1. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 1 Chapter 5 Methods

2. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 2 Motivations A method is a construct for grouping statements together to perform a function. Using a method, you can write the code once for performing the function in a program and reuse it by many other programs. For example, often you need to find the maximum between two numbers. Whenever you need this function, you would have to write the following code: int result; if (num1 > num2) result = num1; else result = num2; If you define this function for finding a maximum number between any two numbers in a method, you don’t have to repeatedly write the same code. You need to define it just once and reuse it by any other programs.

3. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 3 Defining Methods A method is a collection of statements that are grouped together to perform an operation.

4. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 4 Method Signature Method signature is the combination of the method name and the parameter list.

5. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 5 Formal Parameters The variables defined in the method header are known as formal parameters.

6. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 6 Actual Parameters When a method is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument.

7. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 7 Return Value Type A method may return a value. The returnValueType is the data type of the value the method returns. If the method does not return a value, the returnValueType is the keyword void. For example, the returnValueType in the main method is void.

8. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 8 Calling Methods Listing 5.1 Testing the max method This program demonstrates calling a method max to return the largest of the int values TestMax

9. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 9 Calling Methods, cont. animation

10. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 10 Trace Method Invocation invoke max(i, j) Pass the value of i to num1 Pass the value of j to num2 animation

11. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 11 Trace Method Invocation result is now 5 animation

12. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 12 Trace Method Invocation return result, which is 5 animation

13. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 13 Trace Method Invocation return max(i, j) and assign the return value to k animation

14. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 14 CAUTION A return statement is required for a value-returning method. The method shown below in (a) is logically correct, but it has a compilation error because the Java compiler thinks it possible that this method does not return any value. To fix this problem, delete if (n < 0) in (a), so that the compiler will see a return statement to be reached regardless of how the if statement is evaluated.

15. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 15 Reuse Methods from Other Classes NOTE: One of the benefits of methods is for reuse. The max method can be invoked from any class besides TestMax. If you create a new class Test, you can invoke the max method using ClassName.methodName (e.g., TestMax.max).

16. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 16 Call Stacks

17. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 17 Trace Call Stack pass the values of i and j to num1 and num2 animation

18. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 18 Trace Call Stack (num1 > num2) is true animation

19. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 19 Trace Call Stack Return result and assign it to k animation

20. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 20 Trace Call Stack Execute print statement animation

21. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 21 void Method Example This type of method does not return a value. The method performs some actions. TestVoidMethod

22. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 22 Passing Parameters public static void nPrintln(String message, int n) { for (int i = 0; i < n; i++) System.out.println(message); } Suppose you invoke the method using nPrintln(“Welcome to Java”, 5); What is the output? Suppose you invoke the method using nPrintln(“Computer Science”, 15); What is the output?

23. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 23 Pass by Value Listing 5.2 Testing Pass by value This program demonstrates passing values to the methods. TestPassByValue

24. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 24 Pass by Value, cont.

25. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 25 Overloading Methods Listing 5.3 Overloading the max Method public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; } TestMethodOverloading

26. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 26 Ambiguous Invocation Sometimes there may be two or more possible matches for an invocation of a method, but the compiler cannot determine the most specific match. This is referred to as ambiguous invocation. Ambiguous invocation is a compilation error.

27. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 27 Ambiguous Invocation public class AmbiguousOverloading { public static void main(String[] args) { System.out.println(max(1, 2)); } public static double max(int num1, double num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, int num2) { if (num1 > num2) return num1; else return num2; }

28. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 28 Scope of Local Variables A local variable: a variable defined inside a method. Scope: the part of the program where the variable can be referenced. The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared before it can be used.

29. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 29 Scope of Local Variables, cont. You can declare a local variable with the same name multiple times in different non- nesting blocks in a method, but you cannot declare a local variable twice in nested blocks.

30. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 30 Scope of Local Variables, cont. A variable declared in the initial action part of a for loop header has its scope in the entire loop. But a variable declared inside a for loop body has its scope limited in the loop body from its declaration and to the end of the block that contains the variable.

31. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 31 Scope of Local Variables, cont.

32. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 32 Method Abstraction You can think of the method body as a black box that contains the detailed implementation for the method.

33. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 33 Benefits of Methods Write a method once and reuse it anywhere. Information hiding. Hide the implementation from the user. Reduce complexity.

34. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 34 The Math Class F Class constants: –PI –E F Class methods: –Trigonometric Methods –Exponent Methods –Rounding Methods –min, max, abs, and random Methods

35. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 35 Trigonometric Methods F sin(double a) F cos(double a) F tan(double a) F acos(double a) F asin(double a) F atan(double a) Radians toRadians(90) Examples: Math.sin(0) returns 0.0 Math.sin(Math.PI / 6) returns 0.5 Math.sin(Math.PI / 2) returns 1.0 Math.cos(0) returns 1.0 Math.cos(Math.PI / 6) returns 0.866 Math.cos(Math.PI / 2) returns 0

36. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 36 Exponent Methods  exp(double a) Returns e raised to the power of a.  log(double a) Returns the natural logarithm of a.  log10(double a) Returns the 10-based logarithm of a.  pow(double a, double b) Returns a raised to the power of b.  sqrt(double a) Returns the square root of a. Examples: Math.exp(1) returns 2.71 Math.log(2.71) returns 1.0 Math.pow(2, 3) returns 8.0 Math.pow(3, 2) returns 9.0 Math.pow(3.5, 2.5) returns 22.91765 Math.sqrt(4) returns 2.0 Math.sqrt(10.5) returns 3.24

37. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 37 Rounding Methods  double ceil(double x) x rounded up to its nearest integer. This integer is returned as a double value.  double floor(double x) x is rounded down to its nearest integer. This integer is returned as a double value.  double rint(double x) x is rounded to its nearest integer. If x is equally close to two integers, the even one is returned as a double.  int round(float x) Return (int)Math.floor(x+0.5).  long round(double x) Return (long)Math.floor(x+0.5).

38. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 38 Rounding Methods Examples Math.ceil(2.1) returns 3.0 Math.ceil(2.0) returns 2.0 Math.ceil(-2.0) returns –2.0 Math.ceil(-2.1) returns -2.0 Math.floor(2.1) returns 2.0 Math.floor(2.0) returns 2.0 Math.floor(-2.0) returns –2.0 Math.floor(-2.1) returns -3.0 Math.rint(2.1) returns 2.0 Math.rint(2.0) returns 2.0 Math.rint(-2.0) returns –2.0 Math.rint(-2.1) returns -2.0 Math.rint(2.5) returns 2.0 Math.rint(-2.5) returns -2.0 Math.round(2.6f) returns 3 Math.round(2.0) returns 2 Math.round(-2.0f) returns -2 Math.round(-2.6) returns -3

39. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 39 min, max, and abs  max(a, b) and min(a, b) Returns the maximum or minimum of two parameters.  abs(a) Returns the absolute value of the parameter.  random() Returns a random double value in the range [0.0, 1.0). Examples: Math.max(2, 3) returns 3 Math.max(2.5, 3) returns 3.0 Math.min(2.5, 3.6) returns 2.5 Math.abs(-2) returns 2 Math.abs(-2.1) returns 2.1

40. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 40 The random Method Generates a random double value greater than or equal to 0.0 and less than 1.0 (0 <= Math.random() < 1.0). Examples: In general,

41. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 41 Stepwise Refinement (Optional) The concept of method abstraction can be applied to the process of developing programs. When writing a large program, you can use the “divide and conquer” strategy, also known as stepwise refinement, to decompose it into subproblems. The subproblems can be further decomposed into smaller, more manageable problems.

42. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 42 PrintCalender Case Study Let us use the PrintCalendar example to demonstrate the stepwise refinement approach. PrintCalendar

43. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 43 Design Diagram

44. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 44 Design Diagram

45. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 45 Design Diagram

46. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 46 Design Diagram

47. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 47 Design Diagram

48. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 48 Design Diagram

49. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 49 Implementation: Top-Down A Skeleton for printCalendar Top-down approach is to implement one method in the structure chart at a time from the top to the bottom. Stubs can be used for the methods waiting to be implemented. A stub is a simple but incomplete version of a method. The use of stubs enables you to test invoking the method from a caller. Implement the main method first and then use a stub for the printMonth method. For example, let printMonth display the year and the month in the stub. Thus, your program may begin like this:

50. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 50 Implementation: Bottom-Up Bottom-up approach is to implement one method in the structure chart at a time from the bottom to the top. For each method implemented, write a test program to test it. Both top-down and bottom-up methods are fine. Both approaches implement the methods incrementally and help to isolate programming errors and makes debugging easy. Sometimes, they can be used together.

51. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 51 Chapter 6 Arrays

52. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 52 Motivations Often you will have to store a large number of values during the execution of a program. Suppose, for instance, that you need to read one hundred numbers, compute their average, and find out how many numbers are above the average. Your program first reads the numbers and computes their average, and then compares each number with the average to determine whether it is above the average. The numbers must all be stored in variables in order to accomplish this task. You have to declare one hundred variables and repeatedly write almost identical code one hundred times. From the standpoint of practicality, it is impossible to write a program this way. So, how do you solve this problem?

53. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 53 Objectives F To describe why arrays are necessary in programming (§6.1). F To learn the steps involved in using arrays: declaring array reference variables and creating arrays (§§6.2.1-6.2.2). F To initialize the values in an array (§6.2.3). F To access array elements using indexed variables (§6.2.4). F To simplify programming using the JDK 1.5 foreach loops (§6.2.5). F To declare, create, and initialize an array using an array initializer (§6.2.6). F To copy contents from one array to another (§6.3). F To develop and invoke methods with array arguments and return value (§6.4- 6.5). F To declare a method with variable-length argument list (§6.6). F To search elements using the linear (§6.7.1) or binary (§6.7.2) search algorithm. F To sort an array using the selection sort (§6.8.1) F To sort an array using the insertion sort algorithm (§6.8.2). F To use the methods in the Arrays class (§6.9). F To declare and create two-dimensional arrays to solve interesting problems such as Sudoku (§6.10). F To declare and create multidimensional arrays (§6.11).

54. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 54 Introducing Arrays Array is a data structure that represents a collection of the same types of data.

55. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 55 Declaring Array Variables F datatype[] arrayRefVar; Example: double[] myList;  datatype arrayRefVar[]; // This style is allowed, but not preferred Example: double myList[];

56. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 56 Creating Arrays arrayRefVar = new datatype[arraySize]; Example: myList = new double[10]; myList[0] references the first element in the array. myList[9] references the last element in the array.

57. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 57 Declaring and Creating in One Step F datatype[] arrayRefVar = new datatype[arraySize]; double[] myList = new double[10]; F datatype arrayRefVar[] = new datatype[arraySize]; double myList[] = new double[10];

58. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 58 The Length of an Array Once an array is created, its size is fixed. It cannot be changed. You can find its size using arrayRefVar.length For example, myList.length returns 10

59. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 59 Default Values When an array is created, its elements are assigned the default value of 0 for the numeric primitive data types, '\u0000' for char types, and false for boolean types.

60. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 60 Indexed Variables The array elements are accessed through the index. The array indices are 0-based, i.e., it starts from 0 to arrayRefVar.length-1. In the example in Figure 6.1, myList holds ten double values and the indices are from 0 to 9. Each element in the array is represented using the following syntax, known as an indexed variable: arrayRefVar[index];

61. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 61 Using Indexed Variables After an array is created, an indexed variable can be used in the same way as a regular variable. For example, the following code adds the value in myList[0] and myList[1] to myList[2]. myList[2] = myList[0] + myList[1];

62. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 62 Array Initializers F Declaring, creating, initializing in one step: double[] myList = {1.9, 2.9, 3.4, 3.5}; This shorthand syntax must be in one statement.

63. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 63 Declaring, creating, initializing Using the Shorthand Notation double[] myList = {1.9, 2.9, 3.4, 3.5}; This shorthand notation is equivalent to the following statements: double[] myList = new double[4]; myList[0] = 1.9; myList[1] = 2.9; myList[2] = 3.4; myList[3] = 3.5;

64. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 64 CAUTION Using the shorthand notation, you have to declare, create, and initialize the array all in one statement. Splitting it would cause a syntax error. For example, the following is wrong: double[] myList; myList = {1.9, 2.9, 3.4, 3.5};

65. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 65 Canonical Form of the for Loop double sum = 0; for (int i = 0; i < 5; i++) { sum += myList[i]; } System.out.println(sum); Make sure that you do not access illegal indices for (int i = 1; i <= 5; i++) // wrong! { sum += myList[i]; }

66. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 66 Trace Program with Arrays double sum = 0; for (int i = 0; I < 5; i++ double value: myList) { sum += value; } System.out.println(sum); animation

67. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 67 Enhanced for Loop (for-each loop) JDK 1.5 introduced a new for loop that enables you to traverse the complete array sequentially without using an index variable. For example, the following code displays all elements in the array myList: for (double value: myList) System.out.println(value); In general, the syntax is for (elementType value: arrayRefVar) { // Process the value } int sum = 0; for (double value: myList) { sum += value; } System.out.println(sum); You still have to use an index variable if you wish to traverse the array in a different order or change the elements in the array.

68. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 68 Problem: Assigning Grades F Objective: read student scores (int), get the best score, and then assign grades based on the following scheme: –Grade is A if score is >= best–10; –Grade is B if score is >= best–20; –Grade is C if score is >= best–30; –Grade is D if score is >= best–40; –Grade is F otherwise. AssignGrade Run

69. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 69 Copying Arrays Often, in a program, you need to duplicate an array or a part of an array. In such cases you could attempt to use the assignment statement (=), as follows: list2 = list1;

70. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 70 Copying Arrays Using a loop: int[] sourceArray = {2, 3, 1, 5, 10}; int[] targetArray = new int[sourceArray.length]; for (int i = 0; i < sourceArrays.length; i++) targetArray[i] = sourceArray[i];

71. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 71 The arraycopy Utility arraycopy(sourceArray, src_pos, targetArray, tar_pos, length); Example: System.arraycopy(sourceArray, 0, targetArray, 0, sourceArray.length);