Chapter 5 Methods

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 1

Spring 2013

Chapter 5 Methods


Opening ProblemFind the sum of integers from 1 to 10, from 20 to 30, and from 35 to 45, respectively.


Problemint 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);



Problemint sum = 0;for (int i = 1; i <= 10; i++) sum += i;System.out.println("Sum from 1 to 10 is " + sum);




Solutionpublic static int sum(int i1, int i2) { int sum = 0; for (int i = i1; i <= i2; 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));}


Objectives 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-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). 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).


Defining MethodsA method is a collection of statements that are grouped together to perform an operation.

public static int max(int num1, int num2) {

int result; if (num1 > num2) result = num1; else result = num2; return result;

}

modifier return value

type method name

formal parameters

return value

method body

method header

parameter list

Define a method Invoke a method

int z = max(x, y);

actual parameters (arguments)

method signature


Method SignatureMethod signature is the combination of the method name and the parameter list.



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature


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



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature


Actual ParametersWhen a method is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument.



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature


Return Value TypeA 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.



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature


Return Value TypeA 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.



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature


Example Return Value Types

Return type int– public static int max(int a, int b) {

Returns type integer– public static double max(int a, int b) {

Returns type double– public static void printn(String c) {

Does not return a value, so no return statement in this method.


Method bodyThe method body is the code executed to produce the results.



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature


ModifiersIdentifies how the method is used:

•public: any other program has access to this method

•static: static methods can be called without creating an instance of the class



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature


Modifiers Java modifiers

– public: all methods have access to the method– protected: only methods in the same package

have access to the method– private: only methods in the same class has

access to the method Java structure

– Package: contains one or more classes– Class: contains one or more methods– Subclass: derived from a class


Calling MethodsTesting the max method

This program demonstrates calling a method max to return the largest of the int values

TestMax Run


TestMaxpublic class TestMax { /** Main method */ public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println("The maximum between " + i + " and " + j + " is " + k); } /** Return the max between two numbers */ public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; } }


Calling Methods, cont.

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

pass the value of i pass the value of j

animation


Trace Method Invocation



i is now 5

animation

i5





j is now 2

animation

i j5 2





invoke max(i, j)

animation

i j5 2





invoke max(i, j)Pass the value of i to num1Pass the value of j to num2

animation

i j5 2

num1 num25 2





declare variable result

animation

i j5 2

num1 num2 result5 2





(num1 > num2) is true since num1 is 5 and num2 is 2

animation

i j5 2

num1 num2 result5 2





result is now 5

animation

i j5 2

num1 num2 result5 2 5





return result, which is 5

animation

i j5 2






return max(i, j) and assign the return value to k

animation

i j k5 2 5






Execute the print statement

animation

i j k5 2 5

num1, num2, and result no longer exist


CAUTIONA 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.

public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return –1; }

(a)

Should be

(b)

public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else return –1; }


Reuse Methods from Other ClassesNOTE: 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).

public class Test { public static void main(String[] args) { int a = 4; int b = 8; int c = TestMax.max(a,b); System.out.println("The largest of " + a + " and " + b + " is " + c); }}


Call Stacks

(a) The main method is invoked.

Space required for the main method k: j: 2 i: 5

(b) The max method is invoked.

Space required for the max method num2: 2 num1: 5

(d) The max method is finished and the return value is sent to k.

(e) The main method is finished.

Stack is empty


Space required for the main method k: 5 j: 2 i: 5

(c) The max method is being executed.

Space required for the max method result: 5 num2: 2 num1: 5



Trace Call Stack

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

i is declared and initialized

The main method is invoked.

i: 5

animation


Trace Call Stack


j is declared and initialized


j: 2 i: 5

animation


Trace Call Stack


Declare k


Space required for the main method

k: j: 2 i: 5

animation


Trace Call Stack


Invoke max(i, j)



k: j: 2 i: 5

animation


Trace Call Stack


pass the values of i and j to num1 and num2

The max method is invoked.

num2: 2 num1: 5


k: j: 2 i: 5

animation


Trace Call Stack


pass the values of i and j to num1 and num2


result:

num2: 2 num1: 5


k: j: 2 i: 5

animation


Trace Call Stack


(num1 > num2) is true


result:

num2: 2 num1: 5


k: j: 2 i: 5

animation


Trace Call Stack


Assign num1 to result


Space required for the max method result: 5

num2: 2 num1: 5


k: j: 2 i: 5

animation


Trace Call Stack


Return result and assign it to k


Space required for the max method result: 5

num2: 2 num1: 5

Space required for the main method k:5

j: 2 i: 5

animation


Trace Call Stack


Execute print statement


Space required for the main method k:5

j: 2 i: 5

animation


void Method Example

This type of method does not return a value. The method performs some actions.

TestVoidMethod Run


TestVoidMethodpublic class TestVoidMethod { public static void main(String[] args) { System.out.print("The grade is "); printGrade(78.5); } public static void printGrade(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'); } } }


Passing Parameterspublic 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?

Welcome to JavaWelcome to JavaWelcome to JavaWelcome to JavaWelcome to Java


Pass by Value

This program demonstrates passing values to the methods.

Increment Run

• Pass the value of the variable, not the memory location

• Make changes to value will not change the value in the original variable

Listing 5.2 Testing Pass by value



Pass by Value

Testing Pass by value


TestPassByValue Run


TestPassByValuepublic class TestPassByValue { /** Main method */ public static void main(String[] args) { // Declare and initialize variables int num1 = 1; int num2 = 2; System.out.println("Before invoking the swap method, num1 is " + num1 + " and num2 is " + num2); // Invoke the swap method to attempt to swap two variables swap(num1, num2); System.out.println("After invoking the swap method, num1 is " + num1 + " and num2 is " + num2); } /** Swap two variables */ public static void swap(int n1, int n2) { System.out.println("\tInside the swap method"); System.out.println("\t\tBefore swapping n1 is " + n1 + " n2 is " + n2); // Swap n1 with n2 int temp = n1; n1 = n2; n2 = temp; System.out.println("\t\tAfter swapping n1 is " + n1 + " n2 is " + n2); } }


Pass by Value, cont.

The main method is invoked

The values of num1 and num2 are passed to n1 and n2. Executing swap does not affect num1 and num2.


num2: 2 num1: 1

The swap method is invoked


num2: 2 num1: 1

Space required for the swap method temp:

n2: 2 n1: 1

The swap method is finished


num2: 2 num1: 1

The main method is finished

Stack is empty

After the swap, n2 = 1 and n1 = 2

But the values in num1 and num2 do not change


Modularizing CodeMethods 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.

GreatestCommonDivisorMethod

Run

PrimeNumberMethod

Run


GreatestCommonDivisorMethod

import java.util.Scanner; public class GreatestCommonDivisorMethod { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter first integer: "); int n1 = input.nextInt(); System.out.print("Enter second integer: "); int n2 = input.nextInt(); System.out.println("The greatest common divisor for " + n1 + " and " + n2 + " is " + gcd(n1, n2)); }


GreatestCommonDivisorMethod (cont.) /** Return the gcd of two integers */ public static int gcd(int n1, int n2) { int gcd = 1; // Initial gcd is 1 int k = 1; // Possible gcd while (k <= n1 && k <= n2) { if (n1 % k == 0 && n2 % k == 0) gcd = k; // Update gcd k++; } return gcd; // Return gcd } }


PrimeNumberMethodpublic class PrimeNumberMethod { public static void main(String[] args) { final int NUMBER_OF_PRIMES = 50; // Number of primes to display final int NUMBER_OF_PRIMES_PER_LINE = 10; // Display 10 per line int count = 0; // Count the number of prime numbers int number = 2; // A number to be tested for primeness System.out.println("The first 50 prime numbers are \n"); // Repeatedly find prime numbers


PrimeNumberMethod (cont) while (count < NUMBER_OF_PRIMES) { // Print the prime number and increase the count if (isPrime(number)) { count++; // Increase the count if (count % NUMBER_OF_PRIMES_PER_LINE == 0) { // Print the number and advance to the new line System.out.printf("%-5s\n", number); } else System.out.printf("%-5s", number); } // Check if the next number is prime number++; number++; } } /** Check whether number is prime */


PrimeNumberMethod (cont)public static boolean isPrime(int number) { for (int divisor = 2; divisor <= number / 2; divisor++) { if (number % divisor == 0) { // If true, number is not prime return false; // number is not a prime } } return true; // number is prime } }


Overloading MethodsOverloading the max Method

public static double max(double num1, double num2) {

if (num1 > num2) return num1; else return num2;}

TestMethodOverloading Run

Chapter 5, Lesson 2

57


TestMethodOverloadpublic class TestMethodOverloading { /** Main method */ public static void main(String[] args) { // Invoke the max method with int parameters System.out.println("The maximum between 3 and 4 is " + max(3, 4)); // Invoke the max method with the double parameters System.out.println("The maximum between 3.0 and 5.4 is " + max(3.0, 5.4)); // Invoke the max method with three double parameters System.out.println("The maximum between 3.0, 5.4, and 10.14 is " + max(3.0, 5.4, 10.14)); }

/** Return the max between two int values */ public static int max(int num1, int num2) { if (num1 > num2) return num1; else return num2; } /** Find the max between two double values */


TestMethodOverload (cont)

public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; } /** Return the max among three double values */ public static double max(double num1, double num2, double num3) { return max(max(num1, num2), num3); } }


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.


Ambiguous Invocationpublic 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; }}

Compiler cannot determine if 1 is an integer or a double value and if 2 is an int or double value. This will produce a compile error.


Problem: Converting Decimals to Hexadecimals

Write a method that converts a decimal integer to a hexadecimal.

Decimal2HexConversion Run


Scope of Local VariablesA 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.


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.


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.

public static void method1() { . . for (int i = 1; i < 10; i++) { . . int j; . . . } }

The scope of j

The scope of i


Scope of Local Variables, cont.

public static void method1() { int x = 1; int y = 1;

for (int i = 1; i < 10; i++) {

x += i; }

for (int i = 1; i < 10; i++) {

y += i; } }

It is fine to declare i in two non-nesting blocks public static void method2() {

int i = 1; int sum = 0;

for (int i = 1; i < 10; i++) {

sum += i; } }

It is wrong to declare i in two nesting blocks


Scope of Local Variables, cont.// Fine with no errorspublic static void correctMethod() { 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; }}


Scope of Local Variables, cont.// With no errorspublic static void incorrectMethod() { int x = 1; int y = 1; for (int i = 1; i < 10; i++) { int x = 0; x += i; }System.out.println(“X = “ + x);}

X = 1


Method AbstractionYou can think of the method body as a black box that contains the detailed implementation for the method.

Method Header

Method body Black Box

Optional arguments for Input

Optional return value


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

• Information hiding. Hide the implementation from the user.

• Reduce complexity.


The Math Class Java has predefined classes and methods

– Java API http://java.sun.com/j2se/1.5.0/docs/api/index.html http://www.java2s.com/Code/JavaAPI/CatalogJavaAPI.htm

– java.lang contains Math class import java.lang.Math;

Class constants:– PI– E

Class methods: – Trigonometric Methods – Exponent Methods– Rounding Methods– min, max, abs, and random Methods

http://java.sun.com/j2se/1.5.0/docs/api/index.html

http://www.java2s.com/Code/JavaAPI/CatalogJavaAPI.htm


Trigonometric Methods sin(double a) cos(double a)

tan(double a)

acos(double a)

asin(double a)

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 1.0Math.cos(0) returns 1.0Math.cos(Math.PI / 6)

returns 0.866 Math.cos(Math.PI / 2)

returns 0


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 2.0Math.sqrt(10.5) returns 3.24


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).


Rounding Methods ExamplesMath.ceil(2.1) returns 3.0 Math.ceil(2.0) returns 2.0Math.ceil(-2.0) returns –2.0Math.ceil(-2.1) returns -2.0Math.floor(2.1) returns 2.0Math.floor(2.0) returns 2.0Math.floor(-2.0) returns –2.0Math.floor(-2.1) returns -3.0Math.rint(2.1) returns 2.0Math.rint(2.0) returns 2.0Math.rint(-2.0) returns –2.0Math.rint(-2.1) returns -2.0Math.rint(2.5) returns 2.0Math.rint(-2.5) returns -2.0Math.round(2.6f) returns 3 Math.round(2.0) returns 2 Math.round(-2.0f) returns -2

Math.round(-2.6) returns -3


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 valuein 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 2Math.abs(-2.1) returns

2.1


The random MethodGenerates a random double value greater than or equal to 0.0 and less than 1.0 (0 <= Math.random() < 1.0).

Examples:

(int)(Math.random() * 10) Returns a random integer between 0 and 9.

50 + (int)(Math.random() * 50) Returns a random integer between 50 and 99.

In general, a + Math.random() * b Returns a random number between

a and a + b, excluding a + b.


Case Study: Generating Random Characters

Computer programs process numerical data and characters. You have seen many examples that involve numerical data. It is also important to understand characters and how to process them. As introduced in Section 2.9, each character has a unique Unicode between 0 and FFFF in hexadecimal (65535 in decimal). 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.)

(int)(Math.random() * (65535 + 1))


Case Study: Generating Random Characters, cont.

Now let us consider how to generate a random lowercase letter. The Unicode for lowercase letters are consecutive integers starting from the Unicode for 'a', then for 'b', 'c', ..., and 'z'. The Unicode for 'a' is

(int)'a'

So, a random integer between (int)'a' and (int)'z' is(int)((int)'a' + Math.random() * ((int)'z' - (int)'a' + 1)



As discussed in Chapter 2., all numeric operators can be applied to the char operands. The char operand is cast into a number if the other operand is a number or a character. So, the preceding expression can be simplified as follows:

'a' + Math.random() * ('z' - 'a' + 1)

So a random lowercase letter is

(char)('a' + Math.random() * ('z' - 'a' + 1))



To generalize the foregoing discussion, a random character between any two characters ch1 and ch2 with ch1 < ch2 can be generated as follows:

(char)(ch1 + Math.random() * (ch2 – ch1 + 1))


The RandomCharacter Class// RandomCharacter.java: Generate random characterspublic class RandomCharacter { /** Generate a random character between ch1 and ch2 */ public static char getRandomCharacter(char ch1, char ch2) { return (char)(ch1 + Math.random() * (ch2 - ch1 + 1)); } /** Generate a random lowercase letter */ public static char getRandomLowerCaseLetter() { return getRandomCharacter('a', 'z'); } /** Generate a random uppercase letter */ public static char getRandomUpperCaseLetter() { return getRandomCharacter('A', 'Z'); } /** Generate a random digit character */ public static char getRandomDigitCharacter() { return getRandomCharacter('0', '9'); } /** Generate a random character */ public static char getRandomCharacter() { return getRandomCharacter('\u0000', '\uFFFF'); }}

TestRandomCharacter

Run

RandomCharacter


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.


PrintCalender Case Study Let us use the PrintCalendar example to demonstrate the stepwise refinement approach.

PrintCalendar Run


Design Diagram printCalendar

(main)

readInput printMonth

getStartDay

printMonthTitle printMonthBody

getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear



(main)


getStartDay


getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear



(main)


getStartDay


getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear



(main)


getStartDay


getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear



(main)


getStartDay


getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear



(main)


getStartDay


getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear


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:


Implementation: Bottom-UpBottom-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.

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Chapter 5 Methods