Description
Case Study: A “Time” class
• Suppose we want to be able to work with values
representing clock times to 1 minute precision.
– What information to we have to store to represent
a time? How do we store that information?
– What operations might we want to do with a Time
value?
What to store in “Time”?
• Two integers:
– hour: the hour number (0 ≤ hour ≤ 23)
– minute: the minute number (0 ≤ minute ≤ 59)
class Time
{
public int hour;
public int minute;
}
• Alternatives?
Declaring and creating Time objects
// DECLARE VARIABLES
Time time1; // time is null here
…
// USING Time OBJECT
…
time1 = new Time( ); // We now have a Time object
// but it contains ? for values
time1.hour = 17;
time1.minute = 45; // time1 now represents 17:45
What do we have?
time1
17
45
hour
minute
A method to set the time
class Time
{
public int hour;
public int minute;
public void setTime( int h, int m )
{
this.hour = h;
this.minute = m;
}
}
Usage
// DECLARE VARIABLES
Time time1; // time is null here
…
// USING Time OBJECT
…
time1 = new Time( );
time1.setTime( 17, 45 );
// time1 now represents 17:45
This method is different!
• Did anyone see what was “missing” from the method?
public void setTime( int h, int m )
{
this.hour = h;
this.minute = m;
}
• The word static does not appear in the method
header.
Instance methods
• When the word static does not appear in the
method header, this means that the method can be
called via a variable declared to be of a type matching
the class name. This is called an “instance” method.
• An instance method can make use of the variables
defined in the class.
• The result: the method will produce different
results for different object instances.
Instance methods
• For example,
Time time1;
Time time2;
time1 = new Time( );
time2 = new Time( )
time1.setTime( 17, 45 ); // time1 is 17:45
time2.setTime( 14, 30 ); // time2 is 14:30
What do we have?
time1
17
45
hour
minute
time2
14
30
hour
minute
this
• Objects time1 and time2 use the same code in class
Time to set their own copy of hour and minute.
• When we want to refer to “the object on which I was
called”, we use this.
this
time1
17
45
hour
minute
time2 14
30
hour
minute
this
time1.setTime( 17, 45 );
Information Hiding
• If we want to ensure that:
– hour: must be in range 0 ≤ hour ≤ 23
– minute: must be in range 0 ≤ minute ≤ 59
then direct access to these variables should not be permitted.
class Time
{
private int hour;
private int minute;
}
• Instead, access should be provided through setTime, and we
can adjust the values if needed.
Revised version of setTime
public void setTime( int h, int m )
{
// If minutes value is too large, adjust it
// by mod 60, and add to hours value.
if ( m > 59 )
{
h = h + m / 60; // determine hours to add
m = m % 60; // puts minutes in range
}
else
{
; // do nothing
}
this.hour = h % 24; // puts hours in range
this.minute = m;
}
Accessors
• With the variables now declared to be private, we
need to provide a way for other classes to ask for the
values.
public int getHours()
{
return hour;
}
public int getMinute( )
{
return minute;
}
Compare times for equality
• Suppose we want a method that checks whether one
time object is equal to another.
• One approach: a static method
public static boolean isEqual( Time t1, Time t2 )
This would be called as Time.isEqual( t1, t2 )
• Alternative: an instance method
public boolean isEqual( Time t2 )
This would be called as t1.isEqual( t2 )
The static method
public static boolean isEqual( Time t1, Time t2 )
{
return (t1.hour == t2.hour) &&
(t1.minute == t2.minute);
}
• If the method is inside the class Time, it can access
the private variables inside the class.
The instance method
public boolean isEqual( Time t2 )
{
return (this.hour == t2.hour) &&
(this.minute == t2.minute);
}
• In this case, we are comparing “ourself” to another
Time value.
Exercise 1: Add two instance methods to
provided Time class
• public boolean isBefore( Time t )
that returns true is the Time t is before the
current time object
• public Time duration( Time t )
This method calculates the time duration from
the current time to the given time t. Return the
duration as a Time object.
Also test your two methods using Junit tests from
the provided file TimeTest.java. You will need to
use the Test menu button in DrJava instead of the
Run button to execute the tests.
• To do Junit testing, load your Time.java solution and the
provided TimeTest.java to Dr.Java. Compile both files,
and then press “Test” menu button in Dr.Java.
TimeTest.java contains many tests. Those tests for which
your solution computes the correct answer will be
displayed in green and those for which your solution
fails will be displayed in red. Make sure all the
provided tests are displayed in green i.e. that they
pass.
• You will have to do similar testing in your Assignment 5
A “Line” segment class
• Design a Java class called Line that will store
information about a line segment, where the line
segment is specified by the coordinates, (x,y), of its
two endpoints. Provide operations to work with
segments.
• Each Line object starts at a point (xStart, yStart ) and
ends at a point (xEnd, yEnd), where xStart, yStart , xEnd,
and yEnd are real-valued numbers that could be
positive or negative.
(xStart, yStart)
y (xEnd, yEnd)
x
UML diagram for Line
If the method is not underlined that it is an instance method.
Underlined methods are static methods.
Thus, this UML diagram tells us that all the methods in this class
are instance methods
Method descriptions
• setPoints method: Set the start and end points of the
line segment.
• Name of method: setPoints(…)
• Parameters to the method: xs, ys, xe, ye
• Returns: (nothing)
• This method modifies the Line object
• length method: Returns the length of the line segment
• Name of method: length( )
• Input parameters to the method: none.
• Method returns: length of the line segment (a real value)
Method descriptions
Method descriptions
• slope method: Returns the slope of the line segment.
– The slope is (ye − ys) / (xe − xs) (watch out for
vertical lines – their slope is infinite!)
• Name of method: slope( )
• Parameters to the method: none.
• The methods retunrs: the slope of the line
segment (a real value)
Method descriptions
• translate method: Translate the segment by (tx, ty),
where tx and ty are any positive or negative real
values.
– A translation of a line segment represents “sliding”
the entire segment. The value tx is added to the x
coordinates of the start and end of the segment,
and the value ty is added to the y coordinates of
the start and end of the segment.
• Name of method: translate(…)
• Parameters to the method: tx, ty
• Results: (none)
• This method modifies the Line object
Method descriptions
• toString method: Returns a String with information
about the line segment.
– The string that is returned for a line segmetn with
(for example) start point (0.0,1.0) and end point
(3.5,-1.2) should be:
”Line from (0.0, 1.0) to (3.5, -1.2)”
• Name of method: toString ( )
• Parameters to the method: (none)
• Results: a String in the above format
Exercise 2
• Implement the class Line and all of its methods.
Recall, by looking at UML diagram or method
descriptions, that all the methods of the Line class are
supposed to be instance methods.
• Test the class using the main method in the provided
file LineTest.java
Extra Exercise 3
We want to realize a program for the administration of a parking lot formed by a
collection of parking places. For each parking place the following information has to be
stored:
– Whether it is free of occupied;
– Licence plate of the car, when occupied (a string);
– Time since when it is occupied (two integers, for hours and minutes).
Write a class ParkingPlace, for handling a parking place, that implements the following
methods:
– ParkingPlace(): constructor without parameters that constructs a parking place that is initially
free;
– String toString(): that returns “——-” if the parking place is free, and the licence plate of the
car, if the parking place is occupied;
– void carArrives(String plate, int hour, int minutes: modifies the state of the parking place by
setting it to occupied, sets the plate of the car that occupies the place to plate, and sets the
time since when it is occupied to hour and minutes; if the parking place is already occupied, the
method does nothing;
– void carLeaves(): modifies the state of the parking place by setting it to free;
– boolean free(): returns true if the parking place is free, false otherwise;
– String getCar(): returns the plate of the car that occupies the parking place, if the place is
occupied, null otherwise;
– int getHour(): returns the hour since when the parking place is occupied, if the place is
occupied, -1 otherwise;
– int getMinutes(): returns the minutes since when the parking place is occupied, if the place is
occupied, -1 otherwise;
Exercise 3 Testing
Use the example program TestParkingPlace.Java to
test the class you have developed.
