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iluwatar#590 Add explanation for Facade pattern
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‎facade/README.md‎

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Provide a unified interface to a set of interfaces in a subsystem.
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Facade defines a higher-level interface that makes the subsystem easier to use.
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![alt text](./etc/facade_1.png "Facade")
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## Explanation
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Real world example
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> How does a goldmine work? "Well, the miners go down there and dig gold!" you say. That is what you believe because you are using a simple interface that goldmine provides on the outside, internally it has to do a lot of stuff to make it happen. This simple interface to the complex subsystem is a facade.
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In plain words
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> Facade pattern provides a simplified interface to a complex subsystem.
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Wikipedia says
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> A facade is an object that provides a simplified interface to a larger body of code, such as a class library.
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**Programmatic Example**
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Taking our goldmine example from above. Here we have the dwarven mine worker hierarchy
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```
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public abstract class DwarvenMineWorker {
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private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenMineWorker.class);
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public void goToSleep() {
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LOGGER.info("{} goes to sleep.", name());
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}
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public void wakeUp() {
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LOGGER.info("{} wakes up.", name());
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}
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public void goHome() {
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LOGGER.info("{} goes home.", name());
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}
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public void goToMine() {
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LOGGER.info("{} goes to the mine.", name());
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}
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private void action(Action action) {
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switch (action) {
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case GO_TO_SLEEP:
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goToSleep();
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break;
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case WAKE_UP:
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wakeUp();
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break;
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case GO_HOME:
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goHome();
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break;
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case GO_TO_MINE:
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goToMine();
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break;
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case WORK:
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work();
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break;
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default:
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LOGGER.info("Undefined action");
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break;
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}
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}
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public void action(Action... actions) {
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for (Action action : actions) {
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action(action);
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}
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}
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public abstract void work();
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public abstract String name();
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static enum Action {
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GO_TO_SLEEP, WAKE_UP, GO_HOME, GO_TO_MINE, WORK
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}
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}
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public class DwarvenTunnelDigger extends DwarvenMineWorker {
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private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenTunnelDigger.class);
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@Override
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public void work() {
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LOGGER.info("{} creates another promising tunnel.", name());
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}
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@Override
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public String name() {
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return "Dwarven tunnel digger";
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}
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}
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public class DwarvenGoldDigger extends DwarvenMineWorker {
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private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenGoldDigger.class);
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@Override
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public void work() {
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LOGGER.info("{} digs for gold.", name());
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}
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@Override
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public String name() {
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return "Dwarf gold digger";
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}
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}
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public class DwarvenCartOperator extends DwarvenMineWorker {
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private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenCartOperator.class);
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@Override
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public void work() {
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LOGGER.info("{} moves gold chunks out of the mine.", name());
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}
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@Override
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public String name() {
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return "Dwarf cart operator";
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}
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}
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```
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To operate all these goldmine workers we have the facade
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```
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public class DwarvenGoldmineFacade {
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private final List<DwarvenMineWorker> workers;
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public DwarvenGoldmineFacade() {
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workers = new ArrayList<>();
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workers.add(new DwarvenGoldDigger());
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workers.add(new DwarvenCartOperator());
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workers.add(new DwarvenTunnelDigger());
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}
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public void startNewDay() {
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makeActions(workers, DwarvenMineWorker.Action.WAKE_UP, DwarvenMineWorker.Action.GO_TO_MINE);
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}
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public void digOutGold() {
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makeActions(workers, DwarvenMineWorker.Action.WORK);
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}
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public void endDay() {
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makeActions(workers, DwarvenMineWorker.Action.GO_HOME, DwarvenMineWorker.Action.GO_TO_SLEEP);
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}
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private static void makeActions(Collection<DwarvenMineWorker> workers,
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DwarvenMineWorker.Action... actions) {
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for (DwarvenMineWorker worker : workers) {
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worker.action(actions);
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}
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}
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}
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```
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Now to use the facade
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```
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DwarvenGoldmineFacade facade = new DwarvenGoldmineFacade();
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facade.startNewDay();
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// Dwarf gold digger wakes up.
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// Dwarf gold digger goes to the mine.
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// Dwarf cart operator wakes up.
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// Dwarf cart operator goes to the mine.
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// Dwarven tunnel digger wakes up.
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// Dwarven tunnel digger goes to the mine.
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facade.digOutGold();
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// Dwarf gold digger digs for gold.
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// Dwarf cart operator moves gold chunks out of the mine.
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// Dwarven tunnel digger creates another promising tunnel.
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facade.endDay();
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// Dwarf gold digger goes home.
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// Dwarf gold digger goes to sleep.
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// Dwarf cart operator goes home.
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// Dwarf cart operator goes to sleep.
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// Dwarven tunnel digger goes home.
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// Dwarven tunnel digger goes to sleep.
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```
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## Applicability
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Use the Facade pattern when

‎facade/etc/facade.png‎

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‎facade/etc/facade.ucls‎

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‎facade/etc/facade.urm.puml‎

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‎facade/etc/facade_1.png‎

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‎pom.xml‎

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<param>bridge</param>
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<param>composite</param>
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<param>decorator</param>
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<param>facade</param>
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</skipForProjects>
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</configuration>
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</plugin>

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