COMP 14 Introduction to Programming

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COMP 14Introduction to Programming

• User-Defined Methods
• Wednesday, July 12, 2006

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Announcements

• Assignment 4
• 20 deduction per day
• I suggest to turn in today even if you are not
  100 done
• Assignment 5
• Questions?

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Today

• Scope
• Reference variables as parameters
• Overloading Methods
• Constructors
• Unified Modeling Language Class Diagrams

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Scope

• The scope of data is
• The area in a program in which that data can be
  used (referenced)
• Key to determining scope
• Look for blocks of code (surrounded by )
• Variables declared inside cannot be used
  outside
• Variables declared outside can be used inside

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Scope

• Data declared at the class level
• Can be used by all methods in that class
• Data declared within a method
• Can be used only in that method
• Also called local data
• Local identifier
• An identifier that is declared within a method or
  block and that is visible only within that method
  or block.
• Multiple blocks
• A methods definition can contain several blocks
• The body of a loop or an if statement also forms
  a block

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ScopeExample
public class Rectangle // variables declared
here are class-level // available in all methods
in Rectangle class public int computeArea()
// variables declared here are
method-level // only available in
computeArea() public void print() //
variables declared here are method-level //
only available in print()
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Scope

• 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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Static identifiers

• Suppose X is an identifier that is 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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Parameters

• Formal parameters
• variable declarations in the method header
• automatic local variables for the method
• Actual parameters
• actual values that are passed to the method
• can be variables, literals, or expressions

printStars (35) printStars (30 5) int num
35 printStars (num)
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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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Primitive Data Type Parameters

• PrimitiveDataTypeParameters.java

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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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Reference Variable Parameter

• Example7_8.java

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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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Caution!

• Some objects cannot be changed by a method
• Wrapper classes (e.g., Integer, Double)
• Strings
• Why not?
• Some objects are immutable (values can not change)

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Reference Variables as Parameters type String
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Immutable Reference Variables as Parameters

• StringObjectsAsParameters.java

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So what can we do?

• Use mutable classes
• StringBuffer
• Mutable wrapper classes provided by book
• IntClass
• DoubleClass

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Overloading Methods

• Overloading
• the process of using the same method name for
  multiple methods
• The signature of each overloaded method must be
  unique
• At least one of these must be different
• number of parameters
• type of the parameters
• NOT
• the return type of the method
• The compiler determines which version of the
  method is being invoked by analyzing the
  parameters

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Overloading Methods
Invocation result tryMe (25, 4.32)
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Overloaded Methodsprintln Example

• The println method is overloaded
• println (String s)
• println (int i)
• println (double d)
• and so on...
• The following lines invoke different versions of
  the println method
• System.out.println ("The total is")
• System.out.println (total)

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Constructors

• Constructor
• A method of a class that is automatically
  executed when an object of the class is created
• Two types of constructors
• With parameters
• Without parameters (default constructor)

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Constructors
public class Rectangle private int
length private int width
Rectangle r2 new Rectangle (5, 10)
public Rectangle () length 0
width 0
public Rectangle (int l, int w) length
l width w
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Constructor Properties

• Constructors have the following properties
• The name of a constructor is the same as the name
  of the class
• A constructor, even though it is a method, has no
  type
• A class can have more than one constructor
• If a class has more than one constructor, any two
  constructors must have different signatures
• If there are multiple constructors, which
  constructor executes depends on the type of
  values passed to the class object when the class
  object is instantiated

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Creating and Using Objects
r

• Create an object
• Rectangle r
• r new Rectangle(2, 3)
• OR
• Rectangle r new Rectangle(2, 3)
• Use the object and the dot operator to access
  methods
• r.setLength(5)
• r.setWidth(10)

2500
2500
3
2
r
2500
2500
3
2
5
10
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Unified Modeling Language Class Diagrams
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• class Clock
• Data Members (Instance Variables)
• private int hr //store hours
• private int min //store minutes
• private int sec //store seconds
• Methods
• public void setTime(int hours, int minutes,
• int seconds)
• public int getHours()
• public int getMinutes()
• public int getSeconds()
• public void printTime()
• public void incrementSeconds()
• public void incrementMinutes()
• public void incrementHours()
• public boolean equals(Clock otherClock)
• public void makeCopy(Clock otherClock)
• public Clock getCopy()

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To do

• Start reading chapter 8
• Pages 445-471
• Finish Assignment 4 if you havent yet done so
• Get started on Assignment 5