Implement cache entry expiration for CachedWeatherForecaster.

Co-Authored-By: os222

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

@@ -1,5 +1,6 @@
 package ic.doc;
 
+import java.time.Instant;
 import java.util.ArrayDeque;
 import java.util.HashMap;
 import java.util.Queue;
@@ -8,33 +9,43 @@ import java.util.Queue;
 public class CachedWeatherForecaster implements WeatherForecaster {
   private final WeatherForecaster forecaster;
   private final Integer maxCacheSize;
-  private final HashMap<CacheKey, WeatherForecast> cache = new HashMap<>();
+  private final Integer maxCacheAgeSecs;
+  private final HashMap<CacheKey, CacheEntry> cache = new HashMap<>();
   private final Queue<CacheKey> evictionQueue = new ArrayDeque<>();
 
-  CachedWeatherForecaster(WeatherForecaster forecaster, Integer maxCacheSize) {
+  CachedWeatherForecaster(WeatherForecaster forecaster, Integer maxCacheSize,
+                          Integer maxCacheAgeSecs) {
     this.forecaster = forecaster;
     this.maxCacheSize = maxCacheSize;
+    this.maxCacheAgeSecs = maxCacheAgeSecs;
   }
 
   @Override
   public WeatherForecast forecastFor(WeatherRegion region, Weekday day) {
+    Instant now = Instant.now();
     CacheKey key = new CacheKey(region, day);
-    WeatherForecast forecast = cache.get(key);
-    if (forecast == null) {
-      forecast = forecaster.forecastFor(region, day);
-      putCache(key, forecast);
+    CacheEntry entry = cache.get(key);
+    if (entry == null || entry.isExpired(now, maxCacheAgeSecs)) {
+      WeatherForecast forecast = forecaster.forecastFor(region, day);
+      putCache(key, new CacheEntry(now, forecast));
       return forecast;
     }
-    return forecast;
+    return entry.forecast;
   }
 
-  private void putCache(CacheKey key, WeatherForecast forecast) {
+  private void putCache(CacheKey key, CacheEntry entry) {
     if (maxCacheSize != null && cache.size() >= maxCacheSize) {
       cache.remove(evictionQueue.poll());
     }
-    cache.put(key, forecast);
+    cache.put(key, entry);
     evictionQueue.add(key);
   }
 
   private record CacheKey(WeatherRegion region, Weekday day) {}
+
+  private record CacheEntry(Instant timestamp, WeatherForecast forecast) {
+    boolean isExpired(Instant now, Integer maxCacheAgeSecs) {
+      return maxCacheAgeSecs != null && timestamp.plusSeconds(maxCacheAgeSecs).isBefore(now);
+    }
+  }
 }

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

@@ -3,6 +3,7 @@ package ic.doc;
 public class CachedWeatherForecasterBuilder {
   private WeatherForecaster forecaster;
   private Integer maxCacheSize = null;
+  private Integer maxCacheAgeSecs = null;
 
   private CachedWeatherForecasterBuilder() {}
 
@@ -20,7 +21,12 @@ public class CachedWeatherForecasterBuilder {
     return this;
   }
 
+  public CachedWeatherForecasterBuilder withMaxCacheAgeSecs(int maxCacheAgeSecs) {
+    this.maxCacheAgeSecs = maxCacheAgeSecs;
+    return this;
+  }
+
   public CachedWeatherForecaster build() {
-    return new CachedWeatherForecaster(forecaster, maxCacheSize);
+    return new CachedWeatherForecaster(forecaster, maxCacheSize, maxCacheAgeSecs);
   }
 }

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

@@ -85,4 +85,31 @@ public class CachedWeatherForecasterTest {
     actForecast = cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
     assertEquals(expForecast, actForecast);
   }
+
+  @Test
+  public void forecastForCacheExpires() {
+    WeatherForecaster cachedForecaster = cachedWeatherForecaster.withMaxCacheAgeSecs(1).build();
+    WeatherForecast forecast1 = new WeatherForecast("Sunny", 20);
+    WeatherForecast forecast2 = new WeatherForecast("Rainy", 10);
+    context.checking(new Expectations() {{
+      oneOf(forecaster).forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+        will(returnValue(forecast1));
+      oneOf(forecaster).forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+        will(returnValue(forecast2));
+    }});
+    // Cache put and immediate hit
+    cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+    WeatherForecast actForecast =
+      cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+    assertEquals(forecast1, actForecast);
+    // Cache expires, miss, then hit
+    try {
+      Thread.sleep(1100);
+    } catch (InterruptedException e) {
+      fail("Interrupted while sleeping: " + e);
+    }
+    cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+    actForecast = cachedForecaster.forecastFor(WeatherRegion.LONDON, Weekday.MONDAY);
+    assertEquals(forecast2, actForecast);
+  }
 }