Recitation 020207 Defining Classes and Methods

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Recitation 02/02/07Defining Classes and Methods

• Chapter 4

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Miscellany

• Project 2 due last night
• Exam 1 (Ch 1-4)
• Thursday, Feb. 8, 830-930pm PHYS 112
• Sample Exam posted
• Project 3
• due Feb. 15 1000pm
• check newsgroup!

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Methods

• Two kinds of methods
• Methods that return a single value
• nextInt()
• Methods that perform some action other than
  returning a single value
• void methods
• println(m_at_)

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void Method Definitions

• example
• public void writeOuput()
• System.out.println(Name name)
• System.out.println(Age age)
• Such methods are called void methods.

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Methods That Return a Value

• example
• public int fiveFactorial()
• int factorial 54321
• return factorial
• As before, the method definition consists of the
  method heading and the method body.
• The return type replaces void.

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Accessor and Mutator Methods

• Appropriate access to an instance variable
  declared private is provided by an accessor
  method which is declared public.
• Typically, accessor methods begin with the word
  get, as in getName.
• Only provide when access is needed

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Accessor and Mutator Methods, cont.

• Appropriate changes to an instance variable
  declared private are provided by an mutator
  method which is declared public.
• Typically, mutator methods begin with the word
  set, as in setName.
• Only provide when needed

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Allocating Memory for a Reference and an Object

• A declaration such as
• SpeciesFourthTry s
• creates a variable s that can hold a memory
  address (compile time)
• A statement such as
• s new SpeciesFourthTry()
• allocates memory for an object of type
  SpeciesFourthTry. (runtime)

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with Variables of a Class Type
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with Variables of a Class Type, cont.

• When used with variables of a class type,
  tests if the variables are aliases of each other,
  not if they reference objects with identical
  data.
• To test for equality of objects in the intuitive
  sense, define and use an appropriate equals
  method.

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with Variables of a Class Type

• class Species

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with Variables of a Class Type

• class SpeciesEqualsDemo

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Method equals

• The definition of method equals depends on the
  circumstances.
• In some cases, two objects may be equal when
  the values of only one particular instance
  variable match.
• In other cases, two objects may be equal only
  when the values of all instance variables match.
• Always name the method equals.

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Boolean-Valued Methods

• A method that returns a value of type boolean is
  called a boolean-valued method.
• Method equals produces and returns a value of
  type boolean.
• The invocation of a boolean-valued method can be
  used as the condition of an if-else statement, a
  while statement, etc.

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Boolean-Valued Methods, cont.

• The value returned by a boolean-valued method can
  be stored in a variable
• boolean areEqual s1.equals(s2)
• Any method that returns a boolean value can be
  used in the same way.

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Class Parameters

• Recall
• When the assignment operator is used with objects
  of a class type, a memory address is copied,
  creating an alias.
• When the assignment operator is used with a
  primitive type, a copy of the primitive type is
  created.

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Class Parameters, cont.

• When a parameter in a method invocation is a
  primitive type, the corresponding formal
  parameter is a copy of the primitive type.

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Class Parameters, cont.

• When a parameter in a method invocation is a
  reference to a class type (i.e. a named object),
  the corresponding formal parameter is a copy of
  that reference (i.e. an identically valued
  reference to the same memory location).

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Class Parameters, cont.

• Example
• if (s1.equals(s2))
• public boolean equals (Species otherObject)
• causes otherObject to become an alias of s2,
  referring to the same memory location, which is
  equivalent to
• otherObject s2

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Class Parameters, cont.

• Any changes made to the object named otherObject
  will be done to the object named s2, and vice
  versa, because they are the same object.
• If otherObject is a formal parameter of a method,
  the otherObject name exists only as long as the
  method is active.

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Comparing Class Parameters and Primitive-Type
Parameters

• A method cannot change the value of a variable of
  a primitive type passed into the method.
• A method can change the value(s) of the instance
  variable(s) of a class type passed into the
  method.

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The Graphics Class

• An object of the class Graphics represents an
  area of the screen.
• The class Graphics also has methods that allow it
  do draw figures and text in the area of the
  screen it represents.

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The Graphics Class, cont.

• A Graphics object has instance variables that
  specify an area of the screen
• In examples seen previously, the Graphics object
  represented the area corresponding to the inside
  of an applet.

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The Graphics Class, cont.

• When an applet is run, a suitable Graphics object
  is created automatically and is used as an
  argument to the applets paint method when the
  paint method is (automatically) invoked.
• The applet library code does all this for us.
• To add this library code to an applet definition,
  use
• extends JApplet

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Programming Example
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The init Method

• An init method can be defined when an applet is
  written.
• The method init (like the method paint) is called
  automatically when the applet is run.
• The paint method is used only for things like
  drawing.
• All other actions in an applet (adding labels,
  buttons, etc.) either occur or start in the init
  method.

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Adding Labels to an Applet

• A label is another way to add text to an applet.

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Adding Labels to an Applet, cont.

• class LabelDemo

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The Content Pane

• Think of the content pane as inside of the
  applet.
• Container contentPane getContentPane()
• When components are added to an applet, they are
  added to its content pane.
• Think adding picture to picture frame
• The content pane is an object of type Container.

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The Content Pane, cont.

• A named content pane can do things such as
  setting color
• contentPane.setBackground(Color.WHITE)
• or specifying how the components are arranged
• contentPane.setLayout
• (new FlowLayout())

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The Content Pane, cont.

• or adding labels
• JLabel label1 new JLabel(Hello)
• contentPane.add(label1)

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Exam 1 Information

• Time/Location
• Thursday, Feb 8, 830-930pm, Physics 112
• Material Covered
• Chapters 1 - 4
• Format
• Multiple Choice
• Programming
• Old exam on course website

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Exam 1 Information

• Topics
• Encapsulation, Polymorphism, information hiding
• Accessor/Mutator methods
• Objects, Classes
• public/private modifiers
• Java naming conventions
• Primitive types vs. class types

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Exam 1 Information

• Topics, cont
• Defining classes and methods (including the main
  method)
• Void methods vs. methods that return a value
• Looping structures (while, do-while, for)
• If-else, switch
• Primitive types vs. class types (and memory
  representation)
• Scanner class for input, System.out for output

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Exam 1 Information

• Topics, cont
• Arithmetic expressions
• Boolean variables
• String methods
• vs. equals methods
• Basic graphics methods

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Questions