Understanding Java Interfaces for Effective Programming

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 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

9. Total order • Properties: • Irreflexive: For no object o does o come before o. • reflexive: x R x xA • irreflexive: (x R x) xA • Trichotomy: For any two object o1 and o2, one and only one of the following holds true: • 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.

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

11. Double and String implement Comparable • 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

12. 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 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.

13. 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.

14. 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) • No instance variables in interfaces

15. Inconsistent interfaces

16. Inconsistent interfaces • Implement two interfaces which: • have conflicting constants, or • have overloaded methods with different return types

17. Inconsistent interfaces • 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.

18. Inconsistent interfaces • 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

19. Inconsistent interfaces • 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

20. Consistent interfaces • 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 { public int f ( ); } OK

21. The clone() method

22. Back to the Object class • 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.

23. 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

24. 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. But runtime error: error: java.lang.CloneNotSupportedException: TestException in thread "main" java.lang.NullPointerException at Test.main(Test.java:14)

25. Cloneable interface • Has no headings. • Has no constants. • So what does it do? Magic!

26. 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

27. 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

28. 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.

29. 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. Don’t have to trouble ourselves in main() with exceptions. Outputs 12 as expected.

30. 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 ); System.out.println( p2.date ); } } • This example: • Clones p1 into p2. • Prints p2. • Changes p1. • Prints p2 again (which has now magically changed).

31. 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 ); System.out.println( p2.date ); } }

32. Listener interfaces

33. Listener interfaces • ActionListener • MouseListener • MouseMotionListener • Mention sync vs. async event handling.

34. ActionListener interface • 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.

35. MouseListener interface • void mouseClicked ( MouseEvent e ) • Invoked when the mouse button has been clicked (pressed and released) on a component. • void mousePressed ( MouseEvent e ) • Invoked when a mouse button has been pressed on a component. • void mouseReleased ( MouseEvent e ) • Invoked when a mouse button has been released on a component. • void mouseEntered ( MouseEvent e ) • Invoked when the mouse enters a component. • void mouseExited ( MouseEvent e ) • Invoked when the mouse exits a component.

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