipt.show();
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LOGGER.info("[REQUEST] User:"+"ignite1771"+" buy product:"+"tv");
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receipt= applicationServices.loggedInUserPurchase("ignite1771","tv");
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receipt.show();
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LOGGER.info("[REQUEST] User:"+"ignite1771"+" buy product:"+"car");
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receipt= applicationServices.loggedInUserPurchase("ignite1771","car");
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receipt.show();
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LOGGER.info("[REQUEST] User:"+"ignite1771"+" buy product:"+"computer");
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receipt= applicationServices.loggedInUserPurchase("ignite1771","computer");
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receipt.show();
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```
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Program output of every request:
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```
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Down for maintenance
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Invalid user: abc123
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Out of stock: tv for user = ignite1771 to buy
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Insufficient funds: 1000.0 of user: ignite1771 for buying item: car
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Receipt: 800.0 paid
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```
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##Class diagram
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![alt text](./etc/special_case_urm.png"Special Case")
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##Applicability
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Use the Special Case pattern when
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* You have multiple places in the system that have the same behavior after a conditional check
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for a particular class instance, or the same behavior after a null check.
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* Return a real object that performs the real behavior, instead of a null object that performs nothing.
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##Tutorial
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*[Special Case Tutorial](https://www.codinghelmet.com/articles/reduce-cyclomatic-complexity-special-case)
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##Credits
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*[How to Reduce Cyclomatic Complexity Part 2: Special Case Pattern](https://www.codinghelmet.com/articles/reduce-cyclomatic-complexity-special-case)
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*[Patterns of Enterprise Application Architecture](https://martinfowler.com/books/eaa.html)
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*[Special Case](https://www.martinfowler.com/eaaCatalog/specialCase.html)

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