1. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 1 Chapter 6 Arrays (continued)

2. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 2 Passing Arrays to Methods public static void printArray(int[] array) { for (int i = 0; i < array.length; i++) { System.out.print(array[i] + " "); } Invoke the method int[] list = {3, 1, 2, 6, 4, 2}; printArray(list); Invoke the method printArray(new int[]{3, 1, 2, 6, 4, 2}); Anonymous array

3. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 3 Anonymous Array The statement printArray(new int[]{3, 1, 2, 6, 4, 2}); creates an array using the following syntax: new dataType[]{literal0, literal1,..., literalk}; There is no explicit reference variable for the array. Such array is called an anonymous array.

4. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 4 Pass By Value Java uses pass by value to pass arguments to a method. There are important differences between passing a value of variables of primitive data types and passing arrays. F For a parameter of a primitive type value, the actual value is passed. Changing the value of the local parameter inside the method does not affect the value of the variable outside the method. F For a parameter of an array type, the value of the parameter contains a reference to an array; this reference is passed to the method. Any changes to the array that occur inside the method body will affect the original array that was passed as the argument.

5. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 5 public class Test { public static void main(String[] args) { int x = 1; // x represents an int value int[] y = new int[10]; // y represents an array of int values m(x, y); // Invoke m with arguments x and y System.out.println("x is " + x); System.out.println("y[0] is " + y[0]); } public static void m(int number, int[] numbers) { number = 1001; // Assign a new value to number numbers[0] = 5555; // Assign a new value to numbers[0] } Simple Example

6. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 6 Call Stack When invoking m(x, y), the values of x and y are passed to number and numbers. Since y contains the reference value to the array, numbers now contains the same reference value to the same array.

7. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 7 Call Stack When invoking m(x, y), the values of x and y are passed to number and numbers. Since y contains the reference value to the array, numbers now contains the same reference value to the same array.

8. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 8 Heap The JVM stores the array in an area of memory, called heap, which is used for dynamic memory allocation where blocks of memory are allocated and freed in an arbitrary order.

9. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 9 Returning an Array from a Method public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result

10. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 10 Trace the reverse Method, cont. public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result 123456 000001 i = 0 and j = 5 Assign list[0] to result[5] animation

11. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 11 Trace the reverse Method, cont. public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result 123456 000001 After this, i becomes 1 and j becomes 4 animation

12. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 12 Trace the reverse Method, cont. public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result 123456 000001 i (=1) is less than 6 animation

13. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 13 Trace the reverse Method, cont. public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result 123456 000021 i = 1 and j = 4 Assign list[1] to result[4] animation

14. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 14 Trace the reverse Method, cont. public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result 123456 000321 i = 2 and j = 3 Assign list[i] to result[j] animation

15. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 15 Trace the reverse Method, cont. public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result 123456 054321 i (=5) is still less than 6 animation

16. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 16 Trace the reverse Method, cont. public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result 123456 654321 i = 5 and j = 0 Assign list[i] to result[j] animation

17. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 17 Trace the reverse Method, cont. public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result 123456 654321 After this, i becomes 6 and j becomes -1 animation

18. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 18 Trace the reverse Method, cont. public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result 123456 654321 i (=6) < 6 is false. So exit the loop. animation

19. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 19 Trace the reverse Method, cont. public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; } int[] list1 = new int[]{1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1); list result 123456 654321 Return result list2 animation

20. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 20 Problem: Counting Occurrence of Each Letter F Generate 100 lowercase letters randomly and assign to an array of characters. F Count the occurrence of each letter in the array. CountLettersInArrayRun

21. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 21 Searching Arrays Searching is the process of looking for a specific element in an array; for example, discovering whether a certain score is included in a list of scores. Searching is a common task in computer programming. There are many algorithms and data structures devoted to searching. In this section, two commonly used approaches are discussed, linear search and binary search.

22. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 22 Linear Search The linear search approach compares the key element, key, sequentially with each element in the array list. The method continues to do so until the key matches an element in the list or the list is exhausted without a match being found. If a match is made, the linear search returns the index of the element in the array that matches the key. If no match is found, the search returns -1.

23. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 23 Linear Search Animation 64197328 64197328 64197328 64197328 64197328 64197328 3 3 3 3 3 3 animation KeyList

24. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 24 From Idea to Solution /** The method for finding a key in the list */ public static int linearSearch(int[] list, int key) { for (int i = 0; i < list.length; i++) if (key == list[i]) return i; return -1; } int[] list = {1, 4, 4, 2, 5, -3, 6, 2}; int i = linearSearch(list, 4); // returns 1 int j = linearSearch(list, -4); // returns -1 int k = linearSearch(list, -3); // returns 5 Trace the method

25. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 25 Binary Search For binary search to work, the elements in the array must already be ordered. Without loss of generality, assume that the array is in ascending order. e.g., 2 4 7 10 11 45 50 59 60 66 69 70 79 The binary search first compares the key with the element in the middle of the array.

26. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 26 Binary Search, cont. F If the key is less than the middle element, you only need to search the key in the first half of the array. F If the key is equal to the middle element, the search ends with a match. F If the key is greater than the middle element, you only need to search the key in the second half of the array. Consider the following three cases:

27. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 27 Binary Search 12346789 12346789 12346789 8 8 8 KeyList animation

28. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 28 Binary Search, cont.

29. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 29 Binary Search, cont.

30. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 30 Binary Search, cont. The binarySearch method returns the index of the element in the list that matches the search key if it is contained in the list. Otherwise, it returns -insertion point - 1. The insertion point is the point at which the key would be inserted into the list.

31. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 31 From Idea to Soluton /** Use binary search to find the key in the list */ public static int binarySearch(int[] list, int key) { int low = 0; int high = list.length - 1; while (high >= low) { int mid = (low + high) / 2; if (key < list[mid]) high = mid - 1; else if (key == list[mid]) return mid; else low = mid + 1; } return -1 - low; }

32. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 32 The Arrays.binarySearch Method Since binary search is frequently used in programming, Java provides several overloaded binarySearch methods for searching a key in an array of int, double, char, short, long, and float in the java.util.Arrays class. For example, the following code searches the keys in an array of numbers and an array of characters. int[] list = {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79}; System.out.println("Index is " + java.util.Arrays.binarySearch(list, 11)); char[] chars = {'a', 'c', 'g', 'x', 'y', 'z'}; System.out.println("Index is " + java.util.Arrays.binarySearch(chars, 't')); For the binarySearch method to work, the array must be pre-sorted in increasing order. Return is 4 Return is –4 (insertion point is 3, so return is -3-1)

33. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 33 Sorting Arrays Sorting, like searching, is also a common task in computer programming. It would be used, for instance, if you wanted to display the grades from Listing 6.2, “Assigning Grades,” in alphabetical order. Many different algorithms have been developed for sorting. This section introduces two simple, intuitive sorting algorithms: selection sort and insertion sort.

34. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 34 Selection sort finds the largest number in the list and places it last. It then finds the largest number remaining and places it next to last, and so on until the list contains only a single number. Figure 6.17 shows how to sort the list {2, 9, 5, 4, 8, 1, 6} using selection sort. Selection Sort

35. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 35 Selection Sort 2954816 2654819 2654189 2145689 2145689 2154689 1245689 animation int[] myList = {2, 9, 5, 4, 8, 1, 6}; // Unsorted

36. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 36 From Idea to Solution for (int i = list.length - 1; i >= 1; i--) { select the largest element in list[0..i]; swap the largest with list[i], if necessary; // list[i] is in place. The next iteration applies on list[0..i-1] } list[0] list[1] list[2] list[3]... list[10] list[0] list[1] list[2] list[3]... list[9] list[0] list[1] list[2] list[3]... list[8] list[0] list[1] list[2] list[3]... list[7] list[0]...

37. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 37 Expand for (int i = list.length - 1; i >= 1; i--) { select the largest element in list[0..i]; swap the largest with list[i], if necessary; // list[i] is in place. The next iteration applies on list[0..i-1] } // Find the maximum in the list[0..i] double currentMax = list[0]; int currentMaxIndex = 0; for (int j = 1; j <= i; j++) { if (currentMax < list[j]) { currentMax = list[j]; currentMaxIndex = j; }

38. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 38 Expand for (int i = list.length - 1; i >= 1; i--) { select the largest element in list[0..i]; swap the largest with list[i], if necessary; // list[i] is in place. The next iteration applies on list[0..i-1] } // Find the maximum in the list[0..i] double currentMax = list[0]; int currentMaxIndex = 0; for (int j = 1; j <= i; j++) { if (currentMax < list[j]) { currentMax = list[j]; currentMaxIndex = j; }

39. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 39 Expand for (int i = list.length - 1; i >= 1; i--) { select the largest element in list[0..i]; swap the largest with list[i], if necessary; // list[i] is in place. The next iteration applies on list[0..i-1] } // Swap list[i] with list[currentMaxIndex] if necessary; if (currentMaxIndex != i) { list[currentMaxIndex] = list[i]; list[i] = currentMax; }

40. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 40 Wrap it in a Method /** The method for sorting the numbers */ public static void selectionSort(double[] list) { for (int i = list.length - 1; i >= 1; i--) { // Find the maximum in the list[0..i] double currentMax = list[0]; int currentMaxIndex = 0; for (int j = 1; j <= i; j++) { if (currentMax < list[j]) { currentMax = list[j]; currentMaxIndex = j; } // Swap list[i] with list[currentMaxIndex] if necessary; if (currentMaxIndex != i) { list[currentMaxIndex] = list[i]; list[i] = currentMax; } Invoke it selectionSort(yourList)

41. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 41 Insertion Sort int[] myList = {2, 9, 5, 4, 8, 1, 6}; // Unsorted The insertion sort algorithm sorts a list of values by repeatedly inserting an unsorted element into a sorted sublist until the whole list is sorted.

42. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 42 How to Insert? The insertion sort algorithm sorts a list of values by repeatedly inserting an unsorted element into a sorted sublist until the whole list is sorted.

43. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 43 From Idea to Solution for (int i = 1; 1; i < list,length; i++) { insert list[i] into a sorted sublist list[0..i-1] so that list[0..i] is sorted } list[0] list[0] list[1] list[0] list[1] list[2] list[0] list[1] list[2] list[3] list[0] list[1] list[2] list[3]... InsertSort

44. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 44 The Arrays.sort Method Since sorting is frequently used in programming, Java provides several overloaded sort methods for sorting an array of int, double, char, short, long, and float in the java.util.Arrays class. For example, the following code sorts an array of numbers and an array of characters. double[] numbers = {6.0, 4.4, 1.9, 2.9, 3.4, 3.5}; java.util.Arrays.sort(numbers); char[] chars = {'a', 'A', '4', 'F', 'D', 'P'}; java.util.Arrays.sort(chars);

45. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 45 Two-dimensional Arrays Thus far, you have used one-dimensional arrays to model linear collections of elements. You can use a two-dimensional array to represent a matrix or a table. For example, the following table that describes the distances between the cities can be represented using a two-dimensional array.

46. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 46 Declare/Create Two-dimensional Arrays // Declare array ref var dataType[][] refVar; // Create array and assign its reference to variable refVar = new dataType[10][10]; // Combine declaration and creation in one statement dataType[][] refVar = new dataType[10][10]; // Alternative syntax dataType refVar[][] = new dataType[10][10];

47. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 47 Declaring Variables of Two- dimensional Arrays and Creating Two-dimensional Arrays int[][] matrix = new int[10][10]; or int matrix[][] = new int[10][10]; matrix[0][0] = 3; for (int i = 0; i < matrix.length; i++) for (int j = 0; j < matrix[i].length; j++) matrix[i][j] = (int)(Math.random() * 1000); double[][] x;

48. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 48 Two-dimensional Array Illustration array.length? 4 array[0].length? 3 matrix.length? 5 matrix[0].length? 5

49. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 49 Lengths of Two-dimensional Arrays int[][] x = new int[3][4];

50. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 50 Lengths of Two-dimensional Arrays, cont. int[][] array = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12} }; array.length array[0].length array[1].length array[2].length array[3].length array[4].length ArrayIndexOutOfBoundsException

51. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 51 Ragged Arrays Each row in a two-dimensional array is itself an array. So, the rows can have different lengths. Such an array is known as a ragged array. For example, int[][] matrix = { {1, 2, 3, 4, 5}, {2, 3, 4, 5}, {3, 4, 5}, {4, 5}, {5} }; matrix.length is 5 matrix[0].length is 5 matrix[1].length is 4 matrix[2].length is 3 matrix[3].length is 2 matrix[4].length is 1

52. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 52 Ragged Arrays, cont.

53. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 53 Chapter 7 Objects and Classes

54. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 54 Motivations After learning the preceding chapters, you are capable of solving many programming problems using selections, loops, methods, and arrays. However, these Java features are not sufficient for developing graphical user interfaces and large scale software systems. Suppose you want to develop a graphical user interface as shown in Figure 7.1. How do you program it?

55. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 55 OO Programming Concepts Object-oriented programming (OOP) involves programming using objects. An object represents an entity in the real world that can be distinctly identified. For example, a student, a desk, a circle, a button, and even a loan can all be viewed as objects. An object has a unique identity, state, and behaviors. The state of an object consists of a set of data fields (also known as properties) with their current values. The behavior of an object is defined by a set of methods.

56. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 56 Objects An object has both a state and behavior. The state defines the object, and the behavior defines what the object does.

57. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 57 Classes Classes are constructs that define objects of the same type. A Java class uses variables to define data fields and methods to define behaviors. Additionally, a class provides a special type of methods, known as constructors, which are invoked to construct objects from the class.

58. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 58 Classes

59. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 59 UML Class Diagram

60. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 60 Constructors Circle() { } Circle(double newRadius) { radius = newRadius; } Constructors are a special kind of methods that are invoked to construct objects.

61. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 61 Constructors, cont. A constructor with no parameters is referred to as a no-arg constructor. · Constructors must have the same name as the class itself. · Constructors do not have a return type—not even void. · Constructors are invoked using the new operator when an object is created. Constructors play the role of initializing objects.

62. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 62 Default Constructor A class may be declared without constructors. In this case, a no-arg constructor with an empty body is implicitly declared in the class. This constructor, called a default constructor, is provided automatically only if no constructors are explicitly declared in the class.

63. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 63 Declaring Object Reference Variables To reference an object, assign the object to a reference variable. To declare a reference variable, use the syntax: ClassName objectRefVar; Example: Circle myCircle;

64. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 64 Declaring/Creating Objects in a Single Step ClassName objectRefVar = new ClassName(); Example: Circle myCircle = new Circle(); Create an object Assign object reference

65. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 65 Accessing Objects F Referencing the object’s data: objectRefVar.data e.g., myCircle.radius F Invoking the object’s method: objectRefVar.methodName(arguments) e.g., myCircle.getArea()

66. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 66 A Simple Circle Class  Objective: Demonstrate creating objects, accessing data, and using methods. TestCircle1 Run

67. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 67 Trace Code Circle myCircle = new Circle(5.0); SCircle yourCircle = new Circle(); yourCircle.radius = 100; Declare myCircle no value myCircle animation

68. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 68 Trace Code, cont. Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle Assign object reference to myCircle animation

69. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 69 Trace Code, cont. Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle reference value yourCircle Assign object reference to yourCircle animation

70. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 70 Trace Code, cont. Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle reference value yourCircle Change radius in yourCircle animation

71. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 71 Caution Recall that you use Math.methodName(arguments) (e.g., Math.pow(3, 2.5)) to invoke a method in the Math class. Can you invoke getArea() using Circle1.getArea()? The answer is no. All the methods used before this chapter are static methods, which are defined using the static keyword. However, getArea() is non-static. It must be invoked from an object using objectRefVar.methodName(arguments) (e.g., myCircle.getArea()). More explanations will be given in Section 7.6, “Static Variables, Constants, and Methods.”

72. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 72 Reference Data Fields The data fields can be of reference types. For example, the following Student class contains a data field name of the String type. public class Student { String name; // name has default value null int age; // age has default value 0 boolean isScienceMajor; // isScienceMajor has default value false char gender; // c has default value '\u0000' }

73. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 73 The null Value If a data field of a reference type does not reference any object, the data field holds a special literal value, null.

74. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 74 Default Value for a Data Field The default value of a data field is null for a reference type, 0 for a numeric type, false for a boolean type, and '\u0000' for a char type. However, Java assigns no default value to a local variable inside a method. public class Test { public static void main(String[] args) { Student student = new Student(); System.out.println("name? " + student.name); System.out.println("age? " + student.age); System.out.println("isScienceMajor? " + student.isScienceMajor); System.out.println("gender? " + student.gender); }

75. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 75 Example public class Test { public static void main(String[] args) { int x; // x has no default value String y; // y has no default value System.out.println("x is " + x); System.out.println("y is " + y); } Compilation error: variables not initialized Java assigns no default value to a local variable inside a method.

76. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 76 Differences between Variables of Primitive Data Types and Object Types

77. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 77 Copying Variables of Primitive Data Types and Object Types

78. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 78 Garbage Collection As shown in the previous figure, after the assignment statement c1 = c2, c1 points to the same object referenced by c2. The object previously referenced by c1 is no longer referenced. This object is known as garbage. Garbage is automatically collected by JVM.

79. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 79 Garbage Collection, cont TIP: If you know that an object is no longer needed, you can explicitly assign null to a reference variable for the object. The JVM will automatically collect the space if the object is not referenced by any variable.