Chapter 5: Enhancing Classes Chapter 5: Enhancing Classes
Presentation slides for Presentation slides for
Java Software Solutions Java Software Solutions
Foundations of Program Design Foundations of Program Design
Second Edition Second Edition
by John Lewis and William Loftus by John Lewis and William Loftus
Java Software Solutions is published by Addison-Wesley Java Software Solutions is published by Addison-Wesley
Presentation slides are copyright 2000 by John Lewis and William Loftus. All rights reserved.
Presentation slides are copyright 2000 by John Lewis and William Loftus. All rights reserved.
Instructors using the textbook may use and modify these slides for pedagogical purposes.
Instructors using the textbook may use and modify these slides for pedagogical purposes.
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Enhancing Classes Enhancing Classes
We can now explore various aspects of classes and objects We can now explore various aspects of classes and objects in more detail
in more detail
Chapter 5 focuses on: Chapter 5 focuses on:
• object references and aliases object references and aliases
• passing objects as parameters passing objects as parameters
• the static modifier the static modifier
• nested classes nested classes
• interfaces and polymorphism interfaces and polymorphism
• events and listeners events and listeners
• animation animation
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References References
Recall from Chapter 2 that an object reference holds the Recall from Chapter 2 that an object reference holds the memory address of an object
memory address of an object
Rather than dealing with arbitrary addresses, we often Rather than dealing with arbitrary addresses, we often depict a reference graphically as a “pointer” to an object depict a reference graphically as a “pointer” to an object
ChessPiece bishop1 = new ChessPiece();
bishop1
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Assignment Revisited Assignment Revisited
The act of assignment takes a copy of a value and stores it in a The act of assignment takes a copy of a value and stores it in a variable
variable
For primitive types: For primitive types:
num2 = num1;
Before
num1
5
num2
12
After
num1
5
num2
5
5
Reference Assignment Reference Assignment
For object references, assignment copies the memory For object references, assignment copies the memory location:
location:
bishop2 = bishop1;
Before
bishop1 bishop2
After
bishop1 bishop2
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Aliases Aliases
Two or more references that refer to the same object are Two or more references that refer to the same object are called
called aliases aliases of each other of each other
One object (and its data) can be accessed using different One object (and its data) can be accessed using different variables
variables
Aliases can be useful, but should be managed carefully Aliases can be useful, but should be managed carefully
Changing the object’s state (its variables) through one Changing the object’s state (its variables) through one reference changes it for all of its aliases
reference changes it for all of its aliases
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Garbage Collection Garbage Collection
When an object no longer has any valid references to it, it When an object no longer has any valid references to it, it can no longer be accessed by the program
can no longer be accessed by the program
It is useless, and therefore called It is useless, and therefore called garbage garbage
Java performs Java performs automatic garbage collection automatic garbage collection periodically, periodically, returning an object's memory to the system for future use returning an object's memory to the system for future use
In other languages, the programmer has the responsibility In other languages, the programmer has the responsibility for performing garbage collection
for performing garbage collection
Passing Objects to Methods Passing Objects to Methods
Parameters in a Java method are Parameters in a Java method are passed by value passed by value
This means that a copy of the actual parameter (the value This means that a copy of the actual parameter (the value passed in) is stored into the formal parameter (in the
passed in) is stored into the formal parameter (in the method header)
method header)
Passing parameters is essentially an assignment Passing parameters is essentially an assignment
When an object is passed to a method, the actual parameter When an object is passed to a method, the actual parameter and the formal parameter become aliases of each other
and the formal parameter become aliases of each other
Passing Objects to Methods Passing Objects to Methods
What you do to a parameter inside a method may or may What you do to a parameter inside a method may or may not have a permanent effect (outside the method)
not have a permanent effect (outside the method)
See See ParameterPassing ParameterPassing .java .java (page 226) (page 226)
See See ParameterTester ParameterTester .java .java (page 228) (page 228)
See See Num.java Num.java (page 230) (page 230)
Note the difference between changing the reference and Note the difference between changing the reference and changing the object that the reference points to
changing the object that the reference points to
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The static Modifier The static Modifier
In Chapter 2 we discussed static methods (also called class In Chapter 2 we discussed static methods (also called class methods) that can be invoked through the class name rather methods) that can be invoked through the class name rather
than through a particular object than through a particular object
For example, the methods of the For example, the methods of the Math class are static Math class are static
To make a method static, we apply the To make a method static, we apply the static static modifier to modifier to the method definition
the method definition
The The static static modifier can be applied to variables as well modifier can be applied to variables as well
It associates a variable or method with the class rather than It associates a variable or method with the class rather than an object
an object
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Static Methods Static Methods
public static int triple (int num) {
int result;
result = num * 3;
return result;
}
class Helper class Helper
Because it is static, the method could be invoked as:
Because it is static, the method could be invoked as:
value = Helper.triple (5);
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Static Methods Static Methods
The order of the modifiers can be interchanged, but by The order of the modifiers can be interchanged, but by convention visibility modifiers come first
convention visibility modifiers come first
Recall that the Recall that the main main method is static; it is invoked by the method is static; it is invoked by the system without creating an object
system without creating an object
Static methods cannot reference instance variables, because Static methods cannot reference instance variables, because instance variables don't exist until an object exists
instance variables don't exist until an object exists
However, they can reference static variables or local However, they can reference static variables or local variables
variables
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Static Variables Static Variables
Static variables are sometimes called Static variables are sometimes called class variables class variables
Normally, each object has its own data space Normally, each object has its own data space
If a variable is declared as static, only one copy of the If a variable is declared as static, only one copy of the variable exists
variable exists
private static float price;
Memory space for a static variable is created as soon as the Memory space for a static variable is created as soon as the class in which it is declared is loaded
class in which it is declared is loaded
Static Variables Static Variables
All objects created from the class share access to the static All objects created from the class share access to the static variable
variable
Changing the value of a static variable in one object Changing the value of a static variable in one object changes it for all others
changes it for all others
Static methods and variables often work together Static methods and variables often work together
See See CountInstances CountInstances .java .java (page 233) (page 233)
See MyClass.java (page 234) See MyClass.java (page 234)
Nested Classes Nested Classes
In addition to a class containing data and methods, it can In addition to a class containing data and methods, it can also contain other classes
also contain other classes
A class declared within another class is called a A class declared within another class is called a nested class nested class
Outer Class
Nested
Class
Nested Classes Nested Classes
A nested class has access to the variables and methods of A nested class has access to the variables and methods of the outer class, even if they are declared private
the outer class, even if they are declared private
In certain situations this makes the implementation of the In certain situations this makes the implementation of the classes easier because they can easily share information classes easier because they can easily share information
Furthermore, the nested class can be protected by the outer Furthermore, the nested class can be protected by the outer class from external use
class from external use
This is a special relationship and should be used with care This is a special relationship and should be used with care
Nested Classes Nested Classes
A nested class produces a separate bytecode file A nested class produces a separate bytecode file
If a nested class called Inside is declared in an outer class If a nested class called Inside is declared in an outer class called Outside, two bytecode files will be produced:
called Outside, two bytecode files will be produced:
Outside.class
Outside$Inside.class
Nested classes can be declared as static, in which case they Nested classes can be declared as static, in which case they cannot refer to instance variables or methods
cannot refer to instance variables or methods
A nonstatic nested class is called an A nonstatic nested class is called an inner class inner class
Interfaces Interfaces
A Java A Java interface interface is a collection of abstract methods and is a collection of abstract methods and constants
constants
An An abstract method abstract method is a method header without a method is a method header without a method body body
An abstract method can be declared using the modifier An abstract method can be declared using the modifier abstract
abstract , but because all methods in an interface are , but because all methods in an interface are abstract, it is usually left off
abstract, it is usually left off
An interface is used to formally define a set of methods that An interface is used to formally define a set of methods that a class will implement
a class will implement
Interfaces Interfaces
public interface Doable {
public void doThis();
public int doThat();
public void doThis2 (float value, char ch);
public boolean doTheOther (int num);
}
interface is a reserved word interface is a reserved word
None of the methods in an None of the methods in an
interface are given interface are given a definition (body) a definition (body)
A semicolon immediately
A semicolon immediately
follows each method header
follows each method header
Interfaces Interfaces
An interface cannot be instantiated An interface cannot be instantiated
Methods in an interface have public visibility by default Methods in an interface have public visibility by default
A class formally implements an interface by A class formally implements an interface by
• stating so in the class header stating so in the class header
• providing implementations for each abstract method in the providing implementations for each abstract method in the interface
interface
If a class asserts that it implements an interface, it must If a class asserts that it implements an interface, it must define all methods in the interface or the compiler will define all methods in the interface or the compiler will
produce errors.
produce errors.
Interfaces Interfaces
public class CanDo implements Doable {
public void doThis () {
// whatever }
public void doThat () {
// whatever }
// etc.
}
implements is a implements is a reserved word reserved word
Each method listed Each method listed
in Doable is
in Doable is
given a definition
given a definition
Interfaces Interfaces
A class that implements an interface can implement other A class that implements an interface can implement other methods as well
methods as well
See Speaker.java (page 236) See Speaker.java (page 236)
See Philosopher.java (page 237) See Philosopher.java (page 237)
See Dog.java (page 238) See Dog.java (page 238)
A class can implement multiple interfaces A class can implement multiple interfaces
The interfaces are listed in the implements clause, The interfaces are listed in the implements clause, separated by commas
separated by commas
The class must implement all methods in all interfaces The class must implement all methods in all interfaces listed in the header
listed in the header
Polymorphism via Interfaces Polymorphism via Interfaces
An interface name can be used as the type of an object An interface name can be used as the type of an object reference variable
reference variable
Doable obj;
The The obj reference can be used to point to any object of any obj reference can be used to point to any object of any class that implements the
class that implements the Doable Doable interface interface
The version of The version of doThis doThis that the following line invokes that the following line invokes depends on the type of object that
depends on the type of object that obj is referring to: obj is referring to:
obj.doThis();
Polymorphism via Interfaces Polymorphism via Interfaces
That reference is That reference is polymorphic polymorphic , which can be defined as , which can be defined as
"having many forms"
"having many forms"
That line of code might execute different methods at That line of code might execute different methods at different times if the object that
different times if the object that obj obj points to changes points to changes
See Talking.java (page 240) See Talking.java (page 240)
Note that polymorphic references must be resolved at run Note that polymorphic references must be resolved at run time; this is called
time; this is called dynamic binding dynamic binding
Careful use of polymorphic references can lead to elegant, Careful use of polymorphic references can lead to elegant, robust software designs
robust software designs
Interfaces Interfaces
The Java standard class library contains many interfaces The Java standard class library contains many interfaces that are helpful in certain situations
that are helpful in certain situations
The The Comparable Comparable interface contains an abstract method interface contains an abstract method called
called compareTo compareTo , which is used to compare two objects , which is used to compare two objects
The The String class implements String class implements Comparable Comparable which gives which gives us the ability to put strings in alphabetical order
us the ability to put strings in alphabetical order
The The Iterator Iterator interface contains methods that allow the interface contains methods that allow the user to move through a collection of objects easily
user to move through a collection of objects easily
Events Events
An An event event is an object that represents some activity to which is an object that represents some activity to which we may want to respond
we may want to respond
For example, we may want our program to perform some For example, we may want our program to perform some action when the following occurs:
action when the following occurs:
• the mouse is moved the mouse is moved
• a mouse button is clicked a mouse button is clicked
• the mouse is dragged the mouse is dragged
• a graphical button is clicked a graphical button is clicked
• a keyboard key is pressed a keyboard key is pressed
• a timer expires a timer expires
Often events correspond to user actions, but not always Often events correspond to user actions, but not always
Events Events
The Java standard class library contains several classes The Java standard class library contains several classes that represent typical events
that represent typical events
Certain objects, such as an applet or a graphical button, Certain objects, such as an applet or a graphical button, generate (fire) an event when it occurs
generate (fire) an event when it occurs
Other objects, called Other objects, called listeners listeners , respond to events , respond to events
We can write listener objects to do whatever we want when We can write listener objects to do whatever we want when an event occurs
an event occurs
Events and Listeners Events and Listeners
Generator
This object may This object may generate an event generate an event
Listener
This object waits for and This object waits for and
responds to an event responds to an event
Event
When an event occurs, the generator calls When an event occurs, the generator calls
the appropriate method of the listener,
the appropriate method of the listener,
passing an object that describes the event
passing an object that describes the event
Listener Interfaces Listener Interfaces
We can create a listener object by writing a class that We can create a listener object by writing a class that implements a particular
implements a particular listener interface listener interface
The Java standard class library contains several interfaces The Java standard class library contains several interfaces that correspond to particular event categories
that correspond to particular event categories
For example, the For example, the MouseListener MouseListener interface contains interface contains methods that correspond to mouse events
methods that correspond to mouse events
After creating the listener, we After creating the listener, we add the listener to the add the listener to the
component that might generate the event to set up a formal component that might generate the event to set up a formal
relationship between the generator and listener
relationship between the generator and listener
Mouse Events Mouse Events
The following are The following are mouse mouse events events : :
• mouse pressed - the mouse button is pressed down mouse pressed - the mouse button is pressed down
• mouse released - the mouse button is released mouse released - the mouse button is released
• mouse clicked - the mouse button is pressed and released mouse clicked - the mouse button is pressed and released
• mouse entered - the mouse pointer is moved over a particular mouse entered - the mouse pointer is moved over a particular component
component
• mouse exited mouse exited - the mouse pointer is moved off of a particular - the mouse pointer is moved off of a particular component
component
Any given program can listen for some, none, or all of these Any given program can listen for some, none, or all of these
See Dots.java (page 246) See Dots.java (page 246)
