1. COMP 10 Introduction to Programming Mr. Joshua Stough October 29, 2007

2. Debugging Tips Pay attention to detail! –Java is case-sensitive –difference between () and [] and {} Double-check loop conditions Work through errors one at a time Add println statements to show values of variables before/after errors occur Take a break!

3. Spot the Bug public class Errors { public static void main (String[] args) { int[] numbers = {8, 4, 5, 7}; for (int i=0; i<=numbers.length; i++) { System.out.println (numbers(i)); } } Errors.java:8: cannot resolve symbol symbol: method numbers (int) location: class Errors System.out.println (numbers(i)); ^ 1 error

4. Spot the Bug public class Errors { public static void main (String[] args) { int[] numbers = {8, 4, 5, 7}; for (int i=0; i<=numbers.length; i++) { System.out.println (numbers[i]); } } 8 4 5 7 Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 4 at Errors.main(Errors.java:8)

5. Review Visibility Modifiers public visibility –can be accessed from anywhere private visibility –can only be accessed from inside the class (inside the same Java source file) public class Rectangle { private int length; private int width; } public Rectangle () { length = 0; width = 0; }...

6. Review Visibility Modifiers Usually declare data members with private visibility Declare methods that clients (other classes) are supposed to call with public visibility –service methods Declare methods that only other methods in the class are supposed to call with private visibility –support methods

7. Review - Constructor public Rectangle (int l, int w) { length = l; width = w; } public class Rectangle { private int length; private int width; Rectangle r2 = new Rectangle (5, 10); public Rectangle () { length = 0; width = 0; } All formal parameters are local to the method.

8. r 2500 2 3 Review Objects Create an object: Rectangle r; r = new Rectangle(2, 3); OR Rectangle r = new Rectangle(2, 3); Use the object and the dot operator to access methods: r.setLength(5); r.setWidth(10); r 2 3 2500 510

9. Review Accessing Static Members public class Card { public static final int ACE = 14; public class BlackjackGame { public static int calcPoints (Card card) Card.ACE BlackjackGame.calcPoints

10. Review Calling Methods static methods –classname.methodname non-static methods –create an object –objectname.methodname

11. Review this public class Rectangle { private int length; private int width; public Rectangle (int length, int width) { this.length = length; this.width = width; }

12. Review Passing Objects as Parameters public class BlackjackGame { public static int calcPoints(Card c) { } Card card1 = new Card (2, Card.HEARTS); card1 face 2 suit 0 c int points = BlackjackGame.calcPoints(card1); 2 points

13. Array Bounds Arrays have finite size If you access an element outside of the array, you’ll get an ArrayIndexOutOfBounds Exception Example: Int[] grades = {99, 98, 95, 96}; System.out.println (grades[4]); 01 23

14. int arraySize; System.out.print ("Enter the size of the array:"); arraySize = Integer.parseInt(keyboard.readLine()); int[] list = new int[arraySize]; Specify Array Size During Program Execution Example 01 23 (Assume that keyboard has already been declared and instantiated.)

15. Example for (int ind = 0; ind < sale.length; ind++) { sale[ind] = 10.00; } Initialize Array to Specific Value (10.00) (Assume that sale has already been declared and instantiated.) 01 23

16. Example for (int ind = 0; ind < sale.length; ind++) { sale[ind] = Double.parseDouble(keyboard.readLine()); } (Assume that sale has already been declared and instantiated, and that keyboard has already been declared and instantiated.) Read Data into Array 01 23

17. Example for (int ind = 0; ind < sale.length; ind++) { System.out.print(sale[ind] + " "); } Print Array (Assume that sale has already been declared and instantiated.) 01 23

18. Example sum = 0; for(int ind = 0; ind < sale.length; ind++) { sum = sum + sale[ind]; } if(sale.length != 0) average = sum / sale.length; else average = 0.0; Find Sum and Average of Array (Assume that sale has already been declared and instantiated, and that sum and average have already been declared.) 01 23

19. Example maxIndex = 0; for (int ind = 1; ind < sale.length; ind++) { if (sale[maxIndex] < sale[ind]) { maxIndex = ind; } largestSale = sale[maxIndex]; Determining Largest Element in Array (Assume that sale has already been declared and instantiated, and that maxIndex and largestSale have already been declared.) 0 1 2 3 4 5 6 7 12.50 8.35 19.60 25.00 14.00 39.43 35.90 98.23 01 23

20. Parallel Arrays Arrays are parallel if corresponding components hold related information String[] studentName; double[] studentGPA; For example, studentName and studentGPA are parallel if studentGPA[3] is the GPA of the student with studentName[3]. 01 23

21. In-Class Exercises 1.Declare an array of integers called numbers Hint: type[] name; 2.Declare and instantiate an array of 26 characters called alphabet Hint: type[] name = new type[size]; int[] numbers; char[] alphabet = new char[26]; 01 23

22. In-Class Exercises 3.Declare an array of 5 characters called grades and initialize it with the letters: A, B, C, D, F Hint: type[] name = {initialization list}; 4.Write a loop to print the contents of an array named zipCodes Hint: to access array element name[index] char[] grades = {'A', 'B', 'C', 'D', 'F'}; for (int i=0; i<zipCodes.length; i++) { System.out.println (zipCodes[i]); } 01 23

23. In-Class Exercises 5.Write a loop to change all the values of the integer array numbers to index + 1 for (int i=0; i<numbers.length; i++) { numbers[i] = i+1; } 01 23

24. Arrays Summary Why use them? –maintain a list of related items How use them? –first declare a variable to reference the array –when your program knows how many elements, it can then instantiate (create), initialize, and access the array –design code to index the array only within the array bounds 01 23

25. Review Arrays Declaration int[] counts; Instantiation counts = new int[50]; Initialization / Access for (int i=0; i<counts.length; i++) { counts[i] = 0; } Initializer List –declaration, instantiation, and initialization double[] grades = {98.7, 72.4, 87.5}; int[] numbers = {num, num+1, num+2, num+3}; can use variables and expressions as initial values 01 23

26. Arrays and Assignment counter 0 1 2 3 4 int[] counter = new int[5]; int[] temp; temp temp = counter; doesn't make a copy of the array! temp == counter is true since the reference variables contain the same address

27. Copying Arrays counter 0 1 2 3 4 int[] counter = {1, 2, 3, 4, 5}; 1 2 3 4 5 int[] temp = new int[counter.length]; 0 1 2 3 4 temp for (int i=0; i<counter.length; i++) { temp[i] = counter[i]; } 1 2 3 4 5 i 0 1 2 3 4 5

28. References and Assignment counter 0 1 2 3 4 1 2 3 4 5 0 1 2 3 4 temp temp = counter; 1 2 3 4 5 Remember that arrays use reference variables just like objects.

29. References and null counter 0 1 2 3 4 1 2 3 4 5 0 1 2 3 4 temp temp = null; 1 2 3 4 5 Remember that arrays use reference variables just like objects. null is a reserved word that means "empty"

30. Arrays as Parameters Entire array can be passed as a parameter –method can change elements of the array permanently –since we're passing a reference Elements of an array can be passed as parameters, too –normal rules apply…

31. public class Tester { public static void swap (int[] scores, int x, int y) { int temp = scores[x]; scores[x] = scores[y]; scores[y] = temp; } public static void main (String[] args) { int[] grades = new int[4]; for (int i=0; i<grades.length; i++) { grades[i] = i*10; } swap (grades, 2, 3); } grades[2]: 20 grades[3]: 30 grades[2]: 30 grades[3]: 20 scores[2]: 20 scores[3]: 30 temp : 20 scores[2]: 30 scores[3]: 30 temp : 20 scores[2]: 30 scores[3]: 20 temp : 20

32. Arrays as Parameters public static void swap (int[] scores, int x, int y) swap (grades, 2, 3); alias 0 1 2 3 grades 0 10 20 30 scores x 2 y 3 temp int temp = scores[x]; scores[x] = scores[y]; scores[y] = temp; 20 30

33. Arrays of Objects Can use arrays to manipulate objects Create array of objects Must instantiate each object in array classname[] array = new classname[size]; for(int j=0; j <array.length; j++) { array[j] = new classname(); }

34. Yahtzee Game of Yahtzee requires 5 dice Die –member variable: –methods: int face void roll() int getFace() Die()

35. Instantiating Array Objects Die[] dice = new Die[5]; for (int i=0; i<dice.length; i++) { dice[i] = new Die(); } 0 1 2 3 4 dice face...... each element in the array is a reference variable

36. dice[0] == dice[1] Comparing Objects == vs. equals method dice[0].equals(dice[1]) If equals method for the class is undefined are the variables aliases? (i.e., do the variables contain the same address?) If equals method for the class is defined depends on the implementation of equals are the variables aliases? (i.e., do the variables contain the same address?)