1. Interfaces

2. Interfaces An interface is a Java structure similar to a class
an interface defines methods with signatures only (no body)
public interface Plumber {
public void clearDrain();
public double computeBill(); }
An interface defines the essence of something

3. Interfaces (continued)
Other classes can implement an interface
public class Handyman extends Carpenter
implements Plumber {
implementation is a contract, indicating that the class will include the methods specified by the interface (real methods, with bodies)
this creates the same kind of “is a” relationship as with superclasses:
a Handyman is a Handyman
a Handyman is a Carpenter
a Handyman is a Plumber

4. Interfaces (continued)
Interfaces are Java’s approach to multiple inheritance
when extending a superclass, a class inherits the functionality (methods) of the superclass
when implementing an interface, a class agrees to provide the functionality (methods) of the interface
a class may only extend one superclass, but may implement many interfaces
public class Handyman extends Carpenter implements Plumber, Electrician, Painter {…}

5. Iterator Interface Class ArrayList has the method:
public Iterator iterator() Returns an iterator over the elements in
this list in proper sequence.
Iterator is an interface:
public interface Iterator {
public boolean hasNext(); // true if another object exists
public Object next(); // returns next object
public void remove(); // removes last object returned }
What does it mean when the return type is an interface?

6. Iterator Interface Class ArrayList has the method:
public Iterator iterator() Returns an iterator over the elements in
this list in proper sequence.
Iterator is an interface:
public interface Iterator {
public boolean hasNext(); // true if another object exists
public Object next(); // returns next object
public void remove(); // removes last object returned }
What does it mean when the return type is an interface?
It means that the method returns an Object of a class that implements that interface. (We don’t know, or care, what the class name is.). We can interact with the object using the interface methods.

7. Iterator Interface ArrayList list = new ArrayList();
list.add (new Student (“Wemba”, “CSS”, 3.7, 75)); …
Iterator it = list.iterator();
while (it.hasNext()) {
Student s = (Student)it.next();
System.out.println (s.getName() + “ “ + s.getGPA()); }

8. Interfaces as Data Types
an interface can be used as a data type when declaring variables, just as a class type can:
Iterator it;
however, objects of an interface cannot be instantiated:
Iterator it = new Iterator(); // incorrect!
objects of a class which implements the interface can be assigned to such a variable:
it = list.iterator(); // returned object implements Iterator

9. Polymorphism with Interfaces
interface Speaker {
void speak(); }
class Dog implements Speaker {
public void speak() {
System.out.println (“Woof”);
class Cat implements Speaker {
System.out.println (“Meow”);
class Cow implements Speaker {
System.out.println (“Moo”);
Speaker[] critters = new Speaker[3];
int k = 0;
critters[k++] = new Cat();
critters[k++] = new Dog();
critters[k++] = new Cow();
for (int j=0; j<critters.length; j++) {
critters[j].speak(); }

10. More About Interfaces interfaces may contain: methods
public & abstract by default
have no body (end with semi-colon)
constants
public, static, final by default

11. public interface ConversionFactors{
double MM_PER_INCH = 25.4;
double GRAMS_PER_OUNCE = ;
double GRAMS_PER_POUND = ;
double WATTS_PER_HP = 745.7;
double HP_PER_WATT = 1 / WATTS_PER_HP;
} // don't have to say public, static, or final
public interface Conversions
double inchToMM(double inches);
double ounceToGram(double ounces);
double poundToGram(double pounds);
double HPToWatt(double hp);
double wattToHP(double watts);
} // don't have to say public or abstract

12. public interface ConversionFactors{
double MM_PER_INCH = 25.4;
double GRAMS_PER_OUNCE = ;
double GRAMS_PER_POUND = ;
double WATTS_PER_HP = 745.7;
double HP_PER_WATT = 1 / WATTS_PER_HP; }
public class MyClass implements ConversionFactors
private double poundsWeight;
public double getMetricWeight()
return poundsWeight * GRAMS_PER_POUND;
...

13. public interface Conversions{
double inchToMM(double inches);
double ounceToGram(double ounces);
double poundToGram(double pounds);
double HPToWatt(double hp);
double wattToHP(double watts); }
public class MyClass implements Conversions,
ConversionFactors
public double inchToMM(double inches)
return inches * MM_PER_INCH;
...
// need to implement all methods,
// or make class abstract

14. interface Alpha {
void b();
void c(); }
class Beta implements Alpha {
void b() {
System.out.println(“b() called”);
void c() {
System.out.println(“c() called”);
public class InterfaceExample {
public static void main(String[] args) {
Alpha x = new Alpha();
Alpha y = new Beta();
y.b();
y.c();

15. interface Alpha {
void b();
void c(); }
class Beta implements Alpha {
void b() {
System.out.println(“b() called”);
void c() {
System.out.println(“c() called”);
public class InterfaceExample {
public static void main(String[] args) {
Alpha x = new Alpha(); // no way
Alpha y = new Beta(); // OK
y.b();
y.c();