Implement a basic CachedWeatherForecaster proxy to cache all results of a WeatherForecaster.

Co-Authored-By: os222

src/main/java/ic/doc/CachedWeatherForecaster.java

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+package ic.doc;
+
+import java.util.HashMap;
+
+// This class is a decorator for WeatherForecaster that caches all results.
+public class CachedWeatherForecaster implements WeatherForecaster {
+  private final WeatherForecaster forecaster;
+  private final HashMap<CacheKey, WeatherForecast> cache = new HashMap<>();
+
+  CachedWeatherForecaster(WeatherForecaster forecaster) {
+    this.forecaster = forecaster;
+  }
+
+  @Override
+  public WeatherForecast forecastFor(WeatherRegion region, Weekday day) {
+    CacheKey key = new CacheKey(region, day);
+    WeatherForecast forecast = cache.get(key);
+    if (forecast == null) {
+      forecast = forecaster.forecastFor(region, day);
+      cache.put(key, forecast);
+      return forecast;
+    }
+    return forecast;
+  }
+
+  private record CacheKey(WeatherRegion region, Weekday day) {}
+}

src/main/java/ic/doc/CachedWeatherForecasterBuilder.java

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+package ic.doc;
+
+public class CachedWeatherForecasterBuilder {
+  private WeatherForecaster forecaster;
+
+  private CachedWeatherForecasterBuilder() {}
+
+  public static CachedWeatherForecasterBuilder cachedWeatherForecaster() {
+    return new CachedWeatherForecasterBuilder();
+  }
+
+  public CachedWeatherForecasterBuilder withForecaster(WeatherForecaster forecaster) {
+    this.forecaster = forecaster;
+    return this;
+  }
+
+  public CachedWeatherForecaster build() {
+    return new CachedWeatherForecaster(forecaster);
+  }
+}

src/test/java/ic/doc/CachedWeatherForecasterTest.java

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+package ic.doc;
+
+import org.jmock.Expectations;
+import org.jmock.integration.junit4.JUnitRuleMockery;
+import org.junit.Rule;
+import org.junit.Test;
+
+import static ic.doc.CachedWeatherForecasterBuilder.cachedWeatherForecaster;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
+
+// Unit test for CachedWeatherForecaster decorator
+public class CachedWeatherForecasterTest {
+  @Rule
+  public JUnitRuleMockery context = new JUnitRuleMockery();
+
+  private final WeatherForecaster forecaster = context.mock(WeatherForecaster.class);
+  private final CachedWeatherForecasterBuilder cachedWeatherForecaster =
+    cachedWeatherForecaster().withForecaster(forecaster);
+
+  @Test
+  public void forecastForInitialCall() {
+    WeatherForecaster cachedForecaster = cachedWeatherForecaster.build();
+    WeatherForecast expForecast = new WeatherForecast("Sunny", 20);
+    context.checking(new Expectations() {{
+      oneOf(forecaster).forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+        will(returnValue(expForecast));
+    }});
+    WeatherForecast actForecast = cachedForecaster.forecastFor(
+      WeatherRegion.LONDON, Weekday.MONDAY
+    );
+    assertEquals(expForecast, actForecast);
+  }
+
+  @Test
+  public void forecastForCachedCall() {
+    WeatherForecaster cachedForecaster = cachedWeatherForecaster.build();
+    WeatherForecast expForecast = new WeatherForecast("Sunny Spells", 30);
+    WeatherForecast badForecast = new WeatherForecast("Rainy", 10);
+    context.checking(new Expectations() {{
+      // Only one call, second must hit cache
+      oneOf(forecaster).forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+        will(returnValue(expForecast));
+      // Also make some other requests in-between
+      oneOf(forecaster).forecastFor(WeatherRegion.LONDON, Weekday.TUESDAY);
+        will(returnValue(badForecast));
+      oneOf(forecaster).forecastFor(WeatherRegion.WALES, Weekday.MONDAY);
+        will(returnValue(badForecast));
+    }});
+    cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+    cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.TUESDAY);
+    cachedForecaster.forecastFor(WeatherRegion.WALES, Weekday.MONDAY);
+    WeatherForecast actForecast =
+      cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+    assertEquals(expForecast, actForecast);
+  }
+}