9781439035665 ppt ch07

Java Programming: From Problem Analysis to Program Design, 4e

Chapter 7User-Defined Methods

2Java Programming: From Problem Analysis to Program Design, 4e

Chapter Objectives

• Understand how methods are used in Java programming

• Learn about standard (predefined) methods and discover how to use them in a program

• Learn about user-defined methods

• Examine value-returning methods, including actual and formal parameters

Chapter Objectives (continued)

• Explore how to construct and use a value-returning, user-defined method in a program

• Learn how to construct and use user-defined void methods in a program

• Explore variables as parameters

• Learn about the scope of an identifier

• Become aware of method overloading

Predefined Classes

• Methods already written and provided by Java

• Organized as a collection of classes (class libraries)

• To use: import package

• Method type: data type of value returned by method

Predefined Classes (continued)

class Character (Package: java.lang)

class Character (Package: java.lang) (continued)

Syntax: Value-Returning Method

User-Defined Methods

• Value-returning methods– Used in expressions– Calculate and return a value– Can save value for later calculation or print value

• modifiers: public, private, protected, static, abstract, final

• returnType: type of the value that the method calculates and returns (using return statement)

• methodName: Java identifier; name of method

Syntax

• Syntax: formal parameter list-The syntax of the formal parameter list is:

• Method call-The syntax to call a value-returning method is:

Syntax (continued)

• Syntax: return statement -The return statement has the following syntax:

return expr;

• Syntax: actual parameter list-The syntax of the actual parameter list is:

Equivalent Method Definitions

public static double larger(double x, double y){ double max;

if (x >= y) max = x; else max = y;

return max;}

Equivalent Method Definitions (continued)

public static double larger(double x, double y){ if (x >= y) return x; else return y;}

Equivalent Method Definitions (continued)

public static double larger(double x, double y){ if (x >= y) return x; return y;}

The int variable num contains the desired sum to be rolled

Palindrome Number

• Palindrome: integer or string that reads the same forwards and backwards

• The method isPalindrome takes a string as a parameter and returns true if the string is a palindrome, false otherwise

Solution: isPalindrome Method

public static boolean isPalindrome(String str){ int len = str.length(); int i, j; j = len - 1;

for (i = 0; i <= (len - 1) / 2; i++) { if (str.charAt(i) != str.charAt(j)) return false; j--; } return true; }

Flow of Execution• Execution always begins with the first statement

in the method main• User-defined methods execute only when called

• Call to method transfers control from caller to called method

• In method call statement, specify only actual parameters, not data type or method type

• Control goes back to caller when method exits

Programming Example: Largest Number

• Input: set of 10 numbers

• Output: largest of 10 numbers

• Solution– Get numbers one at a time– Method largest number: returns the larger of two

numbers– For loop: calls method largest number on each number

received and compares to current largest number

Solution: Largest Numberstatic Scanner console = new Scanner(System.in);

public static void main(String[] args){ double num; double max; int count; System.out.println("Enter 10 numbers."); num = console.nextDouble(); max = num; for (count = 1; count < 10; count++) { num = console.nextDouble(); max = larger(max, num); } System.out.println("The largest number is " + max); }

Sample Run: Largest Number

• Sample Run

Enter 10 numbers:10.5 56.34 73.3 42 22 67 88.55 26 62 11The largest number is 88.55

Void Methods

• Similar in structure to value-returning methods

• Call to method is always stand-alone statement

• Can use return statement to exit method early

Void Methods with Parameters: Syntax

Void Methods with Parameters: Syntax (continued)

Primitive Data Type Variables as Parameters

• A formal parameter receives a copy of its corresponding actual parameter

• If a formal parameter is a variable of a primitive data type:– Value of actual parameter is directly stored– Cannot pass information outside the method– Provides only a one-way link between actual

parameters and formal parameters

Reference Variables as Parameters

• If a formal parameter is a reference variable:– Copies value of corresponding actual parameter– Value of actual parameter is address of the object

where actual data is stored– Both formal and actual parameter refer to same

object

Uses of Reference Variables as Parameters

• Can return more than one value from a method

• Can change the value of the actual object

• When passing address, would save memory space and time, relative to copying large amount of data

Reference Variables as Parameters: type String

Reference Variables as Parameters: type String (continued)

String str = "Hello"; //Line 5

stringParameter(str); //Line 7

pStr = "Sunny Day"; //Line 14

Variables before the statement in Line 8 executes

• The class StringBuffer contains the method append, which allows you to append a string to an existing string, and the method delete, which allows you to delete all the characters of the string

• The assignment operator cannot be used with StringBuffer variables; you must use the operator new (initially) to allocate memory space for a string

Primitive Type Wrapper Classes as Parameters

• If a formal parameter is of the primitive data type and the corresponding actual parameter is a variable, then the formal parameter cannot change the value of the actual parameter

• Only reference variables can pass values outside the method (except, of course, for the return value)

• Corresponding to each primitive data type, Java provides a class so that the values of primitive data types can be wrapped in objects

• The class Integer does not provide a method to change the value of an existing Integer object

• The same is true of other wrapper classes

Primitive Type Wrapper Classes as Parameters (continued)

• If we want to pass a String object as a parameter and also change that object, we can use the class StringBuffer

• Java does not provide any class that wraps primitive type values in objects and when passed as parameters changes their values

• If a method returns only one value of a primitive type, then you can write a value-returning method

• If you encounter a situation that requires you to write a method that needs to pass more than one value of a primitive type, then you should design your own classes

• Appendix D provides the definitions of such classes and shows how to use them in a program

Scope of an Identifier within a Class

• Local identifier: identifier declared within a method or block, which is visible only within that method or block

• Java does not allow the nesting of methods; you cannot include the definition of one method in the body of another method

• Within a method or a block, an identifier must be declared before it can be used; a block is a set of statements enclosed within braces

Scope of an Identifier within a Class (continued)

• A method’s definition can contain several blocks – The body of a loop or an if statement also

form a block• Within a class, outside of every method

definition (and every block), an identifier can be declared anywhere

Java Programming: From Problem Analysis to Program Design, 4e 51

• Within a method, an identifier used to name a variable in the outer block of the method cannot be used to name any other variable in an inner block of the method

• For example, in the method definition on the next slide, the second declaration of the variable x is illegal

public static void illegalIdentifierDeclaration(){ int x; //block { double x; //illegal declaration, //x is already declared ... }}

Scope Rules

• Scope rules of an identifier declared within a class and accessed within a method (block) of the class

• An identifier, say X, declared within a method (block) is accessible:– Only within the block from the point at which it is

declared until the end of the block– By those blocks that are nested within that block

Scope Rules (continued)

• Suppose X is an identifier declared within a class and outside of every method’s definition (block) – If X is declared without the reserved word static (such

as a named constant or a method name), then it cannot be accessed in a static method

– If X is declared with the reserved word static (such as a named constant or a method name), then it can be accessed within a method (block), provided the method (block) does not have any other identifier named X

Example 7-11public class ScopeRules

{ static final double rate = 10.50; static int z; static double t; public static void main(String[] args) { int num; double x, z; char ch; //... } public static void one(int x, char y) { //... }

public static int w; public static void two(int one, int z) { char ch; int a; //block three { int x = 12; //... } //end block three //... }}

Scope Rules: Demonstrated

Scope Rules: Demonstrated (continued)

Method Overloading: An Introduction

• Method overloading: more than one method can have the same name

• Two methods are said to have different formal parameter lists if both methods have:– A different number of formal parameters, or– If the number of formal parameters is the same,

then the data type of the formal parameters, in the order you list, must differ in at least one position

Method Overloading

public void methodOne(int x)

public void methodTwo(int x, double y)

public void methodThree(double y, int x)

public int methodFour(char ch, int x,

double y)

public int methodFive(char ch, int x,

String name)

• These methods all have different formal parameter lists

Method Overloading (continued)

public void methodSix(int x, double y,

char ch)

public void methodSeven(int one, double u,

char firstCh)

• The methods methodSix and methodSeven both have three formal parameters and the data type of the corresponding parameters is the same

• These methods all have the same formal parameter lists

• Method overloading: creating several methods, within a class, with the same name

• The signature of a method consists of the method name and its formal parameter list

• Two methods have different signatures if they have either different names or different formal parameter lists– Note that the signature of a method does not include

the return type of the method

• The following method headings correctly overload the method methodXYZ:

public void methodXYZ()

public void methodXYZ(int x, double y)

public void methodXYZ(double one, int y)

public void methodXYZ(int x, double y,

char ch)

public void methodABC(int x, double y)

public int methodABC(int x, double y)

• Both these method headings have the same name and same formal parameter list

• These method headings to overload the method methodABC are incorrect

• In this case, the compiler will generate a syntax error– Notice that the return types of these method headings are different

Programming Example: Data Comparison

• Input: data from two different files• Data format: course number followed by scores• Output: course number, group number, course

average• Solution

– Read from more than one file, write output to file– Generate bar graphs– User-defined methods and re-use (calculateAverage and printResult)

– Parameter passing

Sample Output

Course No Group No Course Average CSC 1 83.71 2 80.82

ENG 1 82.00 2 78.20

HIS 1 77.69 2 84.15

MTH 1 83.57 2 84.29

PHY 1 83.22 2 82.60

Avg for group 1: 82.04Avg for group 2: 82.01

Programming Example: Data Comparison (continued)

Chapter Summary

• Predefined methods

• User-defined methods– Value-returning methods– Void methods– Formal parameters– Actual parameters

• Flow of Execution

Chapter Summary (continued)

• Primitive data type variables as parameters– One-way link between actual parameters and

formal parameters (limitations caused)

• Reference variables as parameters – Can pass one or more variables from a method– Can change value of actual parameter

• Scope of an identifier within a class• Method overloading

9781439035665 ppt ch07

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