1. Interfaces in Java

2. What is an interface? public class BankAccount {
public BankAccount() {...}
public void deposit(double amount) {...}
public void withdraw(double amount) {...}
public double getBalance() {...} }
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3. What is an interface? If we have agreed on the interface…
…but do not have any sensible default implementation of the interface…
…then why must we provide a (perhaps useless) class with a default implemen-tation to the user?
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4. What is an interface? We can use interfaces instead!
public class Shape {
public void draw() {???}
public double getArea() {???} }
We can use interfaces instead!
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5. What is an interface? public interface Shape { void draw();
double getArea(); }
New keyword!
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6. Interface properties An interface is different than a class
All methods are abstract; they have no implementation
All methods are automatically public
No instance fields
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7. Why use interfaces? Promotes loose coupling!
Another programmer working with shape classes need only to know the interface type, not concrete shape classes
”Program to an interface, not an implementation”…
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8. Why use interfaces? Shape + draw() + getArea() Circle - radius - x - y
+ getRadius()
Square
- length
+ Square(…)
Point
+ Point(…)
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9. Implementing interfaces
Any class that wishes to implement a given interface, must explicitly state this
Not enough just to implement the methods themselves
For this purpose, we use the implements keyword
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10. Implementing an interface
public class Circle implements Shape {
private double radius;
private double x;
private double y;
public Circle(...) {...}
public void draw() {...} // from Shape
public double getArea() {...} // from Shape
public double getRadius() {...} // New }
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11. Programming with interfaces
Now, only the creator of various shape objects need to know the exact type of a concrete shape
Other parts of the code only need to know the interface
The latter code is loosely coupled to the concrete shape classes
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12. Programming with interfaces
public void produceShapes() {
Shape s1 = new Circle(80,300,300);
Shape s2 = new Square(50,200,200);
Shape s3 = new Point(100,400);
processShape(s1);
processShape(s2);
processShape(s3); }
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13. Programming with interfaces
public void processShape(Shape s) {
double area = s.getArea();
String result = ”Area is ” + area;
System.out.println(result);
s.draw(); }
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14. Important!! This is not possible: Shape s = new Shape(); // NO!
We can never create an object that (only) has an interface type
A variable can have the type ”reference to an interface”
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15. Important!! In other words: Shape s1 = new Square(10,10,10); // OK
Shape s2 = new Point(20,20); // OK
Circle c = new Circle(5,10,20); // OK
Shape s = new Shape(); // NO!
Circle c = new Shape(); // NO!
Square sq = new Circle(5,20,40); // NO!
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16. Conversions Why does this actually work…?
Shape s = new Circle(10,10,10);
The object we create has type Circle
The variable has the type Shape
The class Circle implements the interface Shape
This makes the conversion from Circle to Shape legal
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17. Conversions
”You can always convert from a class type to an interface type, provided the class implements the interface”
Shape
Circle
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18. Conversions There is a price to pay…
We can now only use methods defined in the Shape interface on the object
Shape s = new Circle(10,10,10);
double r = s.getRadius(); // NO!
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19. Conversions How about this…? public void enlarge(Shape s) {
Circle c = s;
double r = c.getRadius();
c.setRadius(2*r); }
Error!
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20. Conversions How about this then…? public void enlarge(Shape s) {
Circle c = (Circle)s;
double r = c.getRadius();
c.setRadius(2*r); }
OK – if s is a Circle!
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21. Conversions
”You can convert from an interface type to a class type, but only if the actual classes are the same – being sure of that is your responsibility!”
Shape
Square
Circle
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22. Using interfaces for callbacks
Consider a simple task – comparing two objects, to see which one is ”best”
Easy…if we know what we mean by ”best”
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23. Using interfaces for callbacks
For numbers, this is a very simple task
public bool AisBest(int a, int b) {
return (a > b); }
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24. Using interfaces for callbacks
For a BankAccount class, perhaps like this:
public bool AisBest(BankAccount a, BankAccount b) {
return (a.getBalance() > b.getBalance()); }
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25. Using interfaces for callbacks
Major problem: We have to write a new method for each type…
Maybe we can use interfaces to help us!
public interface Measurable {
double getMeasure(); }
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26. Using interfaces for callbacks
public bool AisBest(Measurable a, Measurable b) {
return (a.getMeasure() > b.getMeasure()); }
This will work for all classes implementing the Measurable interface!
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27. Using interfaces for callbacks
This is great, but…
…we can only make our own classes implement the Measurable interface
We cannot force e.g. the Rectangle class to implement the Measurable interface
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28. Using interfaces for callbacks
We would like something like this…
public bool AisBest(Object a, Object b) {
return (a.getMeasure() > b.getMeasure()); }
… but Object does not implement the Measurable interface (of course)
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29. Using interfaces for callbacks
A different approach is to create an inter-face for making a measurement, not giving a measurement
Class is then no longer responsible for measuring itself
A bit contrary to OO-philosophy…
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30. Using interfaces for callbacks
Define an interface for being able to make a measurement:
public interface Measurer {
double measure(Object x); }
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31. Using interfaces for callbacks
Define a class being able to measure a Rectangle object
public class RectMeasurer implements Measurer {
public double measure(Object x)
Rectangle rect = (Rectangle)x;
// Code for appropriate measure }
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32. Using interfaces for callbacks
We then get…
public bool AisBest(Object a, Object b, Measurer m) {
return (m.measure(a) > m.measure(b)); }
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33. Using interfaces for callbacks
We call it like this:
Measurer m = new RectMeasurer();
Rectangle r1 = new Rectangle(10,20,30,40);
Rectangle r2 = new Rectangle(15,25,35,45);
boolean aIsBetterThanB = AisBest(r1,r2,m);
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34. Using interfaces for callbacks
Advantages:
Can compare all classes
Can change strategy for measurement, without changing class itself
Drawbacks
A bit risky – danger of mixing up objects and measurers (do we like casting…?)
Somewhat anti-OO…
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