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COS 260 DAY 6
Tony Gauvin
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Agenda
• Questions?
• 2nd Mini quiz results poor
• Assignment 1 Due
– All turned in
• Assignment 2 will be posted soon
– Will include exercises 3.31 & 3.32 (page 81)
• Review Class definitions
• Begin Object Interaction
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Class definition revisited
• Template for a class definition
public class ClassName
{
fields ...
constructors …
methods …
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Fields
• Also called instance variables
accessibility modifier
type
fieldname ;
private int gradeExam1;
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Constructors
• Special method type has a header
and body
visibility modifier
ClassName
( parameter list)
pararmeter  type, namePara
{ assignments statements
}
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Methods
• Many types two most common are
– Accessors
– Mutators
• All methods has a header and a body
– Method header is also called a method
signature since is most be unique within
the class definition
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Accessors
used to retrieve state information
from an object
public type methodName()
{
return a field
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Mutator
• Used to change the sate of an object
public return_type methodName( parameters)
{
at least one assignment statement
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Object interaction
Creating cooperating objects
5.0
Clock review
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Concepts
•
•
•
•
abstraction
modularization
classes define new types
class diagram
•
•
•
•
object diagram
object references
object types
primitive types
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Objects creating objects
public class ClockDisplay
{
private NumberDisplay hours;
private NumberDisplay minutes;
private String displayString;
public ClockDisplay()
{
hours = new NumberDisplay(24);
minutes = new NumberDisplay(60);
…
}
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Objects creating objects
in class ClockDisplay:
hours = new NumberDisplay(24);
actual parameter
in class NumberDisplay:
public NumberDisplay(int rollOverLimit);
formal parameter
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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ClockDisplay object diagram
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Method calling
public void timeTick()
{
minutes.increment();
if(minutes.getValue() == 0) {
// it just rolled over!
hours.increment();
}
updateDisplay();
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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External method call
• external method calls
minutes.increment();
object . methodName ( parameter-list )
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Internal method call
• internal method calls
updateDisplay();
• No variable name is required.
• this
– could be used as a reference to the
invoking object, but not used for
method calls.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Internal method
/**
* Update the internal string that
* represents the display.
*/
private void updateDisplay()
{
displayString =
hours.getDisplayValue() + ":" +
minutes.getDisplayValue();
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Method calls
• NB: A method call on another object
of the same type would be an
external call.
• ‘Internal’ means ‘this object’.
• ‘External’ means ‘any other object’,
regardless of its type.
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null
• null is a special value in Java
• Object fields are initialized to null
by default.
• You can test for and assign null:
private NumberDisplay hours;
if(hours != null) { ... }
hours = null;
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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The debugger
• Useful for gaining insights into
program behavior …
• … whether or not there is a program
error.
• Set breakpoints.
• Examine variables.
• Step through code.
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The debugger
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Concept summary
• object creation
• overloading
• internal/external method
calls
• debugger
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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