1. Interfaces and Polymorphism
Developing Reusable Solutions
Converting between Types
Polymorphism
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2. Interface
A Java interface declares a set of methods and their signatures. Unlike a class, it provides no implementation. (meaning it has no code, just the first line of each method)
To realize (implement) an interface, a class must supply all the methods that the interface requires.
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3. public interface Comparable
Method Summary 
intcompareTo(Object o)
Compares this object with another. 
If you want your class to be Comparable, it needs a compareTo method that takes in an Object and returns an integer.
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4. Example String implements Comparable
We can compare two strings by invoking the String method compareTo.
String makes a promise to "define" what is means to compare two String objects.
String s1 = <some string>;
String s2 = <some string>;
if (s2.compareTo(s1))…….
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5. Examples { } public int compareTo(Object o)
If we want to compare two circles, we must supply the method compareTo.
public class Circle implements Comparable {
public int compareTo(Object o)
// Circle must define what it means to compare two Circle objects. }
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6. Examples public class Circle implements Comparable {
public int compareTo(Object o)
Circle c = (Circle) o;
if (this.equals(c)) return 0;
if (this.radius < c.radius) return -1;
return 1; }
… // we have already defined equals for Circle. What if equals is not defined for Circle?
this is not required here.
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7. How do you compare students?
Examples
public class Student implements Comparable {
public int compareTo(Object o) } …
How do you compare students?
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8. Examples public class Student implements Comparable {
public int compareTo(Object o)
Student s = (Student) o;
if (this.equals(s)) return 0;
if (this.getGPA() < s.getGPA()) return -1;
else return 1; } …
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9. I’m a Person, I’m a Graduate, I’m an Achiever..
An interface…..
"To be useful, an interface must be realized (implemented) by at least one Java class." – Rick Mercer
A class may implement MANY interfaces.
I’m a Person, I’m a Graduate, I’m an Achiever..
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10. A picture (BlueJ) <<Flier>> fly() <<Athlete>>
train()
SkiJumper
fly()
train()
compareTo()
Bird
fly()
findHome()
Airplane
fly()
Engine
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11. Interfaces SHOULD NOT GROW once defined!
Classes have "contracts" with an interface.
If the interface changes, all classes that realize the interface are affected.
Interfaces allow for encapsulating similar behaviors between unrelated classes.
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12. Interface Contains constants, methods or both.
NO implementations. (no code)
No instance variables.
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13. Using interfaces Define a set of methods as an interface.
Classes that realize the interface must support all the methods of the interface.
Create classes that implement those methods.
The Compiler will verify that method calls are valid.
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14. Interface Example public interface Flier { void fly(); }
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15. Interface declarations:
All methods in an interface are public. The methods in the interface are not declared as public because they are public by default.
When implementing the methods of the interface in a class, include the keyword public.
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16. Classes realizing Flier
public class Bird implements Flier {
public void fly()
System.out.println("Using my wings to fly"); }
==================================================================
public class Airplane implements Flier
System.out.println("Using my jet engines to fly");
public class SkiJumper implements Flier, Athlete, Comparable
System.out.println("Using skis to take me into the air");
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17. Abstract An interface is ABSTRACT.
You can’t create an object of that type.
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18. Interfaces
Cannot be instantiated.
Flier f = new Flier();NO
You can use interface name as variable type; variable can refer to an object of any class that implements the interface.
Flier b = new Bird();OK
Flier s = new SkiJumper();OK
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19. Using interfaces Interfaces define types (sets of methods).
A variable of type Flier must refer to an object that implements methods defined by Flier, not necessarily to an instance of Flier.
Actual method invoked is defined by the object’s class, at run-time. (dynamic binding)
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20. polymorphism…
Dynamic Binding (overriding): Behavior can vary depending on the actual type of an object. Occurs at runtime.
Early Binding (overloading) occurs at compile time.
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21. polymorphism
"In the object-oriented world, polymorphism refers to the ability of different kinds of objects to respond differently to the same commands, provided that the objects belong to classes with a common ancestor."
K.N.King
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22. Again…
Early binding of methods occurs if the compiler selects a method from several possible candidates as with overloaded methods.
Late binding occurs if the method selection takes place when the program runs.
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23. The importance of IS-A
You can convert from a class type to an interface type if the class realizes the interface.
String str = "word";
Comparable p = str; // OK
You need a cast to convert from an interface type to a class type.
Comparable p = str;
String t = p; // NO
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24. The importance of IS-A
You can convert from a class type to an interface type if the class realizes the interface.
String str = "word";
Comparable p = str; // OK
You need a cast to convert from an interface type to a class type.
Comparable p = str;
String t = (String)p; // OK
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25. instanceof operator
The instanceof operator tests whether an object belongs to a particular type. What happens?
Flier aFlier = new SkiJumper("Joe","Smith");
Flier aFlier2 = new Airplane();
if (aFlier instanceof SkiJumper)
System.out.println("That's right.");
else
System.out.println("NOT");
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26. instanceof operator
The instanceof operator tests whether an object belongs to a particular type. What happens?
Flier aFlier = new SkiJumper("Joe","Smith");
Flier aFlier2 = new Airplane();
if (aFlier instanceof SkiJumper)
System.out.println("That's right.");
else
System.out.println("NOT");
That's right.
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27. instanceof operator
The instanceof operator tests whether an object belongs to a particular type. What happens?
Flier aFlier = new SkiJumper("Joe","Smith");
Flier aFlier2 = new Airplane();
if (aFlier instanceof Flier)
System.out.println("That's right.");
else
System.out.println("NOT");
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28. instanceof operator
The instanceof operator tests whether an object belongs to a particular type.
Flier aFlier = new SkiJumper("Joe","Smith");
Flier aFlier2 = new Airplane();
if (aFlier instanceof Flier)
System.out.println("That's right.");
else
System.out.println("NOT");
That's right.
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29. instanceof operator
The instanceof operator tests whether an object belongs to a particular type. What happens?
Flier aFlier = new SkiJumper("Joe","Smith");
Flier aFlier2 = new Airplane();
if (aFlier2 instanceof Flier)
System.out.println("That's right.");
else
System.out.println("NOT");
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30. instanceof operator
The instanceof operator tests whether an object belongs to a particular type.
Flier aFlier = new SkiJumper("Joe","Smith");
Flier aFlier2 = new Airplane();
if (aFlier2 instanceof Flier)
System.out.println("That's right.");
else
System.out.println("NOT");
That's right.
Drew University

31. instanceof operator
The instanceof operator tests whether an object belongs to a particular type. What happens?
Flier aFlier = new SkiJumper("Joe","Smith");
Flier aFlier2 = new Airplane();
if (aFlier2 instanceof SkiJumper)
System.out.println("That's right.");
else
System.out.println("NOT");
Drew University

32. instanceof operator
The instanceof operator tests whether an object belongs to a particular type.
Flier aFlier = new SkiJumper("Joe","Smith");
Flier aFlier2 = new Airplane();
if (aFlier2 instanceof SkiJumper)
System.out.println("That's right.");
else
System.out.println("NOT");
NOT
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33. Polymorphism is NOT Overloading.
Method signature
refers to method name and parameters
Overloading means methods have different signatures(different parameters).
Overloading usually occurs in same class.
BankAccount has two constructors. One of the constructors has one double parameter. One of the constructors has no parameters.
The compiler chooses the appropriate method.
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34. Polymorphism
The actual type of the object determines which method is to be called.
This is late binding (dynamic binding).
The virtual machine, not the compiler, selects the appropriate method at run time.
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35. Consider the following DataSet class.
//Computes the average of a set of
// data values.
public class DataSet {
public DataSet()
sum = 0;
count = 0;
maximum = 0; }
//Adds a data value to the data // set
public void add(double x)
sum = sum + x;
if (count == 0 || maximum < x)
maximum = x;
count++;
//Returns the average of the added data.
public double getAverage() {
if (count == 0) return 0;
else return sum / count; }
//Returns the largest of the added data
// or 0 if no data has been added
public double getMaximum()
return maximum;
private double sum;
private double maximum;
private int count;
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36. Concerns
Suppose we wanted to find the BankAccount with the highest balance?
Suppose we wanted to find the Coin with the highest value?
Suppose we wanted to find the Student with the highest gpa?
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37. Solution: //Computes the average of a set of // data values.
public class DataSet {
public DataSet()
sum = 0;
count = 0;
maximum = 0; }
//Adds a data value to the data // set
public void add(BankAccount x)
sum = sum + x.getBalance();
if (count == 0
|| maximum.getBalance() < x.getBalance())
maximum = x;
count++;
//Returns the average of the added data.
public double getAverage() {
if (count == 0) return 0;
else return sum / count; }
//Returns the largest of the added data
// or 0 if no data has been added
public BankAccount getMaximum()
return maximum;
private double sum;
private BankAccount maximum;
private int count;
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38. Solution: //Computes the average of a set of // data values.
public class DataSet {
public DataSet()
sum = 0;
count = 0;
maximum = 0; }
//Adds a data value to the data // set
public void add(Coin x)
sum = sum + x.getValue();
if (count == 0
|| maximum.getValue () < x.getValue ())
maximum = x;
count++;
//Returns the average of the added data.
public double getAverage() {
if (count == 0) return 0;
else return sum / count; }
//Returns the largest of the added data
// or 0 if no data has been added
public Coin getMaximum()
return maximum;
private double sum;
private Coin maximum;
private int count;
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39. Solution: //Computes the average of a set of // data values.
public class DataSet {
public DataSet()
sum = 0;
count = 0;
maximum = 0; }
//Adds a data value to the data // set
public void add(Student x)
sum = sum + x.getGpa();
if (count == 0
|| maximum.getGpa () < x.getGpa ())
maximum = x;
count++;
//Returns the average of the added data.
public double getAverage() {
if (count == 0) return 0;
else return sum / count; }
//Returns the largest of the added data
// or 0 if no data has been added
public Student getMaximum()
return maximum;
private double sum;
private Student maximum;
private int count;
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40. Hmmmmm…
Clearly the mechanics of analyzing the data is the same in ALL cases but the details of measurement are different.
BankAccount : balance
Coin: value
Student: gpa
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41. Suppose we consider the following:
public interface Measurable {
double getMeasure(); }
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42. Solution: //Computes the average of a set of // data values.
public class DataSet {
public DataSet()
sum = 0;
count = 0;
maximum = 0; }
//Adds a data value to the data // set
public void add(Measurable x)
sum = sum + x.getMeasure();
if (count == 0
|| maximum.getMeasure () < x.getMeasure ())
maximum = x;
count++;
//Returns the average of the added data.
public double getAverage() {
if (count == 0) return 0;
else return sum / count; }
//Returns the largest of the added data
// or 0 if no data has been added
public Measurable getMaximum()
return maximum;
private double sum;
private Measurable maximum;
private int count;
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43. And… public class BankAccount implements Measurable { . . . . .
public double getMeasure() {
return balance; }
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44. Test class public class DataSetTest {
public static void main(String[] args)
DataSet bankData = new DataSet();
bankData.add(new BankAccount(0));
bankData.add(new BankAccount(10000));
bankData.add(new BankAccount(2000));
System.out.println("Average balance = " + bankData.getAverage());
Measurable max = bankData.getMaximum();
System.out.println("Highest balance = " + max.getMeasure());
DataSet coinData = new DataSet();
coinData.add(new Coin(0.25, "quarter"));
coinData.add(new Coin(0.1, "dime"));
coinData.add(new Coin(0.05, "nickel"));
System.out.println("Average coin value = " + coinData.getAverage());
max = coinData.getMaximum();
String name = ((Coin)max).getName();
System.out.println(name + " has the highest coin value of " + max.getMeasure()); }
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45. And… public class Coin implements Measurable { . . . . .
public double getMeasure() {
return value; }
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46. And… public class Student implements Measurable { . . . . .
public double getMeasure() {
return gpa; }
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