More on object interaction: Creating cooperating objects

object-oriented programming with java
ahmed al-ajeli
more on object interaction
creating cooperating objects
ahmed al-ajeli
lecture 5
2
concepts to be covered
•overloading
•composition
•object interaction
•internal/external method calls
•the debugger
object-oriented programming with java
ahmed al-ajeli
3
implementation – clockdisplay (1)
public class clockdisplay
{
private numberdisplay hours
private numberdisplay minutes
private string displaystring
// simulates the actual display
public clockdisplay()
{
hours = new numberdisplay(24)
minutes = new numberdisplay(60)
updatingdisplay()
}
4
implementation – clockdisplay (2)
public clockdisplay(int hour, int minute)
{
hours = new numberdisplay(24)
minutes = new numberdisplay(60)
settime(hour, minute)
}
public void timetick()
{
minutes.increment()
if(minutes.getvalue() == 0) {
// it just rolled over!
hours.increment()
}
updatingdisplay()
}
object-oriented programming with java
ahmed al-ajeli
5
implementation – clockdisplay (3)
public void settime(int hour, int minute)
{
hours.setvalue(hour)
minutes.setvalue(minute)
updatingdisplay()
}
public string gettime()
{
return displaystring
}
private void updatingdisplay()
{
displaystring = hours.getdisplayvalue()
+ ":" +
minutes.getdisplayvalue()
}
}
6
overloading
•a class may contain more than one constructor, or more than one method of the same name, as long as each has a distinctive set of parameters.
•example: clockdisplay class has two constructors:
clockdisplay()
clockdisplay(int hour, int minute)
object-oriented programming with java
ahmed al-ajeli
7
object interaction (1)
•two objects interact when one object calls a method on another.
•the interaction is usually all in one direction („client?, „server?).
•the client object can ask the server object to do something.
•the client object can ask for data from the server object.
8
object interaction (2)
•two numberdisplay objects store data on behalf of a clockdisplay object.
–the clockdisplay is the „client? object.
–the numberdisplay objects are the „server? objects.
–the client calls methods in the server objects.
object-oriented programming with java
ahmed al-ajeli
9
method calling
public void timetick()
{
minutes.increment()
if(minutes.getvalue() == 0) {
// it just rolled over!
hours.increment()
}
updatingdisplay()
}
‘client’ method
‘server’ methods
internal/self method call
10
external method calls
•general form: object . methodname ( params )
•examples: hours.increment() minutes.getvalue()
nb: object, not class name
dot notation
object-oriented programming with java
ahmed al-ajeli
11
internal method calls
•no object name is required: updatingdisplay()
•internal methods often have private visibility.
–prevents them from being called from outside their defining class.
12
internal method
private void updatingdisplay()
{
displaystring =
hours.getdisplayvalue() + ":" +
minutes.getdisplayvalue()
}
object-oriented programming with java
ahmed al-ajeli
13
method calls
•nb: a method call on another object of the same type would also be an external call.
•„internal? means „this object?.
•„external? means „any other object?, regardless of its type.
14
the this keyword
•used to distinguish parameters and fields of the same name. e.g.:
•public numberdisplay(int limit) { this.limit = limit value = 0 }
•q- why are we doing this at all?
a- the reason is readability of source code
object-oriented programming with java
ahmed al-ajeli
15
the debugger
•a debugger is a software tool that
helps in examining how an
application executes
•useful to find program errors - errors are commonly known as “bugs”
•set breakpoints.
•examine variables
•step through code