Implemented simple caching for JsonPlantDatabase

Also added Javadoc for the PlantDatabase interface

src/main/java/ch/zhaw/gartenverwaltung/io/HardinessZoneNotSetException.java

@@ -0,0 +1,7 @@
+package ch.zhaw.gartenverwaltung.io;
+
+public class HardinessZoneNotSetException extends Exception {
+    public HardinessZoneNotSetException() {
+        super("HardinessZone must be set to retrieve plants!");
+    }
+}

src/main/java/ch/zhaw/gartenverwaltung/io/JsonPlantDatabase.java

@@ -11,12 +11,25 @@ import java.net.URL;
 import java.time.MonthDay;
 import java.time.format.DateTimeFormatter;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 import java.util.Optional;
 
+/**
+ * Implements the {@link PlantDatabase} interface for loading {@link Plant} objects
+ * from a JSON file.
+ * The reads are cached to minimize file-io operations.
+ */
 public class JsonPlantDatabase implements PlantDatabase {
     private final URL dataSource = getClass().getResource("plantdb.json");
 
+    private HardinessZone currentZone;
+    private Map<Long, Plant> plantMap = Collections.emptyMap();
+
+    /**
+     * Creating constant objects required to deserialize the {@link MonthDay} classes
+     */
     private final static JavaTimeModule timeModule = new JavaTimeModule();
     static {
         DateTimeFormatter dateFormat = DateTimeFormatter.ofPattern("MM-dd");
@@ -24,27 +37,62 @@ public class JsonPlantDatabase implements PlantDatabase {
         timeModule.addDeserializer(MonthDay.class, dateDeserializer);
     }
 
+    /**
+     * If no data is currently loaded, or the specified zone differs
+     * from the {@link #currentZone}, data is loaded from {@link #dataSource}.
+     * In any case, the values of {@link #plantMap} are returned.
+     *
+     * @see PlantDatabase#getPlantList(HardinessZone)
+     */
     @Override
-    public List<Plant> getPlantList(HardinessZone zone) throws IOException {
-        List<Plant> result = Collections.emptyList();
+    public List<Plant> getPlantList(HardinessZone zone) throws IOException, HardinessZoneNotSetException {
+        if (zone == null) {
+            throw new HardinessZoneNotSetException();
+        }
+        if (plantMap.isEmpty() || zone != currentZone) {
+            loadPlantList(zone);
+        }
+        return plantMap.values().stream().toList();
+    }
 
+    /**
+     * @see PlantDatabase#getPlantById(long)
+     */
+    @Override
+    public Optional<Plant> getPlantById(long id) throws HardinessZoneNotSetException, IOException {
+        if (currentZone == null) {
+            throw new HardinessZoneNotSetException();
+        }
+        if (plantMap.isEmpty()) {
+            loadPlantList(currentZone);
+        }
+        return Optional.ofNullable(plantMap.get(id));
+    }
+
+    /**
+     * Loads the database from {@link #dataSource} and updates the cached data.
+     *
+     * @param zone The {@link HardinessZone} for which data is to be loaded
+     * @throws IOException If the database cannot be accessed
+     */
+    private void loadPlantList(HardinessZone zone) throws IOException {
         if (dataSource != null) {
+            currentZone = zone;
             ObjectMapper mapper = new ObjectMapper();
             mapper.registerModule(timeModule);
 
+            List<Plant> result;
             result = mapper.readerForListOf(Plant.class).readValue(dataSource);
-        }
+
             for (Plant plant : result) {
-            plant.inZone(zone);
+                plant.inZone(currentZone);
             }
 
-        return result;
+            // Turn list into a HashMap with structure id => Plant
+            plantMap = result.stream()
+                    .collect(HashMap::new,
+                            (res, plant) -> res.put(plant.id(), plant),
+                            (existing, replacement) -> { });
         }
-
-    @Override
-    public Optional<Plant> getPlantById(long id, HardinessZone zone) throws IOException {
-        return getPlantList(zone).stream()
-                .filter(plant -> plant.id() != id)
-                .findFirst();
     }
 }

src/main/java/ch/zhaw/gartenverwaltung/io/PlantDatabase.java

@@ -7,7 +7,28 @@ import java.io.IOException;
 import java.util.List;
 import java.util.Optional;
 
+/**
+ * A database of {@link Plant}s.
+ * The interface specifies the minimal required operations.
+ */
 public interface PlantDatabase {
-    List<Plant> getPlantList(HardinessZone zone) throws IOException;
-    Optional<Plant> getPlantById(long id, HardinessZone zone) throws IOException;
+    /**
+     * Yields a list of all {@link Plant}s in the database with only data relevant to the specfied {@link HardinessZone}
+     *
+     * @param zone The zone for which data should be fetched
+     * @return A list of {@link Plant}s with data for the specified zone
+     * @throws IOException If the database cannot be accessed
+     * @throws HardinessZoneNotSetException If no {@link HardinessZone} was specified
+     */
+    List<Plant> getPlantList(HardinessZone zone) throws IOException, HardinessZoneNotSetException;
+
+    /**
+     * Attempts to retrieve the {@link Plant} with the specified id.
+     *
+     * @param id The {@link Plant#id()} to look for
+     * @return {@link Optional} of the found {@link Plant}, {@link Optional#empty()} if no entry matched the criteria
+     * @throws IOException If the database cannot be accessed
+     * @throws HardinessZoneNotSetException If no {@link HardinessZone} was specified
+     */
+    Optional<Plant> getPlantById(long id) throws IOException, HardinessZoneNotSetException;
 }