Transcript Feb 25

Chapter 7
User-Defined Methods
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
 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
Advantages of using methods

Methods are the building blocks or modules of
classes and of systems built from many classes
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
Predefined methods are supplied within Java
User defined methods originate from developers
Allows the developer to write, debug, and integrate
one module at a time in a larger system
 Allows many developers to simultaneously develop
different components of a larger system
 Improves readability and ease of maintenance of the
system
Methods
 The
concept of a method is similar to the
mathematical concept of a function
 A function has
 A result
or a value
 A series of arguments
 Functional form
 A method either
 Evaluates
to a result (value) of a particular type
 Modifies or manipulates its arguments (e.g. prints)
Syntax: Value-Returning Method
modifier(s) returnType methodName(formal parameter list)
{
statements
}
Syntax: Void Method
modifier(s) void methodName(formal parameter list)
{
statements
}
Predefined Classes and Methods
 Methods
already written and provided by
Java
 Organized as a collection of classes (class
libraries)
 To use: import package or class
 Classname.methodname(argument list)
class Math (Package: java.lang)
class Math (Package: java.lang)
class Character (Package: java.lang)
class Character (Package: java.lang)
class Character (Package: java.lang)
Value-Returning Method
modifier(s) returnType methodName(formal parameter list)
{
statements;
return expr;
}

The definition of the method has two components


The body of the method, the statements between {}, the
code required to implement the method and return the value
The heading of the method, the line defining modifiers,
returnType, methodName and formal parameters or
arguments
User-Defined Methods

Value-returning methods



Used in expressions
Calculate and return a value
Can be used in the expression in an assignment statement
to save value for later use

Modifiers: public, private, protected, static, abstract,
final
 returnType:



type of value that the method calculates and returns (using
return statement)
Type of expression formed by call of method
methodName: Java identifier; name of method
Syntax
Syntax: Formal Parameter List
The syntax of the formal parameter list is:
dataType identifier, dataType identifier,...
Method Call
The syntax to call a value-returning method is:
methodName(actual parameter list)
Syntax: Actual Parameter List
The syntax of the actual parameter list is:
expression or variable, expression or variable, ...
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
public static double larger(double x, double y)
{
if(x >= y)
return x;
else
return y;
}
Programming Example:
Palindrome Number
 Palindrome: integer
or string that reads the
same forwards and backwards
 Input: integer or string
 Output: Boolean message indicating whether
integer string is a palindrome
Solution: isPalindrome Method
Flow of Execution

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


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 2 numbers
For loop: calls method largest number on each number
received and compares to current largest number
Void Methods
 Similar
in structure to value-returning methods
 No method type
 Call to method is always stand-alone statement
 Can use return statement to exit method early
 Return statement does not return the value of
an expression
Void Methods with no parameters: Syntax
 Method Definition
modifier(s) void methodName( formal
parameter list)
{
statements
}
 Method Call (Within the Class)
methodName();
Void Methods with Parameters: Syntax
Method Definition
Definition of a void method with parameters
modifier(s) void methodName(formal parameter list)
{
statements
}
Formal Parameter List
The formal parameter list has the following syntax:
dataType variable, dataType variable, ...
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, ...
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 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
Scope of an Identifier Within a
Class
 Scope
(of an identifier): refers to those parts
of a program where the identifier is
accessible
 Local variables: variables declared within a
method (or block)
 Within a class
 Any
method can call any other method
 Exception: static method cannot call a nonstatic
method
Scope Rules
 Identifier declared
within a block is
accessible
 Only
within the block from the point at which it
is declared until the end of the block
 By those blocks nested within that block if the
nested block does not have an identifier with
the same name as the identifier in the outside
block
*Outside block: block that encloses nested block
Scope Rules: Demonstrated
public class ScopeRules
{
static final double rate = 10.50;
static int z;
static double t;
public static int w;
public static void two(int one, int z)
{
char ch;
int a;
public static void main(String[] args)
{
int num;
double x, z;
char ch;
//block three
{
int x = 12;
//...
}//end block three
//...
//...
}
public static void one(int x, char y)
{
//...
}
}
Scope Rules: Demonstrated
Method Overloading: An
Introduction

Method overloading: more than one method can have
the same name
 Overloading Rules



Every method must have a different number of formal
parameters
OR
If the number of formal parameters is the same, then the
data type of the formal parameter (in the order listed), must
differ in at least one position
Types of parameters determine which method executes
Programming Example: Data
Comparison

Input: Data from 2 different files
 Data format: Course Number followed by scores
 Output: Course Number, Group Number, Course
Average
 Solution:
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Read from more than one file, write output to file
Generate bar graphs
User-defined methods and re-use (calculateAverage and
printResult)
Parameter passing
Chapter Summary
 Predefined methods
 User-defined methods
 Value-returning
methods
 Void
methods
 Formal parameters
 Actual parameters
 Flow
of Execution
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