Chapter 7

Report
Chapter 7: User-Defined
Methods
Java Programming:
From Problem Analysis to Program Design,
Second Edition
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
 Explore how to construct and use a valuereturning, 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: The data type of the value
returned by the method.
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Predefined Classes
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Predefined Classes
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Predefined Classes
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class Character (Package: java.lang)
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class Character (Package: java.lang)
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Syntax of Value-Returning Method
modifier(s) returnType methodName
(formal parameter list)
{
statements
}
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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 of formal parameter list:
dataType identifier, dataType
identifier,...
 Syntax to call a value-returning method:
methodName(actual parameter list)
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Syntax
 Syntax of the actual parameter list:
expression or variable, expression
or variable, ...
 Syntax of the return statement:
return expr;
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Equivalent Method Definitions
public static double larger(double x, double y)
{
double max;
if (x >= y)
max = x;
else
max = y;
return max;
}
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Equivalent Method Definitions
public static double larger(double x, double
y)
{
if (x >= y)
return x;
else
return y;
}
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Equivalent Method Definitions
public static double larger(double x, double y)
{
if (x >= y)
return x;
return y;
}
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Programming Example:
Palindrome Number
 Palindrome: An integer or string that reads the
same forwards and backwards.
 Input: Integer or string.
 Output: Boolean message indicating whether
integer string is a palindrome.
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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 <= (len - 1) / 2; i++)
{
if (str.charAt(i) != str.charAt(j))
return false;
j--;
}
return true;
}
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Sample Runs: Palindrome Number
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Sample Runs: Palindrome Number
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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 the 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 larger number: Returns the larger of 2 numbers.
 For loop: Calls method larger 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 < 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: Syntax
Method definition:
The general form (syntax) of a void method without
parameters is as follows:
modifier(s) void methodName()
{
statements
}
Method call (within the class):
The method call has the following syntax:
methodName();
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Void Methods with Parameters: Syntax
Method definition:
The definition of a void method with parameters has the
following syntax:
modifier(s) void methodName
(formal parameter list)
{
statements
}
Formal parameter list:
The formal parameter list has the following syntax:
dataType variable, dataType variable, ...
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Void Methods with Parameters:
Syntax
Method call:
The method call has the following syntax:
methodName(actual parameter list);
Actual parameter list:
The actual parameter list has the following syntax:
expression or variable, expression or
variable, ...
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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 parameters 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 an address, saves 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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Scope of an Identifier within a Class
 Local identifier: An identifier that is declared within a
method or block and that is visible only within that
method or block.
 Java does not allow the nesting of methods. That is, 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. Note that a block is a set of
statements enclosed within braces.
 A method’s definition can contain several blocks. The
body of a loop or an if statement also forms a block.
 Within a class, outside of every method definition (and
block), an identifier can be declared anywhere.
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Scope of an Identifier within a Class
 Within a method, an identifier that is 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
following method definition, the second declaration of the
variable x is illegal:
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 that is declared within a class
and accessed within a method (block) of the class.
 An identifier, say X, that is 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.
 Suppose X is an identifier that is 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.
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Scope Rules
Example 7-12
public 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
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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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.
 If the number of formal parameters is the same in
both methods, 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
public
public
public
void methodOne(int x)
void methodTwo(int x, double y)
void methodThree(double y, int x)
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
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
 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
 The following method headings correctly overload
the method methodXYZ:
public
public
public
public
void
void
void
void
methodXYZ()
methodXYZ(int x, double y)
methodXYZ(double one, int y)
methodXYZ(int x, double y,
char ch)
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Method Overloading
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
Sample Output
Course No
CSC
Group No
1
2
Course Average
83.71
80.82
ENG
1
2
82.00
78.20
HIS
1
2
77.69
84.15
MTH
1
2
83.57
84.29
PHY
1
2
83.22
82.60
Avg for group 1: 82.04
Avg for group 2: 82.01
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Programming Example: Data Comparison
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Chapter Summary
 Pre-defined methods
 User-defined methods:




Value-returning methods
Void methods
Formal parameters
Actual parameters
 Flow of execution
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Chapter Summary
 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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