diff --git a/ch08/ArrayExamples.java b/ch08/ArrayExamples.java
index a9a4e9a..a4cb6c3 100644
--- a/ch08/ArrayExamples.java
+++ b/ch08/ArrayExamples.java
@@ -1,5 +1,4 @@
 import java.util.Arrays;
-import java.util.Random;
 
 /**
  * Demonstrates uses of arrays.
@@ -62,8 +61,6 @@ public static void main(String[] args) {
         // reduce
         double total = sum(a);
         System.out.println("total = " + total);
-
-        makeHistogram();
     }
 
     /**
@@ -100,57 +97,4 @@ public static double sum(double[] a) {
         return total;
     }
 
-    /**
-     * Returns an array of random integers.
-     */
-    public static int[] randomArray(int size) {
-        Random random = new Random();
-        int[] a = new int[size];
-        for (int i = 0; i < a.length; i++) {
-            a[i] = random.nextInt(100);
-        }
-        return a;
-    }
-
-    /**
-     * Computes the number of array elements in [low, high).
-     */
-    public static int inRange(int[] a, int low, int high) {
-        int count = 0;
-        for (int i = 0; i < a.length; i++) {
-            if (a[i] >= low && a[i] < high) {
-                count++;
-            }
-        }
-        return count;
-    }
-
-    /**
-     * Example code related to histograms.
-     */
-    public static void makeHistogram() {
-        int numValues = 8;
-        int[] array = randomArray(numValues);
-        printArray(array);
-
-        int[] scores = randomArray(30);
-        int a = inRange(scores, 90, 100);
-        int b = inRange(scores, 80, 90);
-        int c = inRange(scores, 70, 80);
-        int d = inRange(scores, 60, 70);
-        int f = inRange(scores, 0, 60);
-
-        // making a histogram
-        int[] counts = new int[100];
-        for (int i = 0; i < scores.length; i++) {
-            int index = scores[i];
-            counts[index]++;
-        }
-
-        // histogram with enhanced for loop
-        counts = new int[100];
-        for (int score : scores) {
-            counts[score]++;
-        }
-    }
 }
diff --git a/ch08/Fruit.java b/ch08/Fruit.java
new file mode 100644
index 0000000..50428c5
--- /dev/null
+++ b/ch08/Fruit.java
@@ -0,0 +1,35 @@
+/**
+ * Fruit exercise.
+ */
+public class Fruit {
+
+    public static int banana(int[] a) {
+        int kiwi = 1;
+        int i = 0;
+        while (i < a.length) {
+            kiwi = kiwi * a[i];
+            i++;
+        }
+        return kiwi;
+    }
+
+    public static int grapefruit(int[] a, int grape) {
+        for (int i = 0; i < a.length; i++) {
+            if (a[i] == grape) {
+                return i;
+            }
+        }
+        return -1;
+    }
+
+    public static int pineapple(int[] a, int apple) {
+        int pear = 0;
+        for (int pine: a) {
+            if (pine == apple) {
+                pear++;
+            }
+        }
+        return pear;
+    }
+
+}
diff --git a/ch08/Histogram.java b/ch08/Histogram.java
new file mode 100644
index 0000000..32b029c
--- /dev/null
+++ b/ch08/Histogram.java
@@ -0,0 +1,59 @@
+import java.util.Random;
+
+/**
+ * Example code related to histograms.
+ */
+public class Histogram {
+
+    /**
+     * Returns an array of random integers.
+     */
+    public static int[] randomArray(int size) {
+        Random random = new Random();
+        int[] a = new int[size];
+        for (int i = 0; i < a.length; i++) {
+            a[i] = random.nextInt(100);
+        }
+        return a;
+    }
+
+    /**
+     * Computes the number of array elements in [low, high).
+     */
+    public static int inRange(int[] a, int low, int high) {
+        int count = 0;
+        for (int i = 0; i < a.length; i++) {
+            if (a[i] >= low && a[i] < high) {
+                count++;
+            }
+        }
+        return count;
+    }
+
+    public static void main(String[] args) {
+        int numValues = 8;
+        int[] array = randomArray(numValues);
+        ArrayExamples.printArray(array);
+
+        int[] scores = randomArray(30);
+        int a = inRange(scores, 90, 100);
+        int b = inRange(scores, 80, 90);
+        int c = inRange(scores, 70, 80);
+        int d = inRange(scores, 60, 70);
+        int f = inRange(scores, 0, 60);
+
+        // making a histogram
+        int[] counts = new int[100];
+        for (int i = 0; i < scores.length; i++) {
+            int index = scores[i];
+            counts[index]++;
+        }
+
+        // histogram with enhanced for loop
+        counts = new int[100];
+        for (int score : scores) {
+            counts[score]++;
+        }
+    }
+
+}
diff --git a/ch08/MakeDubMus.java b/ch08/MakeDubMus.java
new file mode 100644
index 0000000..9c7e193
--- /dev/null
+++ b/ch08/MakeDubMus.java
@@ -0,0 +1,34 @@
+/**
+ * Stack diagram exercise.
+ */
+public class MakeDubMus {
+
+    public static int[] make(int n) {
+        int[] a = new int[n];
+        for (int i = 0; i < n; i++) {
+            a[i] = i + 1;
+        }
+        return a;
+    }
+
+    public static void dub(int[] jub) {
+        for (int i = 0; i < jub.length; i++) {
+            jub[i] *= 2;
+        }
+    }
+
+    public static int mus(int[] zoo) {
+        int fus = 0;
+        for (int i = 0; i < zoo.length; i++) {
+            fus += zoo[i];
+        }
+        return fus;
+    }
+
+    public static void main(String[] args) {
+        int[] bob = make(5);
+        dub(bob);
+        System.out.println(mus(bob));
+    }
+
+}
diff --git a/ch09/Exercise.java b/ch09/Exercise.java
new file mode 100644
index 0000000..d8a605d
--- /dev/null
+++ b/ch09/Exercise.java
@@ -0,0 +1,22 @@
+/**
+ * Exercise on encapsulation and generalization.
+ */
+public class Exercise {
+
+    public static void main(String[] args) {
+        String s = "((3 + 7) * 2)";
+        int count = 0;
+
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+            if (c == '(') {
+                count++;
+            } else if (c == ')') {
+                count--;
+            }
+        }
+
+        System.out.println(count);
+    }
+
+}
diff --git a/ch09/Max.java b/ch09/Max.java
index 01417f0..2bf923e 100644
--- a/ch09/Max.java
+++ b/ch09/Max.java
@@ -5,6 +5,9 @@
  */
 public class Max {
 
+    /**
+     * Converts args to integers and prints the max.
+     */
     public static void main(String[] args) {
         System.out.println(Arrays.toString(args));
 
@@ -15,5 +18,7 @@ public static void main(String[] args) {
                 max = value;
             }
         }
+        System.out.println("The max is " + max);
     }
+
 }
diff --git a/ch09/Recurse.java b/ch09/Recurse.java
new file mode 100644
index 0000000..c4ba2fb
--- /dev/null
+++ b/ch09/Recurse.java
@@ -0,0 +1,34 @@
+/**
+ * Recursion exercise.
+ */
+public class Recurse {
+
+    /**
+     * Returns the first character of the given String.
+     */
+    public static char first(String s) {
+        return s.charAt(0);
+    }
+
+    /**
+     * Returns all but the first letter of the given String.
+     */
+    public static String rest(String s) {
+        return s.substring(1);
+    }
+
+    /**
+     * Returns all but the first and last letter of the String.
+     */
+    public static String middle(String s) {
+        return s.substring(1, s.length() - 1);
+    }
+
+    /**
+     * Returns the length of the given String.
+     */
+    public static int length(String s) {
+        return s.length();
+    }
+
+}
diff --git a/ch09/StringsThings.java b/ch09/StringsThings.java
index 20357c1..8b95e7e 100644
--- a/ch09/StringsThings.java
+++ b/ch09/StringsThings.java
@@ -3,19 +3,17 @@
  */
 public class StringsThings {
 
-    /**
-     * Reverses a string, returns a new String.
-     */
-    public static String reverse(String s) {
-        String r = "";
-        for (int i = s.length() - 1; i >= 0; i--) {
-            r = r + s.charAt(i);
-        }
-        return r;
-    }
-
     public static void main(String[] args) {
 
+        // Characters
+
+        String fruit = "banana";
+        char letter0 = fruit.charAt(0);
+
+        if (letter0 == 'a') {
+            System.out.println('?');
+        }
+
         System.out.print("Roman alphabet: ");
         for (char c = 'A'; c <= 'Z'; c++) {
             System.out.print(c);
@@ -28,6 +26,47 @@ public static void main(String[] args) {
         }
         System.out.println();
 
+        // Strings are immutable
+
+        String name = "Alan Turing";
+        String upperName = name.toUpperCase();
+
+        String text = "Computer Science is fun!";
+        text = text.replace("Computer Science", "CS");
+
+        // String traversal
+
+        for (int i = 0; i < fruit.length(); i++) {
+            char letter = fruit.charAt(i);
+            System.out.println(letter);
+        }
+
+        for (char letter : fruit.toCharArray()) {
+            System.out.println(letter);
+        }
+
+        int length = fruit.length();
+        char last = fruit.charAt(length - 1);  // correct
+
+        System.out.println(reverse(fruit));
+
+        // Substrings
+
+        System.out.println(fruit.substring(0));
+        System.out.println(fruit.substring(2));
+        System.out.println(fruit.substring(6));
+
+        System.out.println(fruit.substring(0, 3));
+        System.out.println(fruit.substring(2, 5));
+        System.out.println(fruit.substring(6, 6));
+
+        // The indexOf method
+
+        int index = fruit.indexOf('a');
+        int index2 = fruit.indexOf('a', 2);
+
+        // String comparison
+
         String name1 = "Alan Turing";
         String name2 = "Ada Lovelace";
         if (name1.equals(name2)) {
@@ -43,17 +82,24 @@ public static void main(String[] args) {
             System.out.println("name2 comes before name1.");
         }
 
-        String fruit = "banana";
+        // Wrapper classes
 
-        for (int i = 0; i < fruit.length(); i++) {
-            char letter = fruit.charAt(i);
-            System.out.println(letter);
-        }
+        String str = "12345";
+        int num = Integer.parseInt(str);
 
-        for (char letter : fruit.toCharArray()) {
-            System.out.println(letter);
-        }
+        num = 12345;
+        str = Integer.toString(num);
+    }
 
-        System.out.println(reverse(fruit));
+    /**
+     * Reverses a string, returns a new String.
+     */
+    public static String reverse(String s) {
+        String r = "";
+        for (int i = s.length() - 1; i >= 0; i--) {
+            r = r + s.charAt(i);
+        }
+        return r;
     }
+
 }
diff --git a/ch10/Max.java b/ch10/Max.java
deleted file mode 100644
index a7ecea5..0000000
--- a/ch10/Max.java
+++ /dev/null
@@ -1,23 +0,0 @@
-import java.util.Arrays;
-
-/**
- * Demonstrates command-line arguments.
- */
-public class Max {
-
-    /**
-     * Converts the command line arguments to integers and prints the max.
-     */
-    public static void main(String[] args) {
-        System.out.println(Arrays.toString(args));
-
-        int max = Integer.MIN_VALUE;
-        for (String arg : args) {
-            int value = Integer.parseInt(arg);
-            if (value > max) {
-                max = value;
-            }
-        }
-        System.out.println("The max is " + max);
-    }
-}
diff --git a/ch10/PointRect.java b/ch10/PointRect.java
index d0da1df..36801a5 100644
--- a/ch10/PointRect.java
+++ b/ch10/PointRect.java
@@ -6,6 +6,28 @@
  */
 public class PointRect {
 
+    public static void main(String[] args) {
+        Point blank;
+        blank = new Point(3, 4);
+        System.out.println(blank);
+
+        int x = blank.x;
+        System.out.println(blank.x + ", " + blank.y);
+        int sum = blank.x * blank.x + blank.y * blank.y;
+
+        Rectangle box = new Rectangle(0, 0, 100, 200);
+        moveRect(box, 50, 100);
+        System.out.println(box);
+        box.translate(50, 100);
+
+        Rectangle box1 = new Rectangle(0, 0, 100, 200);
+        Rectangle box2 = box1;
+
+        System.out.println(box2.width);
+        box1.grow(50, 50);
+        System.out.println(box2.width);
+    }
+
     /**
      * Prints the attributes of a Point object.
      */
@@ -17,8 +39,8 @@ public static void printPoint(Point p) {
      * Computes the distance between two points.
      */
     public static double distance(Point p1, Point p2) {
-        double dx = (double) (p2.x - p1.x);
-        double dy = (double) (p2.y - p1.y);
+        int dx = p2.x - p1.x;
+        int dy = p2.y - p1.y;
         return Math.sqrt(dx * dx + dy * dy);
     }
 
@@ -40,22 +62,29 @@ public static void moveRect(Rectangle box, int dx, int dy) {
     }
 
     /**
-     * Tests the methods in this class.
+     * Exercise on returning objects.
      */
-    public static void main(String[] args) {
-        Point blank;
-        blank = new Point(3, 4);
-        System.out.println(blank);
+    public static void exercise2() {
+        Point blank = new Point(5, 8);
 
-        Rectangle box = new Rectangle(0, 0, 100, 200);
-        moveRect(box, 50, 100);
-        System.out.println(box);
+        Rectangle rect = new Rectangle(0, 2, 4, 4);
+        Point center = findCenter(rect);
 
-        Rectangle box1 = new Rectangle(0, 0, 100, 200);
-        Rectangle box2 = box1;
+        double dist = distance(center, blank);
+        System.out.println(dist);
+    }
 
-        System.out.println(box2.width);
-        box1.grow(50, 50);
-        System.out.println(box2.width);
+    /**
+     * Exercise on aliasing.
+     */
+    public static void exercise3() {
+        Rectangle box1 = new Rectangle(2, 4, 7, 9);
+        Point p1 = findCenter(box1);
+        printPoint(p1);
+
+        box1.grow(1, 1);
+        Point p2 = findCenter(box1);
+        printPoint(p2);
     }
+
 }
diff --git a/ch10/Pow.java b/ch10/Pow.java
new file mode 100644
index 0000000..9ba0952
--- /dev/null
+++ b/ch10/Pow.java
@@ -0,0 +1,24 @@
+/**
+ * BigInteger exercise.
+ */
+public class Pow {
+
+    /**
+     * Integer exponentiation.
+     */
+    public static int pow(int x, int n) {
+        if (n == 0) return 1;
+
+        // find x to the n/2 recursively
+        int t = pow(x, n / 2);
+
+        // if n is even, the result is t squared
+        // if n is odd, the result is t squared times x
+        if (n % 2 == 0) {
+            return t * t;
+        } else {
+            return t * t * x;
+        }
+    }
+
+}
diff --git a/ch10/Riddle.java b/ch10/Riddle.java
new file mode 100644
index 0000000..cd04a16
--- /dev/null
+++ b/ch10/Riddle.java
@@ -0,0 +1,23 @@
+import java.awt.Point;
+
+/**
+ * Exercise on passing objects as parameters.
+ */
+public class Riddle {
+
+    public static int riddle(int x, Point p) {
+        x = x + 7;
+        return x + p.x + p.y;
+    }
+
+    public static void main(String[] args) {
+        int x = 5;
+        Point blank = new Point(1, 2);
+
+        System.out.println(riddle(x, blank));
+        System.out.println(x);
+        System.out.println(blank.x);
+        System.out.println(blank.y);
+    }
+
+}
diff --git a/ch11/Time.java b/ch11/Time.java
index 99e75db..53afa0d 100644
--- a/ch11/Time.java
+++ b/ch11/Time.java
@@ -99,20 +99,4 @@ public void increment(double seconds) {
         }
     }
 
-    /**
-     * Test the methods in this class.
-     */
-    public static void main(String[] args) {
-        Time time = new Time(11, 59, 59.9);
-        System.out.println(time);
-
-        Time time1 = new Time(9, 30, 0.0);
-        Time time2 = time1;
-        Time time3 = new Time(9, 30, 0.0);
-
-        System.out.println(time1 == time2);
-        System.out.println(time1 == time3);
-        System.out.println(time1.equals(time2));
-        System.out.println(time1.equals(time3));
-    }
 }
diff --git a/ch11/TimeClient.java b/ch11/TimeClient.java
index 59ef451..aa90401 100644
--- a/ch11/TimeClient.java
+++ b/ch11/TimeClient.java
@@ -3,12 +3,31 @@
  */
 public class TimeClient {
 
-    /**
-     * Demonstrates that we cannot access private variables from another
-     * class.
-     */
     public static void main(String[] args) {
         Time time = new Time(11, 59, 59.9);
-        System.out.println(time.hour);          // ERROR
+        System.out.println(time);
+
+        // cannot access private variables from another class
+        // System.out.println(time.hour);
+
+        String s = time.toString();
+        System.out.println(s);
+
+        Time time1 = new Time(9, 30, 0.0);
+        Time time2 = time1;
+        Time time3 = new Time(9, 30, 0.0);
+
+        System.out.println(time1 == time2);
+        System.out.println(time1 == time3);
+        System.out.println(time1.equals(time2));
+        System.out.println(time1.equals(time3));
+
+        Time startTime = new Time(18, 50, 0.0);
+        Time runningTime = new Time(2, 16, 0.0);
+        Time endTime = Time.add(startTime, runningTime);
+
+        // using the instance method
+        endTime = startTime.add(runningTime);
     }
+
 }