Adding initialization-on-demand idiom and noninstantiable class instead of interface constant idiom

caching/src/main/java/com/iluwatar/caching/LruCache.java

@@ -167,7 +167,7 @@ public void clear() {
    * Returns cache data in list form.
    */
   public List<UserAccount> getCacheDataInListForm() {
-    ArrayList<UserAccount> listOfCacheData = new ArrayList<>();
+    List<UserAccount> listOfCacheData = new ArrayList<>();
     Node temp = head;
     while (temp != null) {
       listOfCacheData.add(temp.userAccount);

dao/src/main/java/com/iluwatar/dao/CustomerSchemaSql.java

@@ -22,10 +22,13 @@
  */
 package com.iluwatar.dao;
 
-public interface CustomerSchemaSql {
+public final class CustomerSchemaSql {
 
-  String CREATE_SCHEMA_SQL = "CREATE TABLE CUSTOMERS (ID NUMBER, FNAME VARCHAR(100), " 
+  private CustomerSchemaSql() {}
+
+  public static final String CREATE_SCHEMA_SQL = "CREATE TABLE CUSTOMERS (ID NUMBER, FNAME VARCHAR(100), " 
       + "LNAME VARCHAR(100))";
 
-  String DELETE_SCHEMA_SQL = "DROP TABLE CUSTOMERS";
+  public static final String DELETE_SCHEMA_SQL = "DROP TABLE CUSTOMERS";
+
 }

factory-kit/src/main/java/com/iluwatar/factorykit/WeaponFactory.java

@@ -23,6 +23,7 @@
 package com.iluwatar.factorykit;
 
 import java.util.HashMap;
+import java.util.Map;
 import java.util.function.Consumer;
 import java.util.function.Supplier;
 
@@ -48,7 +49,7 @@ public interface WeaponFactory {
    * @return factory with specified {@link Builder}s
    */
   static WeaponFactory factory(Consumer<Builder> consumer) {
-    HashMap<WeaponType, Supplier<Weapon>> map = new HashMap<>();
+    Map<WeaponType, Supplier<Weapon>> map = new HashMap<>();
     consumer.accept(map::put);
     return name -> map.get(name).get();
   }

hexagonal/src/main/java/com/iluwatar/hexagonal/banking/MongoBank.java

@@ -29,6 +29,7 @@
 import org.bson.Document;
 
 import java.util.ArrayList;
+import java.util.List;
 
 /**
  * Mongo based banking adapter
@@ -110,7 +111,7 @@ public void setFunds(String bankAccount, int amount) {
   @Override
   public int getFunds(String bankAccount) {
     Document search = new Document("_id", bankAccount);
-    ArrayList<Document> results = accountsCollection.find(search).limit(1).into(new ArrayList<Document>());
+    List<Document> results = accountsCollection.find(search).limit(1).into(new ArrayList<Document>());
     if (results.size() > 0) {
       return results.get(0).getInteger("funds");
     } else {

hexagonal/src/main/java/com/iluwatar/hexagonal/database/MongoTicketRepository.java

@@ -35,8 +35,10 @@
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.Set;
 
 /**
  * Mongo lottery ticket database
@@ -142,7 +144,7 @@ public MongoCollection<Document> getCountersCollection() {
   @Override
   public Optional<LotteryTicket> findById(LotteryTicketId id) {
     Document find = new Document("ticketId", id.getId());
-    ArrayList<Document> results = ticketsCollection.find(find).limit(1).into(new ArrayList<Document>());
+    List<Document> results = ticketsCollection.find(find).limit(1).into(new ArrayList<Document>());
     if (results.size() > 0) {
       LotteryTicket lotteryTicket = docToTicket(results.get(0));
       return Optional.of(lotteryTicket);
@@ -166,7 +168,7 @@ public Optional<LotteryTicketId> save(LotteryTicket ticket) {
   @Override
   public Map<LotteryTicketId, LotteryTicket> findAll() {
     Map<LotteryTicketId, LotteryTicket> map = new HashMap<>();
-    ArrayList<Document> docs = ticketsCollection.find(new Document()).into(new ArrayList<Document>());
+    List<Document> docs = ticketsCollection.find(new Document()).into(new ArrayList<Document>());
     for (Document doc: docs) {
       LotteryTicket lotteryTicket = docToTicket(doc);
       map.put(lotteryTicket.getId(), lotteryTicket);
@@ -183,7 +185,7 @@ private LotteryTicket docToTicket(Document doc) {
     PlayerDetails playerDetails = new PlayerDetails(doc.getString("email"), doc.getString("bank"),
         doc.getString("phone"));
     int[] numArray = Arrays.asList(doc.getString("numbers").split(",")).stream().mapToInt(Integer::parseInt).toArray();
-    HashSet<Integer> numbers = new HashSet<>();
+    Set<Integer> numbers = new HashSet<>();
     for (int num: numArray) {
       numbers.add(num);
     }

layers/src/main/java/com/iluwatar/layers/CakeBakingServiceImpl.java

@@ -163,7 +163,7 @@ public List<CakeInfo> getAllCakes() {
       CakeToppingInfo cakeToppingInfo =
           new CakeToppingInfo(cake.getTopping().getId(), cake.getTopping().getName(), cake
               .getTopping().getCalories());
-      ArrayList<CakeLayerInfo> cakeLayerInfos = new ArrayList<>();
+      List<CakeLayerInfo> cakeLayerInfos = new ArrayList<>();
       for (CakeLayer layer : cake.getLayers()) {
         cakeLayerInfos.add(new CakeLayerInfo(layer.getId(), layer.getName(), layer.getCalories()));
       }

object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java

@@ -23,15 +23,16 @@
 package com.iluwatar.object.pool;
 
 import java.util.HashSet;
+import java.util.Set;
 
 /**
  * 
  * Generic object pool
  */
 public abstract class ObjectPool<T> {
 
-  private HashSet<T> available = new HashSet<>();
-  private HashSet<T> inUse = new HashSet<>();
+  private Set<T> available = new HashSet<>();
+  private Set<T> inUse = new HashSet<>();
 
   protected abstract T create();
 

semaphore/src/main/java/com/iluwatar/semaphore/FruitBowl.java

@@ -22,14 +22,15 @@
  */
 package com.iluwatar.semaphore;
 
+import java.util.List;
 import java.util.ArrayList;
 
 /**
  * A FruitBowl contains Fruit. 
  */
 public class FruitBowl {
 
-  private ArrayList<Fruit> fruit = new ArrayList<>();
+  private List<Fruit> fruit = new ArrayList<>();
 
   /**
    * 

servant/src/main/java/com/iluwatar/servant/App.java

@@ -26,6 +26,7 @@
 import org.slf4j.LoggerFactory;
 
 import java.util.ArrayList;
+import java.util.List;
 
 
 /**
@@ -58,7 +59,7 @@ public static void scenario(Servant servant, int compliment) {
     King k = new King();
     Queen q = new Queen();
 
-    ArrayList<Royalty> guests = new ArrayList<>();
+    List<Royalty> guests = new ArrayList<>();
     guests.add(k);
     guests.add(q);
 

servant/src/test/java/com/iluwatar/servant/ServantTest.java

@@ -25,6 +25,7 @@
 import org.junit.Test;
 
 import java.util.ArrayList;
+import java.util.List;
 
 import static org.junit.Assert.*;
 import static org.mockito.Mockito.mock;
@@ -74,12 +75,12 @@ public void testCheckIfYouWillBeHanged() throws Exception {
     final Royalty badMoodRoyalty = mock(Royalty.class);
     when(badMoodRoyalty.getMood()).thenReturn(true);
 
-    final ArrayList<Royalty> goodCompany = new ArrayList<>();
+    final List<Royalty> goodCompany = new ArrayList<>();
     goodCompany.add(goodMoodRoyalty);
     goodCompany.add(goodMoodRoyalty);
     goodCompany.add(goodMoodRoyalty);
 
-    final ArrayList<Royalty> badCompany = new ArrayList<>();
+    final List<Royalty> badCompany = new ArrayList<>();
     goodCompany.add(goodMoodRoyalty);
     goodCompany.add(goodMoodRoyalty);
     goodCompany.add(badMoodRoyalty);

singleton/src/main/java/com/iluwatar/singleton/IvoryTower.java

@@ -28,14 +28,14 @@
 public final class IvoryTower {
 
   /**
-   * Static to class instance of the class.
+   * Private constructor so nobody can instantiate the class.
    */
-  private static final IvoryTower INSTANCE = new IvoryTower();
+  private IvoryTower() {}
 
   /**
-   * Private constructor so nobody can instantiate the class.
+   * Static to class instance of the class.
    */
-  private IvoryTower() {}
+  private static final IvoryTower INSTANCE = new IvoryTower();
 
   /**
    * To be called by user to obtain instance of the class.