1. Interfaces

2. 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.)

3. Interfaces
Abstract or concrete classes may implement interfaces.

4. Interfaces To implement an interface, a concrete class must do:
State “implements InterfaceName” or “implements InterfaceName1, …, InterfaceNamen”
You must implement all of the method headings listed in the definition(s) of the interface(s).

5. Interfaces To implement an interface, an abstract class must do:
State “implements InterfaceName” or “implements InterfaceName1, …, InterfaceNamen”
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).

6. Interfaces and interfaces
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.

7. The Comparable interface

8. The Comparable interface
See
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

9. The Comparable interface
“This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class's natural ordering, and the class's compareTo method is referred to as its natural comparison method.”

10. Totally ordered set. from http://mathworld. wolfram
A total order (or "totally ordered set," or "linearly ordered set") is a set plus a relation on the set (called a total order) that satisfies the conditions for a partial order plus an additional condition known as the comparability condition. A relation <= is a total order on a set S ("<= totally orders S") if the following properties hold.
Reflexivity: a<=a for all a in S.
Antisymmetry: a<=b and b<=a implies a=b.
Transitivity: a<=b and b<=c implies a<=c.
Comparability (trichotomy law):
For any a,b in S, either a<=b or b<=a.
The first three are the axioms of a partial order, while addition of the trichotomy law defines a total order.

11. Using the Comparable interface
if (a.compareTo(b) < 0) { … } else if (a.compareTo(b) == 0) { } else { //must be a.compareTo(b)>0 }

12. Double and String implement Comparable
See
See

13. Problem
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 getValue ( ) {
return value;
You can think of the underlying “comes-before” relationship as saying that for any objects d1 and d2, d1 comes-before d2.

14. Problem
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 getValue ( ) {
return value;
You can think of the underlying “comes before” relationship as saying that for any objects d1 and d2, d1 comes before d2.
Problems:
1. compareTo indicates that a<a. This is not true. When a==a, it should return 0.
2. When this is a and other is b, compareTo indicates a<b. But when this is b and other is a, compareTo indicates b<a. It cannot be both!

15. Problem
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.)

16. Problem
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.)
Problem: not reflexive
Reflexivity: a<=a for all a in S.

17. Constants and interfaces

18. Constants and interfaces
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!)
Also, no instance variables in interfaces.

19. Inconsistent interfaces

20. Inconsistent interfaces
Let’s mplement two interfaces which:
have conflicting constants, or
have overloaded methods with different return types

21. Inconsistent interfaces
Let’s 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.

22. Inconsistent interfaces
Let’s 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

23. Inconsistent interfaces
Let’s 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

24. Consistent interfaces
Let’s 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 ( ) {
return 0;
interface I1 {
public int f ( );
interface I2 { OK

25. The clone() method

26. Back to the Object class
Note that the Object class has a clone() method.
See
protected Object clone ( )
Creates and returns a copy of this object.
Note that it’s protected.
Interesting! Let’s give it a try.

27. clone() method
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(); System.out.println( t2.x ); }
Compiler error:
Test.java:8: unreported exception java.lang.CloneNotSupportedException; must be caught or declared to be thrown Test t2 = (Test)t1.clone(); ^ 1 error
We discover that the clone method throws an exception (that we aren’t/forgot to handle).

28. clone() method
public class Test { 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 );
Compiles OK (we handle the exception). But get a runtime error:
error: java.lang.CloneNotSupportedException: Test Exception in thread "main" java.lang.NullPointerException at Test.main(Test.java:14)
Now we discover that Object’s clone seems to always thrown an exception no matter what we do. (As we will see, it’s because we failed to implement the Cloneable interface – which Object’s clone method checks!)

29. Cloneable interface has no headings has no constants
So what does it do? Magic!
Actually, implementing the Cloneable interface allows us to pass Object’s check.

30. clone() method revisited
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

31. Cloneable interface Object.clone()
exact, bit-by-bit copy
may cause privacy leaks!
To avoid privacy leaks, your clone() method should:
Invoke super.clone(), and
Create new instances of mutable types

32. Cloneable interface Simplest case
You have no mutable instance variables (but you may have primitive and/or immutable instance variables), and
You are derived from Object.

33. Cloneable interface: simplest case
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.
We don’t have to trouble ourselves in main() with exceptions.
Outputs 12 as expected.

34. Cloneable interface – more extensive case (w/ problem)
import java.util.Date; import java.awt.Point; public class Point3D extends Point implements Cloneable { private int z = 0; private Date date = new Date(); public Object clone ( ) { Point3D copy = null; try { copy = (Point3D)super.clone(); } catch (Exception e) { return null; } return copy;
public static void main ( String args[] ) { Point3D p1 = new Point3D(); Point3D p2 = (Point3D)p1.clone(); System.out.println( p2.date ); //month becomes March p1.date.setMonth( 2 ); }
This example:
Clones p1 into p2.
Prints p2.
Changes p1’s date.
Prints p2 again (which has now magically changed).

35. Cloneable interface – more extensive case (problem fixed)
import java.util.Date; import java.awt.Point; public class Point3D extends Point implements Cloneable { private int z = 0; private Date date = new Date(); public Object clone ( ) { Point3D copy = null; try { copy = (Point3D)super.clone(); } catch (Exception e) { return null; } copy.date = (Date)date.clone(); return copy;
public static void main ( String args[] ) { Point3D p1 = new Point3D(); Point3D p2 = (Point3D)p1.clone(); System.out.println( p2.date ); //month becomes March p1.date.setMonth( 2 ); }

36. Listener interfaces

37. Listener interfaces ActionListener MouseListener MouseMotionListener
(Mention sync vs. async event handling.)

38. ActionListener interface
See
One method:
void actionPerformed ( ActionEvent e );
Invoked when an action occurs such as a button press.

39. Using the ActionListener
import java.awt.event.ActionEvent; import java.awt.event.ActionListener; class Test implements ActionListener { public void actionPerformed ( ActionEvent e ) { } public static void main ( String p[] ) { Test t = new Test();

40. MouseListener interface

41. MouseListener interface
void mouseClicked ( MouseEvent e )
Invoked when the mouse button has been clicked (pressed and released) on a component.

42. MouseListener interface
void mousePressed ( MouseEvent e )
Invoked when a mouse button has been pressed on a component.

43. MouseListener interface
void mouseReleased ( MouseEvent e )
Invoked when a mouse button has been released on a component.

44. MouseListener interface
void mouseEntered ( MouseEvent e )
Invoked when the mouse enters a component.

45. MouseListener interface
void mouseExited ( MouseEvent e )
Invoked when the mouse exits a component.

46. Using the MouseListener
import java.awt.event.MouseEvent; import java.awt.event.MouseListener; class Test implements MouseListener { public void mouseClicked ( MouseEvent e ) { } public void mouseEntered ( MouseEvent e ) { } public void mouseExited ( MouseEvent e ) { } public void mousePressed ( MouseEvent e ) { } public void mouseReleased ( MouseEvent e ) { System.out.println( "mouseReleased: " + e ); } public static void main ( String p[] ) { Test t = new Test();

47. MouseMotionListener interface

48. MouseMotionListener interface
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.

49. MouseMotionListener interface
void mouseMoved ( MouseEvent e )
Invoked when the mouse cursor has been moved onto a component but no buttons have been pushed.

50. Using the MouseMotionListener
import java.awt.event.MouseEvent; import java.awt.event.MouseMotionListener; class Test implements MouseMotionListener { public void mouseDragged ( MouseEvent e ) { } public void mouseMoved ( MouseEvent e ) { } public static void main ( String p[] ) { Test t = new Test(); }