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

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Java Programming From Problem Analysis to
Program Design, 4e

• Chapter 7
• User-Defined Methods

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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

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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

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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

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Predefined Classes (continued)
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Predefined Classes (continued)
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Predefined Classes (continued)
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Predefined Classes (continued)
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class Character (Package java.lang)
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class Character (Package java.lang) (continued)
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class Character (Package java.lang) (continued)
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Syntax Value-Returning Method
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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

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Syntax

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

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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

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

• public static double larger(double x, double y)
• if (x gt y)
• return x
• else
• return y

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Equivalent Method Definitions (continued)

• public static double larger(double x, double y)
• if (x gt y)
• return x
• return y

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

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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

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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

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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

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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)
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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

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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

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Void Methods with Parameters Syntax
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Void Methods with Parameters Syntax (continued)
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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

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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

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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

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

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Java Programming From Problem Analysis to
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• 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

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• 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

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Java Programming From Problem Analysis to
Program Design, 4e
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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

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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

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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

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Scope of an Identifier within a Class (continued)

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

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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

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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

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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)
• //...

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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
• //...

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Scope Rules Demonstrated
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Scope Rules Demonstrated (continued)
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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

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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

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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

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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

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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)

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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

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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

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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

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Programming Example Data Comparison (continued)
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Chapter Summary

• Predefined methods
• User-defined methods
• Value-returning methods
• Void methods
• Formal parameters
• Actual parameters
• Flow of Execution

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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