diff --git a/caching/src/test/java/com/iluwatar/caching/CachingTest.java b/caching/src/test/java/com/iluwatar/caching/CachingTest.java
index 4db4085d4..22130951a 100644
--- a/caching/src/test/java/com/iluwatar/caching/CachingTest.java
+++ b/caching/src/test/java/com/iluwatar/caching/CachingTest.java
@@ -23,13 +23,15 @@
 
 package com.iluwatar.caching;
 
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 /**
  * Application test
  */
-public class CachingTest {
+class CachingTest {
   private App app;
 
   /**
@@ -47,22 +49,26 @@ public class CachingTest {
   }
 
   @Test
-  public void testReadAndWriteThroughStrategy() {
+  void testReadAndWriteThroughStrategy() {
+	assertNotNull(app);
     app.useReadAndWriteThroughStrategy();
   }
 
   @Test
-  public void testReadThroughAndWriteAroundStrategy() {
+  void testReadThroughAndWriteAroundStrategy() {
+	assertNotNull(app);
     app.useReadThroughAndWriteAroundStrategy();
   }
 
   @Test
-  public void testReadThroughAndWriteBehindStrategy() {
+  void testReadThroughAndWriteBehindStrategy() {
+	assertNotNull(app);
     app.useReadThroughAndWriteBehindStrategy();
   }
 
   @Test
-  public void testCacheAsideStrategy() {
+  void testCacheAsideStrategy() {
+	assertNotNull(app);
     app.useCacheAsideStategy();
   }
 }
diff --git a/spatial-partition/src/main/java/com/iluwatar/spatialpartition/App.java b/spatial-partition/src/main/java/com/iluwatar/spatialpartition/App.java
index 083d6a019..69a2852d7 100644
--- a/spatial-partition/src/main/java/com/iluwatar/spatialpartition/App.java
+++ b/spatial-partition/src/main/java/com/iluwatar/spatialpartition/App.java
@@ -23,7 +23,7 @@
 
 package com.iluwatar.spatialpartition;
 
-import java.util.Hashtable;
+import java.util.HashMap;
 import java.util.Random;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -44,11 +44,11 @@ import org.slf4j.LoggerFactory;
  * <b>{@link Rect}</b> class to define the boundary of the quadtree. We use an abstract class
  * <b>{@link Point}</b>
  * with x and y coordinate fields and also an id field so that it can easily be put and looked up in
- * the hashtable. This class has abstract methods to define how the object moves (move()), when to
+ * the hashmap. This class has abstract methods to define how the object moves (move()), when to
  * check for collision with any object (touches(obj)) and how to handle collision
  * (handleCollision(obj)), and will be extended by any object whose position has to be kept track of
  * in the quadtree. The <b>{@link SpatialPartitionGeneric}</b> abstract class has 2 fields - a
- * hashtable containing all objects (we use hashtable for faster lookups, insertion and deletion)
+ * hashmap containing all objects (we use hashmap for faster lookups, insertion and deletion)
  * and a quadtree, and contains an abstract method which defines how to handle interactions between
  * objects using the quadtree.</p>
  * <p>Using the quadtree data structure will reduce the time complexity of finding the objects
@@ -61,7 +61,7 @@ public class App {
   private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
   private static final String BUBBLE = "Bubble ";
 
-  static void noSpatialPartition(int numOfMovements, Hashtable<Integer, Bubble> bubbles) {
+  static void noSpatialPartition(int numOfMovements, HashMap<Integer, Bubble> bubbles) {
     //all bubbles have to be checked for collision for all bubbles
     var bubblesToCheck = bubbles.values();
 
@@ -81,9 +81,9 @@ public class App {
   }
 
   static void withSpatialPartition(
-      int height, int width, int numOfMovements, Hashtable<Integer, Bubble> bubbles) {
+      int height, int width, int numOfMovements, HashMap<Integer, Bubble> bubbles) {
     //creating quadtree
-    var rect = new Rect(width / 2, height / 2, width, height);
+    var rect = new Rect(width / 2D, height / 2D, width, height);
     var quadTree = new QuadTree(rect, 4);
 
     //will run numOfMovement times or till all bubbles have popped
@@ -110,15 +110,15 @@ public class App {
    */
 
   public static void main(String[] args) {
-    var bubbles1 = new Hashtable<Integer, Bubble>();
-    var bubbles2 = new Hashtable<Integer, Bubble>();
+    var bubbles1 = new HashMap<Integer, Bubble>();
+    var bubbles2 = new HashMap<Integer, Bubble>();
     var rand = new Random();
     for (int i = 0; i < 10000; i++) {
       var b = new Bubble(rand.nextInt(300), rand.nextInt(300), i, rand.nextInt(2) + 1);
       bubbles1.put(i, b);
       bubbles2.put(i, b);
-      LOGGER.info(BUBBLE + i + " with radius " + b.radius
-          + " added at (" + b.coordinateX + "," + b.coordinateY + ")");
+      LOGGER.info(BUBBLE, i, " with radius ", b.radius,
+          " added at (", b.coordinateX, ",", b.coordinateY + ")");
     }
 
     var start1 = System.currentTimeMillis();
@@ -127,8 +127,8 @@ public class App {
     var start2 = System.currentTimeMillis();
     App.withSpatialPartition(300, 300, 20, bubbles2);
     var end2 = System.currentTimeMillis();
-    LOGGER.info("Without spatial partition takes " + (end1 - start1) + "ms");
-    LOGGER.info("With spatial partition takes " + (end2 - start2) + "ms");
+    LOGGER.info("Without spatial partition takes ", (end1 - start1), "ms");
+    LOGGER.info("With spatial partition takes ", (end2 - start2), "ms");
   }
 }
 
diff --git a/spatial-partition/src/main/java/com/iluwatar/spatialpartition/Bubble.java b/spatial-partition/src/main/java/com/iluwatar/spatialpartition/Bubble.java
index 9c39c29c8..4ff166420 100644
--- a/spatial-partition/src/main/java/com/iluwatar/spatialpartition/Bubble.java
+++ b/spatial-partition/src/main/java/com/iluwatar/spatialpartition/Bubble.java
@@ -24,7 +24,7 @@
 package com.iluwatar.spatialpartition;
 
 import java.util.Collection;
-import java.util.Hashtable;
+import java.util.HashMap;
 import java.util.Random;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -58,13 +58,13 @@ public class Bubble extends Point<Bubble> {
         <= (this.radius + b.radius) * (this.radius + b.radius);
   }
 
-  void pop(Hashtable<Integer, Bubble> allBubbles) {
-    LOGGER.info("Bubble " + this.id
-        + " popped at (" + this.coordinateX + "," + this.coordinateY + ")!");
+  void pop(HashMap<Integer, Bubble> allBubbles) {
+    LOGGER.info("Bubble ", this.id,
+        " popped at (", this.coordinateX, ",", this.coordinateY, ")!");
     allBubbles.remove(this.id);
   }
 
-  void handleCollision(Collection<? extends Point> toCheck, Hashtable<Integer, Bubble> allBubbles) {
+  void handleCollision(Collection<? extends Point> toCheck, HashMap<Integer, Bubble> allBubbles) {
     var toBePopped = false; //if any other bubble collides with it, made true
     for (var point : toCheck) {
       var otherId = point.id;
diff --git a/spatial-partition/src/main/java/com/iluwatar/spatialpartition/Point.java b/spatial-partition/src/main/java/com/iluwatar/spatialpartition/Point.java
index 410beae34..a2547b7a1 100644
--- a/spatial-partition/src/main/java/com/iluwatar/spatialpartition/Point.java
+++ b/spatial-partition/src/main/java/com/iluwatar/spatialpartition/Point.java
@@ -24,7 +24,7 @@
 package com.iluwatar.spatialpartition;
 
 import java.util.Collection;
-import java.util.Hashtable;
+import java.util.HashMap;
 
 /**
  * The abstract Point class which will be extended by any object in the field whose location has to
@@ -64,5 +64,5 @@ public abstract class Point<T> {
    * @param toCheck contains the objects which need to be checked
    * @param all     contains hashtable of all points on field at this time
    */
-  abstract void handleCollision(Collection<? extends Point> toCheck, Hashtable<Integer, T> all);
+  abstract void handleCollision(Collection<? extends Point> toCheck, HashMap<Integer, T> all);
 }
diff --git a/spatial-partition/src/main/java/com/iluwatar/spatialpartition/SpatialPartitionBubbles.java b/spatial-partition/src/main/java/com/iluwatar/spatialpartition/SpatialPartitionBubbles.java
index c543da12c..1a9a0d91e 100644
--- a/spatial-partition/src/main/java/com/iluwatar/spatialpartition/SpatialPartitionBubbles.java
+++ b/spatial-partition/src/main/java/com/iluwatar/spatialpartition/SpatialPartitionBubbles.java
@@ -24,7 +24,7 @@
 package com.iluwatar.spatialpartition;
 
 import java.util.ArrayList;
-import java.util.Hashtable;
+import java.util.HashMap;
 
 /**
  * This class extends the generic SpatialPartition abstract class and is used in our example to keep
@@ -33,10 +33,10 @@ import java.util.Hashtable;
 
 public class SpatialPartitionBubbles extends SpatialPartitionGeneric<Bubble> {
 
-  private final Hashtable<Integer, Bubble> bubbles;
+  private final HashMap<Integer, Bubble> bubbles;
   private final QuadTree quadTree;
 
-  SpatialPartitionBubbles(Hashtable<Integer, Bubble> bubbles, QuadTree quadTree) {
+  SpatialPartitionBubbles(HashMap<Integer, Bubble> bubbles, QuadTree quadTree) {
     this.bubbles = bubbles;
     this.quadTree = quadTree;
   }
@@ -44,7 +44,7 @@ public class SpatialPartitionBubbles extends SpatialPartitionGeneric<Bubble> {
   void handleCollisionsUsingQt(Bubble b) {
     // finding points within area of a square drawn with centre same as
     // centre of bubble and length = radius of bubble
-    var rect = new Rect(b.coordinateX, b.coordinateY, 2 * b.radius, 2 * b.radius);
+    var rect = new Rect(b.coordinateX, b.coordinateY, 2D * b.radius, 2D * b.radius);
     var quadTreeQueryResult = new ArrayList<Point>();
     this.quadTree.query(rect, quadTreeQueryResult);
     //handling these collisions
diff --git a/spatial-partition/src/test/java/com/iluwatar/spatialpartition/BubbleTest.java b/spatial-partition/src/test/java/com/iluwatar/spatialpartition/BubbleTest.java
index 957a36ce3..f02b5e961 100644
--- a/spatial-partition/src/test/java/com/iluwatar/spatialpartition/BubbleTest.java
+++ b/spatial-partition/src/test/java/com/iluwatar/spatialpartition/BubbleTest.java
@@ -29,7 +29,7 @@ import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.ArrayList;
-import java.util.Hashtable;
+import java.util.HashMap;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -63,11 +63,11 @@ class BubbleTest {
   void popTest() {
     var b1 = new Bubble(10, 10, 1, 2);
     var b2 = new Bubble(0, 0, 2, 2);
-    var bubbles = new Hashtable<Integer, Bubble>();
+    var bubbles = new HashMap<Integer, Bubble>();
     bubbles.put(1, b1);
     bubbles.put(2, b2);
     b1.pop(bubbles);
-    //after popping, bubble no longer in hashtable containing all bubbles
+    //after popping, bubble no longer in hashMap containing all bubbles
     assertNull(bubbles.get(1));
     assertNotNull(bubbles.get(2));
   }
@@ -77,7 +77,7 @@ class BubbleTest {
     var b1 = new Bubble(0, 0, 1, 2);
     var b2 = new Bubble(1, 1, 2, 1);
     var b3 = new Bubble(10, 10, 3, 1);
-    var bubbles = new Hashtable<Integer, Bubble>();
+    var bubbles = new HashMap<Integer, Bubble>();
     bubbles.put(1, b1);
     bubbles.put(2, b2);
     bubbles.put(3, b3);
diff --git a/spatial-partition/src/test/java/com/iluwatar/spatialpartition/SpatialPartitionBubblesTest.java b/spatial-partition/src/test/java/com/iluwatar/spatialpartition/SpatialPartitionBubblesTest.java
index 3470b27cc..f36995938 100644
--- a/spatial-partition/src/test/java/com/iluwatar/spatialpartition/SpatialPartitionBubblesTest.java
+++ b/spatial-partition/src/test/java/com/iluwatar/spatialpartition/SpatialPartitionBubblesTest.java
@@ -26,7 +26,7 @@ package com.iluwatar.spatialpartition;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNull;
 
-import java.util.Hashtable;
+import java.util.HashMap;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -41,7 +41,7 @@ class SpatialPartitionBubblesTest {
     var b2 = new Bubble(5, 5, 2, 1);
     var b3 = new Bubble(9, 9, 3, 1);
     var b4 = new Bubble(8, 8, 4, 2);
-    var bubbles = new Hashtable<Integer, Bubble>();
+    var bubbles = new HashMap<Integer, Bubble>();
     bubbles.put(1, b1);
     bubbles.put(2, b2);
     bubbles.put(3, b3);