Transcript Interfaces
Specifies a set of methods (i.e., method
headings) that any class that implements that
interface must have.
An interface is a type (but is not a class).
Interface can be parameter types.
Java’s way of approximating multiple
inheritance.
Abstract or concrete classes may implement
interfaces.
To implement an interface, a concrete class must
do:
1.
State “implements InterfaceName” or “implements
InterfaceName1, …, InterfaceNamen”
2.
You must implement all of the method headings
listed in the definition(s) of the interface(s).
To implement an interface, an abstract class
must do:
1.
State “implements InterfaceName” or “implements
InterfaceName1, …, InterfaceNamen”
2.
You must either implement all of the method
headings listed in the definition(s) of the
interface(s), or you must define as abstract the
method headings in the interface(s).
An interface B may extend an interface A and
specify additional method headings.
Any concrete class that implements the derived
interface B must implement all of the methods
in both interfaces A and B.
See
http://java.sun.com/j2se/1.3/docs/api/java/
lang/Comparable.html
Used for sorting. If things can be compared,
they can be sorted.
One method:
public int compareTo ( Object other );
-1 means that this comes before (is less than) other
0 means that this and other are equal
+1 means that this comes after (is greater than) other
Properties:
Irreflexive: For no object o does o come before o.
1.
reflexive:
irreflexive:
2.
Trichotomy: For any two object o1 and o2, one and
only one of the following holds true:
3.
x R x xA
(x R x) xA
o1 comes before o2, or
o1 comes after o2, or
o1 equals o2.
Transitivity: If o1 comes before o2 and o2 comes
before o3, then o1 comes before o3.
if (a.compareTo(b) < 0) {
…
} else if (a.compareTo(b) == 0) {
…
} else { //must be a.compareTo(b)>0
…
}
See
http://java.sun.com/j2se/1.3/docs/api/java/
lang/Double.html
See
http://java.sun.com/j2se/1.3/docs/api/java/
lang/String.html
Is the following a suitable implementation of the Comparable
interface?
public class Double2 implements Comparable {
private double value;
public Double2 ( double theValue ) {
value = theValue;
}
public int compareTo ( Object other ) {
return -1;
}
public double doubleValue ( ) {
return value;
}
}
You can think of the underlying “comes before” relationship as saying
that for any objects d1 and d2, d1 comes before d2.
Suppose you have a class Circle that represents circles all
of whose centers are at the same point. (To make it
concrete you can take the circles to be in the usual x,y
plane and all have their centers at the origin.) Suppose
there is a boolean valued method inside of the class
Circle such that, for circles c1 and c2, c1.inside(c2)
returns true if c1 is completely inside of c2 (and c2 is
not the same as c1). Is the following a total ordering?
c1 comes before c2 if c1 is inside of c2
(that is, c1.inside(c2) returns true).
You could represent objects of the class Circle by a single
value of type double that gives the radius of the circle,
but the answer does not depend on such details.
Constants may be defined in interfaces but ...
Not really in the spirit of an interface
Must be public static final (and will be, even if
omitted)
No instance variables in interfaces
Implement two interfaces which:
1.
have conflicting constants, or
2.
have overloaded methods with different return
types
Implement two interfaces which have
conflicting constants.
class TestInterface implements I1, I2 {
public static void main ( String[] args ) {
System.out.println( A );
}
}
interface I1 {
int A = 100;
}
interface I2 {
int A = 500;
}
Compiler error –
ambiguous.
Implement two interfaces which have
overloaded methods with different return
types.
class TestInterface implements I1, I2 {
public static void main ( String[] args ) {
}
public int f ( ) {
return 0;
}
}
interface I1 {
public int f ( );
}
interface I2 {
public String f ( );
}
Compiler error – TestInterface is
not abstract and does not override
abstract method f() in I2
Implement two interfaces which have
overloaded methods with different return
types.
class TestInterface implements I1, I2 {
public static void main ( String[] args ) {
}
public int f ( ) {
return 0; }
public String f ( ) {
return null;
}
}
interface I1 {
public int f ( );
}
interface I2 {
public String f ( );
}
Compiler error – f() is already
defined in TestInterface
Implement two interfaces which have
overloaded methods with same return types.
class TestInterface implements I1, I2 {
public static void main ( String[] args ) {
}
public int f ( ) {
OK
return 0;
}
}
interface I1 {
public int f ( );
}
interface I2 {
public int f ( );
}
Note that the Object class has a clone() method.
See
http://java.sun.com/j2se/1.5.0/docs/api/java/lang
/Object.html
protected Object clone ( )
Creates and returns a copy of this object.
Note that it’s protected.
Interesting! Let’s give it a try.
public class Test {
private int x=5;
public static void main ( String args[] ) {
Test t1 = new Test();
t1.x = 12;
Test t2 = (Test)t1.clone(); Compiler error:
System.out.println( t2.x ); Test.java:8: unreported exception
java.lang.CloneNotSupportedException;
}
must be caught or declared to be
}
thrown
Test t2 = (Test)t1.clone();
^
1 error
public class Test {
private int x=5;
public static void main ( String args[] ) {
Test t1 = new Test();
Compiles OK. But runtime error:
t1.x = 12;
error:
Test t2 = null;
java.lang.CloneNotSupportedException:
try {
Test
Exception in thread "main"
t2 = (Test)t1.clone();
java.lang.NullPointerException
} catch (Exception e) {
at Test.main(Test.java:14)
System.out.println( "error: " + e );
}
System.out.println( t2.x );
}
}
Has no headings.
Has no constants.
So what does it do? Magic!
public class Test implements Cloneable {
private int x=5;
public static void main ( String args[] ) {
Test t1 = new Test();
t1.x = 12;
Test t2 = null;
try {
t2 = (Test)t1.clone();
} catch (Exception e) {
System.out.println( "error: " + e );
}
System.out.println( t2.x );
}
}
Compile OK.
Runs OK.
Outputs:
12
Object.clone()
Exact, bit-by-bit copy
May cause privacy leaks
To avoid privacy leaks, your clone() method
should:
1.
2.
Invoke super.clone(), and
Create new instances of mutable types
Simplest case
1.
2.
You have no mutable instance variables (but you
may have primitive and/or immutable instance
variables), and
You are derived from Object.
public class Test implements Cloneable {
private int x=5;
public Object clone ( ) {
try {
return super.clone();
} catch (Exception e) {
return null;
}
}
public static void main ( String args[] ) {
Test t1 = new Test();
t1.x = 12;
Test t2 = (Test)t1.clone();
System.out.println( t2.x ); //outputs 12
}
}
Compiles OK.
Runs w/out error/exception.
Don’t have to trouble ourselves in
main() with exceptions.
Outputs 12 as expected.
import java.util.Date;
import java.awt.Point;
public static void main ( String args[] ) {
Point3D p1 = new Point3D();
Point3D p2 = (Point3D)p1.clone();
public class Point3D extends Point
implements Cloneable
{
private int z = 0;
private Date date = new Date();
System.out.println( p2.date );
//month becomes March
p1.date.setMonth( 2 );
System.out.println( p2.date );
}
public Object clone ( ) {
Point3D copy = null;
try {
copy = (Point3D)super.clone();
} catch (Exception e) {
return null;
}
return copy;
}
}
This example:
(a) Clones p1 into p2.
(b) Prints p2.
(c) Changes p1.
(d) Prints p2 again (which has now
magically changed).
import java.util.Date;
import java.awt.Point;
public static void main ( String args[] ) {
Point3D p1 = new Point3D();
Point3D p2 = (Point3D)p1.clone();
public class Point3D extends Point
implements Cloneable
{
private int z = 0;
private Date date = new Date();
System.out.println( p2.date );
//month becomes March
p1.date.setMonth( 2 );
System.out.println( p2.date );
}
public Object clone ( ) {
Point3D copy = null;
try {
copy = (Point3D)super.clone();
} catch (Exception e) {
return null;
}
copy.date = (Date)date.clone();
return copy;
}
}
1.
ActionListener
2.
MouseListener
3.
MouseMotionListener
Mention sync vs. async event handling.
See
http://java.sun.com/j2se/1.5.0/docs/api/java
/awt/event/ActionListener.html.
One method:
void actionPerformed ( ActionEvent e );
Invoked when an action occurs such as a button press.
void mouseClicked ( MouseEvent e )
void mousePressed ( MouseEvent e )
Invoked when a mouse button has been released on a
component.
void mouseEntered ( MouseEvent e )
Invoked when a mouse button has been pressed on a
component.
void mouseReleased ( MouseEvent e )
Invoked when the mouse button has been clicked (pressed and
released) on a component.
Invoked when the mouse enters a component.
void mouseExited ( MouseEvent e )
Invoked when the mouse exits a component.
void mouseDragged ( MouseEvent e )
Invoked when a mouse button is pressed on a component and
then dragged. MOUSE_DRAGGED events will continue to be
delivered to the component where the drag originated until the
mouse button is released (regardless of whether the mouse
position is within the bounds of the component).
Due to platform-dependent Drag&Drop implementations,
MOUSE_DRAGGED events may not be delivered during a
native Drag&Drop operation.
void mouseMoved ( MouseEvent e )
Invoked when the mouse cursor has been moved onto a
component but no buttons have been pushed.