UserDefined Methods

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User-Defined Methods
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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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Objectives

• 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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class Math (Package java.lang)
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class Math (Package java.lang)
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class Character (Package java.lang)
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class Character (Package java.lang)
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class Character (Package java.lang)
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Syntax 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 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
• dataType identifier, dataType identifier,...
• Method Call
• The syntax to call a value-returning method is
• methodName(actual parameter list)

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Syntax

• Syntax Actual Parameter List
• The syntax of the actual parameter list is
• expression or variable, expression or variable,
  ...

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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Equivalent Method Definitions

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

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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 2
  numbers
• For loop calls method largest number on each
  number received and compares to current largest
  number

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Solution Largest Number
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Sample Run Largest Number
Sample Run (When you execute this program, enter
all numbers in the same line.) Enter 10 numbers
in the same line. 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
• No method type
• 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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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

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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

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Scope Rules Demonstrated
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Scope Rules Demonstrated

• 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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• 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
• 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