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

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Title: Chapter 7 Author: Course Technology Last modified by: user Created Date: 11/15/2002 7:59:11 AM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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


1
Java Programming From Problem Analysis to
Program Design, 4e
  • Chapter 7
  • User-Defined Methods

2
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

3
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

4
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

5
Predefined Classes (continued)
6
Predefined Classes (continued)
7
Predefined Classes (continued)
8
Predefined Classes (continued)
9
class Character (Package java.lang)
10
class Character (Package java.lang) (continued)
11
class Character (Package java.lang) (continued)
12
Syntax Value-Returning Method
13
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

14
Syntax
  • Syntax formal parameter list
  • -The syntax of the formal parameter list is
  • Method call
  • -The syntax to call a value-returning method is

15
Syntax (continued)
  • Syntax actual parameter list
  • -The syntax of the actual parameter list is
  • Syntax return statement
  • -The return statement has the following
    syntax
  • return expr

16
Equivalent Method Definitions
public static double larger(double x, double
y) double max if (x gt y)
max x else max y return
max
17
17
Java Programming From Problem Analysis to
Program Design, 4e
18
18
Java Programming From Problem Analysis to
Program Design, 4e
19
Equivalent Method Definitions (continued)
  • public static double larger(double x, double y)
  • if (x gt y)
  • return x
  • else
  • return y

20
Equivalent Method Definitions (continued)
  • public static double larger(double x, double y)
  • if (x gt y)
  • return x
  • return y

21
The int variable num contains the desired sum to
be rolled
21
Java Programming From Problem Analysis to
Program Design, 4e
22
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

23
Solution isPalindrome Method
public static boolean isPalindrome(String str)
int len str.length()
int i, j j len - 1 for (i 0 i lt
(len - 1) / 2 i) if
(str.charAt(i) ! str.charAt(j))
return false j--
return true

24
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

25
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

26
Solution Largest Number
static 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 lt 10 count)
num console.nextDouble() max
larger(max, num) System.out.println("
The largest number is "
max)
27
Sample Run Largest Number
  • Sample Run
  • Enter 10 numbers
  • 10.5 56.34 73.3 42 22 67 88.55 26 62 11
  • The largest number is 88.55

28
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

29
Void Methods with Parameters Syntax
30
Void Methods with Parameters Syntax (continued)
31
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

32
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

33
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

34
Reference Variables as Parameters type String
35
Reference Variables as Parameters type String
(continued)
36
Reference Variables as Parameters type String
(continued)
37
Reference Variables as Parameters type String
(continued)
38
Reference Variables as Parameters type String
(continued)
String str "Hello" //Line 5
38
Java Programming From Problem Analysis to
Program Design, 4e
39
Reference Variables as Parameters type String
(continued)
stringParameter(str) //Line 7
39
Java Programming From Problem Analysis to
Program Design, 4e
40
Reference Variables as Parameters type String
(continued)
pStr "Sunny Day" //Line 14
40
Java Programming From Problem Analysis to
Program Design, 4e
41
Reference Variables as Parameters type String
(continued)
Variables before the statement in Line 8 executes
42
  • 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

43
43
Java Programming From Problem Analysis to
Program Design, 4e
44
44
Java Programming From Problem Analysis to
Program Design, 4e
45
45
Java Programming From Problem Analysis to
Program Design, 4e
46
46
Java Programming From Problem Analysis to
Program Design, 4e
47
47
Java Programming From Problem Analysis to
Program Design, 4e
48
  • 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

49
  • 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

49
Java Programming From Problem Analysis to
Program Design, 4e
50
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

51
Scope of an Identifier within a Class (continued)
  • A methods 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

52
Scope of an Identifier within a Class (continued)
  • 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

53
Scope of an Identifier within a Class (continued)
  • public static void illegalIdentifierDeclaration()
  • int x
  • //block
  • double x //illegal declaration,
  • //x is already declared
  • ...

54
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

55
Scope Rules (continued)
  • Suppose X is an identifier declared within a
    class and outside of every methods 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

56
Scope Rules (continued)
  • 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)
  • //...

57
Scope Rules (continued)
  • public static int w
  • public static void two(int one, int z)
  • char ch
  • int a
  • //block three
  • int x 12
  • //...
  • //end block three
  • //...

58
Scope Rules Demonstrated
59
Scope Rules Demonstrated (continued)
60
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

61
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

62
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

63
Method Overloading (continued)
  • 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

64
Method Overloading (continued)
  • 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)

65
Method Overloading (continued)
  • 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

66
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

67
Programming Example Data Comparison (continued)
  • 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 82.04

68
Programming Example Data Comparison (continued)
69
Chapter Summary
  • Predefined methods
  • User-defined methods
  • Value-returning methods
  • Void methods
  • Formal parameters
  • Actual parameters
  • Flow of Execution

70
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
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