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Interfaces and Polymorphism

9.1 Developing Reusable Solutions9.2 Converting between Types

9.3 PolymorphismCommon Error 9.1Advanced Topic 9.1

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Interface

A Java interface declare a set of methods and their signatures. Unlike a class, it provides no implementation.

To realize (implement) an interface, a class must supply all the methods that the interface requires.

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public interface Comparable

Method Summary  intcompareTo(Object o)

Compares this object with the specified object for order. 

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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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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Examples

public class Circle implements Comparable

{ public int compareTo(Object o)

// Circle must define what it means to compare two Circle objects.

}

If we want to compare two circles, we must supply the method

compareTo.

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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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Examples

public class Student implements Comparable

{ public int compareTo(Object o)

{

}

…

}

How do you compare students?

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Examples

public class Student implements Comparable

{ public int compareTo(Object o)

{

Student s = (Student) o;

if (equals(s)) return 0;

if (gpa < s.gpa) return -1;

return 1;

}

…

}

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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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A picture (BlueJ)

<<Flier>>

fly()

SkiJumper

fly()train()

compareTo()

Bird

fly()findHome()

Airplane

fly()

Engine

<<Athlete>>train()

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Interfaces

SHOULD NOT GROW! 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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Interface

Contains constants, methods or both.

NO implementations.

No instance variables.

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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.

Compiler verifies that method calls are valid.

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Interface Example

public interface Flier

{

void fly();

}

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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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Classes realizing Flierpublic class Bird implements Flier

{ public void fly() { System.out.println("Using my wings to fly"); }}

==================================================================public class Airplane implements Flier

{ public void fly() { System.out.println("Using my jet engines to fly"); }}

==================================================================public class SkiJumper implements Flier, Athlete, Comparable

{ public void fly() { System.out.println("Using skis to take me into the air"); }}

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Abstract

An interface is ABSTRACT. You can’t create an object of that type.

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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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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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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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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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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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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.

String str = "word";

Comparable p = str;

String t = p; // NO

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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.

String str = "word";

Comparable p = str;

String t = (String)p; // OK

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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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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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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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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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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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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.

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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");

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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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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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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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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 addedpublic double getMaximum() { return maximum; }

private double sum; private double maximum; private int count;}

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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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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 addedpublic BankAccount getMaximum() { return maximum; }

private double sum; private BankAccount maximum; private int count;}

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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 addedpublic Coin getMaximum() { return maximum; }

private double sum; private Coin maximum; private int count;}

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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 addedpublic Student getMaximum() { return maximum; }

private double sum; private Student maximum; private int count;}

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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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Suppose we consider the following:

public interface Measurable

{

double getMeasure();

}

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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 addedpublic Measurable getMaximum() { return maximum; }

private double sum; private Measurable maximum; private int count;}

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And…

public class BankAccount implements Measurable{

. . . . .

public double getMeasure()

{

return balance;

}

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Test classpublic 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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And…

public class Coin implements Measurable{

. . . . .

public double getMeasure()

{

return value;

}

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And…

public class Student implements Measurable{

. . . . .

public double getMeasure()

{

return gpa;

}