COMP 14 Introduction to Programming

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COMP 14Introduction to Programming

• Miguel A. Otaduy
• May 28, 2004

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Today

• Defining classes, using objects
• public, private, static
• Assignment operator and objects
• jGRASP projects, multiple files, Java archives

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Multiple Classes

• So far, we have added methods to the main class
• Classes extend the concept of data types data
  operations
• Define new classes and instantiate objects

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Multiple Classes

• Concept Basketball team
• players (objects of class Player)
• coach (object of class Coach)
• games
• Define each class in one file, and instantiate
  objects in other classes or in the main method

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Basketball.java

• Create main class Basketball
• Define Player class
• player name
• year
• operations

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Multiple files

• Good style 1 class per file
• Store all files in 1 directory
• Create jGRASP project (.gpj)

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BasketballProject.gpj

• Example of project creation

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Multiple Objects

• A class has 1 single definition
• We can instantiate multiple objects of 1 class
• Each object keeps its own copy of the data
  members
• Objects are accessed using reference variables
• To perform operations on objects

referenceVariable.method (parameter1, parameter2,
)
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Basketball.java

• Instantiate multiple Player objects
• Call methods of the Player objects from
  Basketball.main()
• Demonstrate flow through different methods with
  the debugger

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Visibility Modifiersprivate and public

• Public
• For members that should be accessible outside the
  class (i.e. from other classes or the main
  method)
• Usually employed with methods
• Private
• For members that should not be accessed outside
  the class
• Usually employed with data members
• Also for methods that are only used internally
• Typically, data members are declared as private,
  so we need methods to set and get data values

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Player.java

• Make data members private
• Make methods public
• Demonstrate accessibility from Basketball.main()

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Assignment Operator

• Assignment expressions that involve reference
  variables do not copy object data!!
• In order to copy object data, we need additional
  methods

Player player1 new Player(Felton) Player
player2 new Player() player2 player1
//player2 points to the same obj.
public void Player.copy(Player otherPlayer)
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Player.java

• Write Player.copy()

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Visibility Modifiersstatic

• static data members
• Data related to a class that does not need to be
  defined for each object

public class Player static final int FRESHMAN
1 static final int SOPHOMORE 2 private
int year
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Visibility Modifiersstatic

• static methods
• Operations that do not use object-dependent data
• They can only use static data members!! (besides
  local variables)

public static int factorial(int x)
static BufferedReader keyboard public static
void main() String strkeyboard.readLine()

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Visibility Modifiersstatic

• To access static data members and methods from
  outside the class
• To access non-static data members and methods
  from outside the class
• In general, from inside the class, no need to
  write class name or reference variable.

double res Math.pow(2.0, 3.0) //ClassName.meth
od()
player1.printInfo() //referenceVariable.method()

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Player.java

• Add static data members and methods

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The toString Method

• Special method in Java classes
• Produces a String object based on the current
  object, suitable for printing
• Mapped to the '' operator (concatenation)
• Also called when the object is a parameter in a
  print() or println() method
• There is a default toString method, but it's
  better if we write our own

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Player.java

• Write Player.toString()

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To do

• Homework 5 (due Wednesday night)
• Dont wait till Tuesday!!
• Homework 6 is going to be rather long
• Read ch. 9