UserDefined Classes

1
Chapter 7

• User-Defined Classes
• An Example
• The TimeType Class

2
Requirements for TimeType

• Data?
• Operations?
• Access?

3
Data Members

• We will need to represent the hours, minutes, and
  seconds
• What should the data type be?
• What are the ranges of each?

4
Data Members

• public class TimeType
• private int hrs // 3 data members
• private int mins
• private int secs

5
Accessors

• Now that we have data members, we need a way to
  retrieve their values
• The client code might want to
• TimeType t new TimeType( )
• System.out.println(t.hrs) // is this legal?
• System.out.println(t.getHrs()) // what about
  this?

6
Accessors

• The easiest way is to create a member function
  (method) for each to retrieve them
• getHrs
• getMins
• getSecs
• Each function will return a value of data type
  _______
• Any arguments?
• What will the logic look like?

7
Accessors

• public class TimeType
• // accessor methods
• public int getHrs( ) return hrs
• public int getMins( ) return mins
• public int getSecs( ) return secs
• private int hrs // 3 data members
• private int mins
• private int secs

8
Modifiers

• We also need a way to set the data members
  values
• We could write a function to set each value
• setHrs (int h)
• setMins (int m)
• setSecs (int s)
• Or we can write a single one to set all three
• set (int h, int m, int s)
• Do you ever want to set just the hour or minute
  or would you always set all three at one time?

9

• Client code might be
• t.Set(10,10,10) // what would this be?
• t.Set(50, 50, 50) // what should this do?

10
Modifiers

• Do we need to do any validation of the input
  arguments?
• What should the validations be?
• What should we do if an input is invalid?

11
Modifier Function

• public class TimeType
• // modifier method with validations
• public void set(int h, int m, int s)
• // validate 0 lt initHrs lt 23 0 lt
  initMins lt 59 0 lt initSecs lt 59
• if (( 0 lt h h lt 24) (0 lt m m lt 60)
• (0 lt s s lt 60))
• hrs h
• mins m
• secs s
• else
• System.out.println(Invalid data,
  defaulting to 000000)
• set(0, 0, 0)

12
Creation of Objects

• Now we need to be able to create objects of type
  TimeType
• To do that we want to declare a constructor
  method to initialize the values in the object
  (set it to a valid state)
• First, lets look at what the client code might
  look like to create an object

13
Invocation of Constructors

• TimeType departureTime new TimeType( ) //
  default constructor invoked
• TimeType movieTime new TimeType(19, 30, 0 )
  // parameterized constructor
• departureTime movieTime

set
set
Private data hrs mins secs
Private data hrs mins secs
increment
increment
0 0 0
19 30 0
toString
toString
LessThan
LessThan
equals
equals
14
Implementation of Default Constructor

• public class TimeType
• public TimeType ( ) // Default Constructor
• hrs 0 // hrs 0 mins 0
  secs 0
• mins 0
• secs 0 // alternate set(0, 0, 0)
• What is the return type of a constructor?

15
Implementation of Another Constructor

• public class TimeType
• public TimeType ( int h, int m, int s ) //
  Constructor
• // validate input 0 lt initHrs lt 23 0 lt
  initMins lt 59
• // 0 lt initSecs lt 59
• set(h, m, s) // why not just set values
  or repeat validation?

16
toString

• Now we need a way to print our object
• System.out.println(t)
• // implicit invocation of toString method
• How should we print the time?

17
Implementation of toString

• public class TimeType
• public String toString ( )
• String holder
• if (hrs lt 10)
• holder 0
• holder hrs
• if (mins lt 10)
• holder 0
• holder (mins )
• if (secs lt 10)
• holder 0
• holder secs
• return holder

18
Increment Method

• Now lets think about the increment method
• time1.set(22, 0, 0)
• time1.increment( )
• System.out.println(time is now time1)
• for (int i 0 i lt 125 i)
• time1.increment( )
• System.out.println(time is now time1)

19
Increment Method

• What is the return type?
• Any arguments?
• What kind of logic do we have to think about?

20
Implementation of Increment

• public class TimeType
• public void increment ( )
• secs
• if (secs gt 59)
• secs 0
• mins
• if (mins gt 59)
• mins 0
• hrs
• if (hrs gt 23)
• hrs 0

21
equals Function

• We may want to be able to determine if two times
  are equal
• time1.set(1, 0, 0)
• time2.set(1, 0, 0)
• if (time1.equals (time2))
• System.out.println(time1 is equal to
  time2)
• What does it mean for two times to be equal?
• What is the return type?
• What data type is the argument?

22
Implementation of equalTo

• public class TimeType
• public boolean equals (TimeType t )
• if (hrs t. hrs mins t. mins
• secs t.secs)
• return true
• else
• return false

23
Object Relationships

• Some use associations occur between objects of
  the same class (equals)
• For example, we might add two Rational number
  objects together as follows
• r3 r1.add(r2)
• One object (r1) is executing the method and
  another (r2) is passed as a parameter

24
lessThan Function

• We may want to be able to determine if one time
  is less than another time
• time1.set(1, 0, 0)
• time2.set(1, 0, 0)
• if (time1.lessThan (time2))
• System.out.println(time1 is less than
  time2)
• What does it mean for one time to be less than
  another time?
• What is the return type?
• What data type is the argument?
• Exercise You write this function!!

25
Subtract Method

• Write the Java code for the subtract method for
  our timer application.