Programming Fundamentals I COSC 1336 Lecture 5 prepared

Programming Fundamentals I (COSC 1336), Lecture 5 (prepared after Chapter 5 of Liang’s 2011 textbook) Stefan Andrei 11/3/2020 COSC-1336, Lecture 5 1

Overview of Previous Lecture F F F F 11/3/2020 To write programs for executing statements repeatedly using a while loop (§ 4. 2). To develop a program for Guess. Number and Subtraction. Quiz. Loop (§ 4. 2. 1). To follow the loop design strategy to develop loops (§ 4. 2. 2). To develop a program for Subtraction. Quiz. Loop (§ 4. 2. 3). To control a loop with a sentinel value (§ 4. 2. 3). To obtain large input from a file using input redirection rather than typing from the keyboard (§ 4. 2. 4). To write loops using do-while statements (§ 4. 3). COSC-1336, Lecture 5 2

Overview of Previous Lecture F To write loops using for statements (§ 4. 4). (cont) F F F 11/3/2020 To discover the similarities and differences of three types of loop statements (§ 4. 5). To write nested loops (§ 4. 6). To learn the techniques for minimizing numerical errors (§ 4. 7). To learn loops from a variety of examples (GCD, Future. Tuition, Monte. Carlo. Simulation) (§ 4. 8). To implement program control with break and continue (§ 4. 9). (GUI) To control a loop with a confirmation dialog (§ 4. 10). COSC-1336, Lecture 5 3

Motivation of the current lecture n Find the sum of integers from 1 to 10, from 20 to 30, and from 35 to 45, respectively. n 11/3/2020 COSC-1336, Lecture 5 4

First Attempt int sum = 0; for (int i = 1; i <= 10; i++) sum += i; System. out. println("Sum from 1 to 10 is " + sum); sum = 0; for (int i = 20; i <= 30; i++) sum += i; System. out. println("Sum from 20 to 30 is " + sum); sum = 0; for (int i = 35; i <= 45; i++) sum += i; System. out. println("Sum from 35 to 45 is " + sum); 11/3/2020 COSC-1336, Lecture 5 5

Problem int sum = 0; for (int i = 1; i <= 10; i++) sum += i; System. out. println("Sum from 1 to 10 is " + sum); sum = 0; for (int i = 20; i <= 30; i++) sum += i; System. out. println("Sum from 20 to 30 is " + sum); sum = 0; for (int i = 35; i <= 45; i++) sum += i; System. out. println("Sum from 35 to 45 is " + sum); 11/3/2020 COSC-1336, Lecture 5 6

Solution public static int sum(int i 1, int i 2) { int sum = 0; for (int i = i 1; i <= i 2; i++) sum += i; return sum; } public static void main(String[] args) { System. out. println("Sum from 1 to 10 is " + sum(1, 10)); System. out. println("Sum from 20 to 30 is " + sum(20, 30)); System. out. println("Sum from 35 to 45 is " + sum(35, 45)); } 11/3/2020 COSC-1336, Lecture 5 7

Overview of This Lecture n n To define methods, invoke methods, and pass arguments to a method (§ 5. 2 -5. 5). To develop reusable code that is modular, easy-toread, easy-to-debug, and easy-to-maintain. (§ 5. 6). To use method overloading and understand ambiguous overloading (§ 5. 7). To design and implement overloaded methods (§ 5. 8). 11/3/2020 COSC-1336, Lecture 5 8

Overview of This Lecture (cont. ) n n To determine the scope of variables (§ 5. 9). To know how to use the methods in the Math class (§§ 5. 10 -5. 11). To learn the concept of method abstraction (§ 5. 12). To design and implement methods using stepwise refinement (§ 5. 12). 11/3/2020 COSC-1336, Lecture 5 9

Defining Methods §A method is a collection of statements that are grouped together to perform an operation. 11/3/2020 COSC-1336, Lecture 5 10

Method Signature §Method signature is the combination of the method name and the parameter list. 11/3/2020 COSC-1336, Lecture 5 11

Formal Parameters §The variables defined in the method header are known as formal parameters. 11/3/2020 COSC-1336, Lecture 5 12

Actual Parameters § When a method is invoked, you pass a value to the parameter. § This value is referred to as actual parameter or argument. 11/3/2020 COSC-1336, Lecture 5 13

Return Value Type § A method may return a value. § The return value type is the data type of the value the method returns. § If the method does not return a value, the return value type is the keyword void. § For example, the return value type in the main() method is void. 11/3/2020 COSC-1336, Lecture 5 14

Calling Methods § Testing the max() method; § This program demonstrates calling a method max to return the largest of the int values. 11/3/2020 COSC-1336, Lecture 5 15

animation Calling Methods (cont. ) 11/3/2020 COSC-1336, Lecture 5 16

animation Trace Method Invocation i is now 5 11/3/2020 COSC-1336, Lecture 5 17

animation Trace Method Invocation j is now 2 11/3/2020 COSC-1336, Lecture 5 18

animation Trace Method Invocation invoke max(i, j) 11/3/2020 COSC-1336, Lecture 5 19

animation Trace Method Invocation invoke max(i, j) Pass the value of i to num 1 Pass the value of j to num 2 11/3/2020 COSC-1336, Lecture 5 20

animation Trace Method Invocation declare variable result 11/3/2020 COSC-1336, Lecture 5 21

animation Trace Method Invocation (num 1 > num 2) is true since num 1 is 5 and num 2 is 2 11/3/2020 COSC-1336, Lecture 5 22

animation Trace Method Invocation result is now 5 11/3/2020 COSC-1336, Lecture 5 23

animation Trace Method Invocation return result, which is 5 11/3/2020 COSC-1336, Lecture 5 24

animation Trace Method Invocation return max(i, j) and assign the return value to k 11/3/2020 COSC-1336, Lecture 5 25

animation Trace Method Invocation Call the println() method 11/3/2020 COSC-1336, Lecture 5 26

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. 11/3/2020 COSC-1336, Lecture 5 27

Reuse Methods from Other Classes n NOTE: One of the benefits of methods is for reuse. n The max() method can be invoked from any class besides Test. Max. n If you create a new class Test, you can invoke the max() method using Class. Name. method. Name (e. g. , Test. Max. max). 11/3/2020 COSC-1336, Lecture 5 28

Call Stacks 11/3/2020 COSC-1336, Lecture 5 29

animation Trace Call Stack i is declared and initialized 11/3/2020 COSC-1336, Lecture 5 30

animation Trace Call Stack j is declared and initialized 11/3/2020 COSC-1336, Lecture 5 31

animation Trace Call Stack Declare k 11/3/2020 COSC-1336, Lecture 5 32

animation Trace Call Stack Invoke max(i, j) 11/3/2020 COSC-1336, Lecture 5 33

animation Trace Call Stack pass the values of i and j to num 1 and num 2 11/3/2020 COSC-1336, Lecture 5 34

animation Trace Call Stack declare variable result 11/3/2020 COSC-1336, Lecture 5 35

animation Trace Call Stack (num 1 > num 2) is true 11/3/2020 COSC-1336, Lecture 5 36

animation Trace Call Stack Assign num 1 to result 11/3/2020 COSC-1336, Lecture 5 37

animation Trace Call Stack Return result and assign it to k 11/3/2020 COSC-1336, Lecture 5 38

animation Trace Call Stack Call the println() method 11/3/2020 COSC-1336, Lecture 5 39

void Method Example § This type of method does not return a value. § The method performs some actions. public class Test. Void. Method { public static void main(String[] args) { System. out. print("The grade is "); print. Grade(78. 5); System. out. print("The grade is "); print. Grade(59. 5); } //. . . public static void print. Grade() } 11/3/2020 COSC-1336, Lecture 5 40

Method public static void print. Grade() public static void print. Grade(double score) { if (score >= 90. 0) { System. out. println('A'); } else if (score >= 80. 0) { System. out. println('B'); } else if (score >= 70. 0) { System. out. println('C'); } else if (score >= 60. 0) { System. out. println('D'); } else { System. out. println('F'); } } 11/3/2020 COSC-1336, Lecture 5 41

Compiling and running Test. Void. Method. java 11/3/2020 COSC-1336, Lecture 5 42

Passing Parameters public static void n. Println(String message, int n) { for (int i = 0; i < n; i++) System. out. println(message); } § Suppose you invoke the method using n. Println("Welcome to Java", 5); § What is the output? § Suppose you invoke the method using n. Println("Computer Science", 15); § What is the output? 11/3/2020 COSC-1336, Lecture 5 43

Pass by Value § This program demonstrates passing values to the methods. public class Increment { public static void main(String[] args) { int x = 1; System. out. println("Before the call, x is " + x); increment(x); System. out. println("after the call, x is " + x); } public static void increment(int n) { n++; System. out. println("n inside the method is " + n); } } 11/3/2020 COSC-1336, Lecture 5 44

Compiling and running Increment. java 11/3/2020 COSC-1336, Lecture 5 45

Pass by Value § Testing Pass by value. § This program demonstrates passing values to the methods. public class Test. Pass. By. Value { public static void main(String[] args) { int num 1 = 1, num 2 = 2; System. out. println("Before invoking the swap method, num 1 is " + num 1 + " and num 2 is " + num 2); swap(num 1, num 2); System. out. println("After invoking the swap method, num 1 is " + num 1 + " and num 2 is " + num 2); } //. . . } 11/3/2020 COSC-1336, Lecture 5 46

public static void swap() /** Swap two variables */ public static void swap(int n 1, int n 2) { System. out. println("t. Inside the swap method"); System. out. println("tt. Before swapping n 1 is " + n 1 + " n 2 is " + n 2); int temp = n 1; n 1 = n 2; n 2 = temp; System. out. println("tt. After swapping n 1 is " + n 1 + " n 2 is " + n 2); } 11/3/2020 COSC-1336, Lecture 5 47

Compiling and running Test. Pass. By. Value. java 11/3/2020 COSC-1336, Lecture 5 48

Pass by Value (cont. ) 11/3/2020 COSC-1336, Lecture 5 49

Modularizing Code n Methods can be used to reduce redundant coding and enable code reuse. n Methods can also be used to modularize code and improve the quality of the program. 11/3/2020 COSC-1336, Lecture 5 50

Method Overloading Method overloading is the process of giving a single method name multiple definitions. n If a method is overloaded, the method name is not sufficient to determine which method is being called. n The signature of each overloaded method must be unique. n The signature includes the number, type, and order of the parameters. n The return type of the method is not part of the signature. n That is, overloaded methods cannot differ only by their return type. n 11/3/2020 COSC-1336, Lecture 5 51

Method Overloading n The compiler determines which method is being invoked by analyzing the parameters (the first signature is (int) and the second one is (int, double): float method 1(int x) { return x +. 375; } Invocation result = method 1(25, 4. 32) float method 1(int x, double y) { return x * y; } 11/3/2020 COSC-1336, Lecture 5 52

Method Overloading n The println() method is overloaded: println(String s) println(int i) println(double d) and so on. . . n The following lines invoke different versions of the println() method: System. out. println("The total is: "); System. out. println(total); 11/3/2020 COSC-1336, Lecture 5 53

Overloading Methods n Overloading the max() method: public static double max(double num 1, double num 2) { if (num 1 > num 2) return num 1; else return num 2; } 11/3/2020 COSC-1336, Lecture 5 54

Test. Method. Overloading. java public class Test. Method. Overloading { public static void main(String[] args) { System. out. println("The maximum between 3 and 4 is " + max(3, 4)); System. out. println("The maximum between 3. 0 and 5. 4 is " + max(3. 0, 5. 4)); System. out. println("The maximum between 3. 0, 5. 4, and 10. 14 is " + max(3. 0, 5. 4, 10. 14)); } //. . . The max() methods come next } 11/3/2020 COSC-1336, Lecture 5 55

Test. Method. Overloading. java (cont. ) public static int max(int num 1, int num 2) { if (num 1 > num 2) return num 1; else return num 2; } public static double max(double num 1, double num 2){ if (num 1 > num 2) return num 1; else return num 2; } public static double max(double num 1, double num 2, double num 3) { return max(num 1, num 2), num 3); } 11/3/2020 COSC-1336, Lecture 5 56

Ambiguous Invocation n Sometimes there may be two or more possible matches for an invocation of a method, but the compiler cannot determine the most specific match. n This is referred to as ambiguous invocation. n Ambiguous invocation is a compilation error. 11/3/2020 COSC-1336, Lecture 5 57

Ambiguous Invocation public class Ambiguous. Overloading { public static void main(String[] args) { System. out. println(max(1, 2)); } public static double max(int num 1, double num 2) { if (num 1 > num 2) return num 1; else return num 2; } public static double max(double num 1, int num 2) { if (num 1 > num 2) return num 1; else return num 2; } } 11/3/2020 COSC-1336, Lecture 5 58

Type error during compilation 11/3/2020 COSC-1336, Lecture 5 59

Scope of Local Variables n n 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. 11/3/2020 COSC-1336, Lecture 5 60

Scope of Local Variables (cont. ) n 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. 11/3/2020 COSC-1336, Lecture 5 61

Scope of Local Variables (cont. ) n A variable declared in the initial action part of a for loop header has its scope in the entire loop. n 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. 11/3/2020 COSC-1336, Lecture 5 62

Scope of Local Variables (cont. ) 11/3/2020 COSC-1336, Lecture 5 63

Scope of Local Variables (cont. ) // Fine with no errors public static void correct. Method() { int x = 1; int y = 1; // i is declared for (int i = 1; i < 10; i++) { x += i; } // i is declared again for (int i = 1; i < 10; i++) { y += i; } } 11/3/2020 COSC-1336, Lecture 5 64

Scope of Local Variables (cont. ) // With no errors public static void incorrect. Method() { int x = 1; int y = 1; for (int i = 1; i < 10; i++) { int x = 0; x += i; } } 11/3/2020 COSC-1336, Lecture 5 65

Method Abstraction n You can think of the method body as a black box that contains the detailed implementation for the method. 11/3/2020 COSC-1336, Lecture 5 66

Benefits of Methods § § Write a method once and reuse it anywhere. Information hiding: hide the implementation from the user. § Reduce complexity. 11/3/2020 COSC-1336, Lecture 5 67

The Math Class n Class constants: q q n PI E Class methods: Trigonometric Methods q Exponent Methods q Rounding Methods q min(), max(), abs(), and random() Methods q 11/3/2020 COSC-1336, Lecture 5 68

Trigonometric Methods n sin(double a) § Examples: n cos(double a) Math. sin(0) returns 0. 0 n tan(double a) Math. sin(Math. PI / 6) returns 0. 5 n asin(double a) n acos(double a) n atan(double a) Radians 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 to. Radians(90) 11/3/2020 COSC-1336, Lecture 5 69

Exponent Methods n exp(double a) Returns e raised to the power of a. n log(double a) Returns the natural logarithm of a. n n n Examples: Math. exp(1) returns 2. 71 Math. log(2. 71) returns 1. 0 Math. pow(2, 3) returns 8. 0 log 10(double a) Math. pow(3, 2) returns 9. 0 Returns the 10 -based logarithm of a. Math. pow(3. 5, 2. 5) returns 22. 91765 pow(double a, double b) Math. sqrt(4) returns 2. 0 Returns a raised to the power of b. Math. sqrt(10. 5) returns 3. 24 sqrt(double a) Returns the square root of a. 11/3/2020 COSC-1336, Lecture 5 70

Rounding Methods n double ceil(double x) x rounded up to its nearest integer. This integer is returned as a double value. n double floor(double x) x is rounded down to its nearest integer. This integer is returned as a double value. n 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. n int round(float x) Return (int)Math. floor(x+0. 5). n long round(double x) Return (long)Math. floor(x+0. 5). 11/3/2020 COSC-1336, Lecture 5 71

Rounding Methods Examples Math. ceil(2. 1) returns 3. 0 Math. rint(2. 0) returns 2. 0 Math. ceil(2. 0) returns 2. 0 Math. rint(-2. 0) returns – 2. 0 Math. ceil(-2. 0) returns – 2. 0 Math. rint(-2. 1) returns -2. 0 Math. ceil(-2. 1) returns -2. 0 Math. rint(2. 5) returns 2. 0 Math. floor(2. 1) returns 2. 0 Math. rint(-2. 5) returns -2. 0 Math. floor(2. 0) returns 2. 0 Math. round(2. 6 f) returns 3 Math. floor(-2. 0) returns – 2. 0 Math. round(2. 0) returns 2 Math. floor(-2. 1) returns -3. 0 Math. round(-2. 0 f) returns -2 Math. rint(2. 1) returns 2. 0 Math. round(-2. 6) returns -3 11/3/2020 COSC-1336, Lecture 5 72

min(), max(), and abs() n max(a, b)and min(a, b) Examples: Returns the maximum or minimum of two parameters. n Math. max(2, 3) returns 3 abs(a) Returns the absolute value of the parameter. n random() Returns a random double value in the range [0. 0, 1. 0). 11/3/2020 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 COSC-1336, Lecture 5 73

The random() Method n 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, 11/3/2020 COSC-1336, Lecture 5 74

Case Study: Generating Random Characters n Computer programs process numerical data and characters. n You have seen many examples that involve numerical data. n It is also important to understand characters and how to process them. n As introduced in Section 2. 9, each character has a unique Unicode between 0 and FFFF in hexadecimal (65535 in decimal). 11/3/2020 COSC-1336, Lecture 5 75

Case Study: Generating Random Characters (cont. ) n To generate a random character is to generate a random integer between 0 and 65535 using the following expression: note that since 0 <= Math. random() < 1. 0, you have to add 1 to 65535. q (int)(Math. random() * (65535 + 1)) q 11/3/2020 COSC-1336, Lecture 5 76

Case Study: Generating Random Characters (cont. ) n Now let us consider how to generate a random lowercase letter. n The Unicode for lowercase letters are consecutive integers starting from the Unicode for 'a', then for 'b', 'c', . . . , and 'z'. n The Unicode for 'a' is (int)'a'. n So, a random integer between (int)'a' and (int)'z' is: (int)((int)'a' + Math. random() * ((int)'z' – (int)'a' + 1) q 11/3/2020 COSC-1336, Lecture 5 77

Case Study: Generating Random Characters (cont. ) n As discussed in Chapter 2, all numeric operators can be applied to the char operands. n The char operand is cast into a number if the other operand is a number or a character. n So, the preceding expression can be simplified as follows: 'a' + Math. random() * ('z' - 'a' + 1) n So a random lowercase letter is (char)('a' + Math. random() * ('z' - 'a' + 1)) 11/3/2020 COSC-1336, Lecture 5 78

Case Study: Generating Random Characters (cont. ) n To generalize the foregoing discussion, a random character between any two characters ch 1 and ch 2 with ch 1 < ch 2 can be generated as follows: (char)(ch 1 + Math. random() * (ch 2 – ch 1 + 1)) 11/3/2020 COSC-1336, Lecture 5 79

The Random. Character Class // Random. Character. java: Generate random characters public class Random. Character { /** Generate a random character between ch 1 and ch 2 */ public static char get. Random. Character(char ch 1, char ch 2) { return (char)(ch 1 + Math. random() * (ch 2 - ch 1 + 1)); } /** Generate a random lowercase letter */ public static char get. Random. Lower. Case. Letter() { return get. Random. Character('a', 'z'); } /** Generate a random uppercase letter */ public static char get. Random. Upper. Case. Letter() { return get. Random. Character('A', 'Z'); } /** Generate a random digit character */ public static char get. Random. Digit. Character() { return get. Random. Character('0', '9'); } /** Generate a random character */ public static char get. Random. Character() { return get. Random. Character('u 0000', 'u. FFFF'); } } 11/3/2020 COSC-1336, Lecture 5 80

Stepwise Refinement (Optional) n The concept of method abstraction can be applied to the process of developing programs. n When writing a large program, you can use the “divide and conquer” strategy, also known as stepwise refinement, to decompose it into subproblems. n The subproblems can be further decomposed into smaller, more manageable problems. 11/3/2020 COSC-1336, Lecture 5 81

Print. Calendar Case Study n Let us use the Print. Calendar example to demonstrate the stepwise refinement approach. 11/3/2020 COSC-1336, Lecture 5 82

Design Diagram 11/3/2020 COSC-1336, Lecture 5 83

Design Diagram 11/3/2020 COSC-1336, Lecture 5 84

Design Diagram 11/3/2020 COSC-1336, Lecture 5 85

Design Diagram 11/3/2020 COSC-1336, Lecture 5 86

Design Diagram 11/3/2020 COSC-1336, Lecture 5 87

Design Diagram 11/3/2020 COSC-1336, Lecture 5 88

Implementation: Top-Down § 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 print. Month() method. § For example, let print. Month() display the year and the month in the stub. 11/3/2020 COSC-1336, Lecture 5 89

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. 11/3/2020 COSC-1336, Lecture 5 90

Summary n n To define methods, invoke methods, and pass arguments to a method (§ 5. 2 -5. 5). To develop reusable code that is modular, easy-toread, easy-to-debug, and easy-to-maintain. (§ 5. 6). To use method overloading and understand ambiguous overloading (§ 5. 7). To design and implement overloaded methods (§ 5. 8). 11/3/2020 COSC-1336, Lecture 5 91

Summary (cont. ) n n To determine the scope of variables (§ 5. 9). To know how to use the methods in the Math class (§§ 5. 10 -5. 11). To learn the concept of method abstraction (§ 5. 12). To design and implement methods using stepwise refinement (§ 5. 12). 11/3/2020 COSC-1336, Lecture 5 92

Reading suggestions n From [Liang: Introduction to Java programming: Eight Edition, 2011 Pearson Education, 0132130807] q Chapter 5 (Methods) 11/3/2020 COSC-1336, Lecture 5 93

Coming up next n From [Liang: Introduction to Java programming: Eight Edition, 2011 Pearson Education, 0132130807] q 11/3/2020 Chapter 9 (Strings) COSC-1336, Lecture 5 94

Thank you for your attention! Questions? 11/3/2020 COSC-1336, Lecture 5 95