Fixes based on code review feedback

dirty-flag/src/main/java/com/iluwatar/dirtyflag/App.java

@@ -57,7 +57,7 @@ public class App {
     executorService.scheduleAtFixedRate(new Runnable() {
       @Override
       public void run() {
-        World world = World.getInstance();
+        World world = new World();
         List<String> countries = world.fetch();
         System.out.println("Our world currently has the following countries:-");
         for (String country : countries) {

dirty-flag/src/main/java/com/iluwatar/dirtyflag/DataFetcher.java

@@ -15,23 +15,11 @@ import java.util.List;
  */
 public class DataFetcher {
 
-  private static DataFetcher df;
   private final String filename = "world.txt";
-  private long lastFetched = -1;
+  private long lastFetched;
 
-  private DataFetcher() {
-  }
-
-  /**
-   * Init.
-   * 
-   * @return DataFetcher instance
-   */
-  public static DataFetcher getInstance() {
-    if (df == null) {
-      df = new DataFetcher();
-    }
-    return df;
+  public DataFetcher() {
+    this.lastFetched = -1;
   }
 
   private boolean isDirty(long fileLastModified) {
@@ -66,6 +54,6 @@ public class DataFetcher {
       return data;
     }
 
-    return null;
+    return new ArrayList<String>();
   }
 }

dirty-flag/src/main/java/com/iluwatar/dirtyflag/World.java

@@ -12,22 +12,12 @@ import java.util.List;
  */
 public class World {
 
-  private static World world;
-  private static List<String> countries = new ArrayList<String>();
+  private List<String> countries;
+  private DataFetcher df;
 
-  private World() {
-  }
-
-  /**
-   * Init.
-   * 
-   * @return World instance
-   */
-  public static World getInstance() {
-    if (world == null) {
-      world = new World();
-    }
-    return world;
+  public World() {
+    this.countries = new ArrayList<String>();
+    this.df = new DataFetcher();
   }
 
   /**
@@ -37,10 +27,9 @@ public class World {
    * @return List of strings
    */
   public List<String> fetch() {
-    DataFetcher df = DataFetcher.getInstance();
     List<String> data = df.fetch();
 
-    countries = data == null ? countries : data;
+    countries = data.isEmpty() ? countries : data;
 
     return countries;
   }

dirty-flag/src/test/java/org/dirty/flag/DirtyFlagTest.java

@@ -24,10 +24,8 @@ package org.dirty.flag;
 
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
-import java.lang.reflect.Field;
 import java.util.List;
 
-import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import com.iluwatar.dirtyflag.DataFetcher;
@@ -39,23 +37,16 @@ import com.iluwatar.dirtyflag.DataFetcher;
  */
 public class DirtyFlagTest {
 
-  @BeforeEach
-  public void reset() throws SecurityException, NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
-    Field instance = DataFetcher.class.getDeclaredField("df");
-    instance.setAccessible(true);
-    instance.set(null, null);
-  }
-
   @Test
   public void testIsDirty() {
-    DataFetcher df = DataFetcher.getInstance();
+    DataFetcher df = new DataFetcher();
     List<String> countries = df.fetch();
     assertTrue(!countries.isEmpty());
   }
 
   @Test
   public void testIsNotDirty() {
-    DataFetcher df = DataFetcher.getInstance();
+    DataFetcher df = new DataFetcher();
     df.fetch();
     List<String> countries = df.fetch();
     assertTrue(countries == null);