Chapter 5 Methods Liang Introduction to Java Programming

Opening Problem Find the sum of integers from 1 to 10, from 20 to

Problem int sum = 0; for (int i = 1; i <= 10; i++)

Solution public static int sum(int i 1, int i 2) { int sum =

Objectives F F F F To define methods, invoke methods, and pass arguments to

Defining Methods A method is a collection of statements that are grouped together to

Method Signature Method signature is the combination of the method name and the parameter

Formal Parameters The variables defined in the method header are known as formal parameters.

Actual Parameters When a method is invoked, you pass a value to the parameter.

Return Value Type A method may return a value. The return. Value. Type is

Calling Methods Testing the max method This program demonstrates calling a method max to

animation Calling Methods, cont. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson

animation Trace Method Invocation i is now 5 Liang, Introduction to Java Programming, Eighth

animation Trace Method Invocation j is now 2 Liang, Introduction to Java Programming, Eighth

animation Trace Method Invocation invoke max(i, j) Liang, Introduction to Java Programming, Eighth Edition,

animation Trace Method Invocation invoke max(i, j) Pass the value of i to num

animation Trace Method Invocation declare variable result Liang, Introduction to Java Programming, Eighth Edition,

animation Trace Method Invocation (num 1 > num 2) is true since num 1

animation Trace Method Invocation result is now 5 Liang, Introduction to Java Programming, Eighth

animation Trace Method Invocation return result, which is 5 Liang, Introduction to Java Programming,

animation Trace Method Invocation return max(i, j) and assign the return value to k

animation Trace Method Invocation Execute the print statement Liang, Introduction to Java Programming, Eighth

CAUTION A return statement is required for a value-returning method. The method shown below

Reuse Methods from Other Classes NOTE: One of the benefits of methods is for

Call Stacks Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc.

animation Trace Call Stack i is declared and initialized Liang, Introduction to Java Programming,

animation Trace Call Stack j is declared and initialized Liang, Introduction to Java Programming,

animation Trace Call Stack Declare k Liang, Introduction to Java Programming, Eighth Edition, (c)

animation Trace Call Stack Invoke max(i, j) Liang, Introduction to Java Programming, Eighth Edition,

animation Trace Call Stack pass the values of i and j to num 1

animation Trace Call Stack (num 1 > num 2) is true Liang, Introduction to

animation Trace Call Stack Assign num 1 to result Liang, Introduction to Java Programming,

animation Trace Call Stack Return result and assign it to k Liang, Introduction to

animation Trace Call Stack Execute print statement Liang, Introduction to Java Programming, Eighth Edition,

void Method Example This type of method does not return a value. The method

F public class Test. Void. Method { F public static void print. Grade(double score)

Passing Parameters public static void n. Println(String message, int n) { for (int i

Pass by Value This program demonstrates passing values to the methods. Increment Liang, Introduction

F F public class Increment { public static void main(String[] args) { int x

Pass by Value Testing Pass by value This program demonstrates passing values to the

F F public class Test. Pass. By. Value { public static void main(String[] args)

Pass by Value, cont. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson

Modularizing Code Methods can be used to reduce redundant coding and enable code reuse.

F F F F import java. util. Scanner; public class Greatest. Common. Divisor. Method

Overloading Methods Overloading the max Method public static double max(double num 1, double num

F F public class Test. Method. Overloading { public static void main(String[] args) {

Ambiguous Invocation Sometimes there may be two or more possible matches for an invocation

Ambiguous Invocation public class Ambiguous. Overloading { public static void main(String[] args) { System.

Scope of Local Variables A local variable: a variable defined inside a method. Scope:

Scope of Local Variables, cont. You can declare a local variable with the same

Scope of Local Variables, cont. A variable declared in the initial action part of

Scope of Local Variables, cont. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011

Scope of Local Variables, cont. // Fine with no errors public static void correct.

Scope of Local Variables, cont. // With no errors public static void incorrect. Method()

Method Abstraction You can think of the method body as a black box that

Benefits of Methods • Write a method once and reuse it anywhere. • Information

The Math Class F Class constants: – PI –E F Class methods: – Trigonometric

Trigonometric Methods F sin(double a) F cos(double a) F tan(double a) F acos(double a)

Exponent Methods F exp(double a) Returns e raised to the power of a. Examples:

Rounding Methods F double ceil(double x) x rounded up to its nearest integer. This

Rounding Methods Examples Math. ceil(2. 1) returns 3. 0 Math. ceil(2. 0) returns 2.

min, max, and abs F max(a, b)and min(a, b) Returns the maximum or minimum

The random Method Generates a random double value greater than or equal to 0.

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Opening Problem Find the sum of integers from 1 to 10, from 20 to 30, and from 35 to 45, respectively. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 2

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); Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 3

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)); } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 5

Objectives F F F F To define methods, invoke methods, and pass arguments to a method (§ 5. 2 -5. 5). To develop reusable code that is modular, easy-to-read, easy-todebug, and easy-to-maintain. (§ 5. 6). To use method overloading and understand ambiguous overloading (§ 5. 7). To design and implement overloaded methods (§ 5. 8). 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). Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 6

Defining Methods A method is a collection of statements that are grouped together to perform an operation. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 7

Method Signature Method signature is the combination of the method name and the parameter list. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 8

Formal Parameters The variables defined in the method header are known as formal parameters. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 9

Actual Parameters When a method is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 10

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. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 11

Calling Methods Testing the max method This program demonstrates calling a method max to return the largest of the int values Test. Max Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 12

animation Trace Method Invocation invoke max(i, j) Pass the value of i to num 1 Pass the value of j to num 2 Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 17

animation Trace Method Invocation (num 1 > num 2) is true since num 1 is 5 and num 2 is 2 Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 19

animation Trace Method Invocation return max(i, j) and assign the return value to k Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 22

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. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 24

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 Test. Max. If you create a new class Test, you can invoke the max method using Class. Name. method. Name (e. g. , Test. Max. max). Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 25

void Method Example This type of method does not return a value. The method performs some actions. Test. Void. Method Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 37

F public class Test. Void. Method { F 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'); } } public static void main(String[] args) { System. out. print("The grade is "); print. Grade(78. 5); F F F F F F System. out. print("The grade is "); print. Grade(59. 5); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 38

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? Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 39

Pass by Value This program demonstrates passing values to the methods. Increment Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 40

F F public class Increment { public static void main(String[] args) { int x = 1; System. out. println("Before the call, x is " + x); F increment(x); F System. out. println("after the call, x is " + x); F } F public static void increment(int n) { n++; System. out. println("n inside the method is " + n); } F F } F OUTPUT F Before the call, x is n inside the method is after the call, x is F F 1 2 1 Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 41

Pass by Value Testing Pass by value This program demonstrates passing values to the methods. Test. Pass. By. Value Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 42

F F public class Test. Pass. By. Value { public static void main(String[] args) { int num 1 = 1; int num 2 = 2; System. out. println("Before Calling, num 1 is " + num 1 + " and num 2 is " + num 2); swap(num 1, num 2); F F F System. out. println("After Calling, num 1 is " + num 1 + " and num 2 is " + num 2); F F } public static void swap(int n 1, int n 2) { System. out. println("Before swapping n 1 is " + n 1 + " n 2 is " + n 2); F int temp = n 1; n 1 = n 2; n 2 = temp; F F F System. out. println("After swapping n 1 is " + n 1+ " n 2 is " + n 2); } F F F } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 43

Modularizing Code Methods can be used to reduce redundant coding and enable code reuse. Methods can also be used to modularize code and improve the quality of the program. Greatest. Common. Divisor. Method Prime. Number. Method Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 45

F F F F import java. util. Scanner; public class Greatest. Common. Divisor. Method { public static void main(String[] arg) Scanner input = new Scanner(System. in); System. out. print("Enter first integer: "); int n 1 = input. next. Int(); System. out. print("Enter second integer: "); int n 2 = input. next. Int(); System. out. println("The greatest common divisor for " + n 1 + " and " + n 2 + " is " + gcd(n 1, n 2)); } public static int gcd(int n 1, int n 2) { int gcd = 1; // Initial gcd is 1 int k = 1; // Possible gcd F while (k <= n 1 && k <= n 2) { if (n 1 % k == 0 && n 2 % k == 0) gcd = k; // Update gcd k++; } return gcd; // Return gcd F F F } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 46

Overloading Methods 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; } Test. Method. Overloading Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 47

F F public class Test. Method. Overloading { public static void main(String[] args) { F 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 F System. out. println("The maximum between 3. 0, 5. 4, 10. 14 is“+ max(3. 0, 5. 4, 10. 14)); } public static int max(int num 1, int num 2) { if (num 1 > num 2) return num 1; else return num 2; F F F } F public static double max(double num 1, double num 2) { if (num 1 > num 2) return num 1; else return num 2; } F F F public static double max(double num 1, double num 2, double num 3) { return max(num 1, num 2), num 3); } F F + max(3. 0, 5. 4)); } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 48

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. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 49

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; } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 50

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. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 51

Scope of Local Variables, cont. You can declare a local variable with the same name multiple times in different nonnesting blocks in a method, but you cannot declare a local variable twice in nested blocks. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 52

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. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 53

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; } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 55

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; } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 56

Method Abstraction You can think of the method body as a black box that contains the detailed implementation for the method. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 57

Benefits of Methods • Write a method once and reuse it anywhere. • Information hiding. Hide the implementation from the user. • Reduce complexity. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 58

The Math Class F Class constants: – PI –E F Class methods: – Trigonometric Methods – Exponent Methods – Rounding Methods – min, max, abs, and random Methods Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 59

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 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 to. Radians(90) Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 60

Exponent Methods F exp(double a) Returns e raised to the power of a. Examples: F log(double a) Returns the natural logarithm of a. F log 10(double a) 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 Returns the 10 -based logarithm of a. F pow(double a, double b) Returns a raised to the power of b. F sqrt(double a) Returns the square root of a. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 61

Rounding Methods F double ceil(double x) x rounded up to its nearest integer. This integer is returned as a double value. F double floor(double x) x is rounded down to its nearest integer. This integer is returned as a double value. F 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. F int round(float x) Return (int)Math. floor(x+0. 5). F long round(double x) Return (long)Math. floor(x+0. 5). Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 62

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. 6 f) returns 3 Math. round(2. 0) returns 2 Math. round(-2. 0 f) returns -2 Math. round(-2. 6) returns -3 Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 63

min, max, and abs F max(a, b)and min(a, b) Returns the maximum or minimum of two parameters. F abs(a) Returns the absolute value of the parameter. F 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 Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 64

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, Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 65