Implement cache size limitation and eviction for CachedWeatherForecaster.

Co-Authored-By: os222

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

@@ -1,14 +1,19 @@
 package ic.doc;
 
+import java.util.ArrayDeque;
 import java.util.HashMap;
+import java.util.Queue;
 
 // This class is a decorator for WeatherForecaster that caches all results.
 public class CachedWeatherForecaster implements WeatherForecaster {
   private final WeatherForecaster forecaster;
+  private final Integer maxCacheSize;
   private final HashMap<CacheKey, WeatherForecast> cache = new HashMap<>();
+  private final Queue<CacheKey> evictionQueue = new ArrayDeque<>();
 
-  CachedWeatherForecaster(WeatherForecaster forecaster) {
+  CachedWeatherForecaster(WeatherForecaster forecaster, Integer maxCacheSize) {
     this.forecaster = forecaster;
+    this.maxCacheSize = maxCacheSize;
   }
 
   @Override
@@ -17,11 +22,19 @@ public class CachedWeatherForecaster implements WeatherForecaster {
     WeatherForecast forecast = cache.get(key);
     if (forecast == null) {
       forecast = forecaster.forecastFor(region, day);
-      cache.put(key, forecast);
+      putCache(key, forecast);
       return forecast;
     }
     return forecast;
   }
 
+  private void putCache(CacheKey key, WeatherForecast forecast) {
+    if (maxCacheSize != null && cache.size() >= maxCacheSize) {
+      cache.remove(evictionQueue.poll());
+    }
+    cache.put(key, forecast);
+    evictionQueue.add(key);
+  }
+
   private record CacheKey(WeatherRegion region, Weekday day) {}
 }

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

@@ -2,6 +2,7 @@ package ic.doc;
 
 public class CachedWeatherForecasterBuilder {
   private WeatherForecaster forecaster;
+  private Integer maxCacheSize = null;
 
   private CachedWeatherForecasterBuilder() {}
 
@@ -14,7 +15,12 @@ public class CachedWeatherForecasterBuilder {
     return this;
   }
 
+  public CachedWeatherForecasterBuilder withMaxCacheSize(int maxCacheSize) {
+    this.maxCacheSize = maxCacheSize;
+    return this;
+  }
+
   public CachedWeatherForecaster build() {
-    return new CachedWeatherForecaster(forecaster);
+    return new CachedWeatherForecaster(forecaster, maxCacheSize);
   }
 }

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

@@ -55,4 +55,34 @@ public class CachedWeatherForecasterTest {
       cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
     assertEquals(expForecast, actForecast);
   }
+
+  @Test
+  public void forecastForCacheEvictedCall() {
+    WeatherForecaster cachedForecaster = cachedWeatherForecaster.withMaxCacheSize(2).build();
+    WeatherForecast expForecast = new WeatherForecast("Sunny Spells", 30);
+    WeatherForecast badForecast = new WeatherForecast("Rainy", 10);
+    context.checking(new Expectations() {{
+      // Two calls since cache gets evicted
+      exactly(2).of(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));
+    }});
+    // Cache put
+    cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+    // Cache put
+    cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.TUESDAY);
+    // Size exceeded, (London, Monday) entry evicted
+    cachedForecaster.forecastFor(WeatherRegion.WALES, Weekday.MONDAY);
+    // Size exceeded, (London, Tuesday) entry evicted
+    WeatherForecast actForecast =
+      cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+    assertEquals(expForecast, actForecast);
+    // Caching still works (3rd call not made)
+    actForecast = cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+    assertEquals(expForecast, actForecast);
+  }
 }