Interfaces Java

Interfaces Java
Bruce Eckel, Thinking in Java, 4th edition, PrenticeHall,
New Jersey, cf. http://mindview.net/Books/TIJ4
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José Valente de Oliveira
11-1
Overview
„
Java interfaces
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‰
‰
‰
‰
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An introduction
Sintaxe
UML notation
Multi-inheritance of interfaces
(Some) Java pre-defined interfaces
Design pattern: Strategy
José Valente de Oliveira
11-2
1
Recall: Data type definition
„
A data type is a set of values and a collection
of operations on the set of values.
Example: The int data type has the following
set of values
{ …, -2, -1, 0, 1, 2, … },
and operations on these values such as: +, *,
etc.
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José Valente de Oliveira
4-3
Recall: Abstract Data Type definition
„
An ADT (abstract data type) is a data type
whose values are only available to clients
through the operations in the ADT interface
„
In C, a string is not an ADT. Why?
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4-4
2
Recall: Some advantages of ADT
1.
2.
3.
4.
5.
6.
Clients of an ADT do not know the representation
therefore do not mess up the representation
Even if implementation changes, as long as the interface
remains stable, no changes are required in the client.
It is easy to ensure invariants,
The user of an ADT can focus on the problem domain
and forget about representation and implementation
details.
We have now ADT that can be easily reused in other
problems
An ADT based program is (much) more stable than
another based solely on procedures/functions.
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4-5
Even in problems as simple as the distance
between 2 points ADT can be useful.
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11-6
3
The interface IPoint
interface IPoint {
int getX();
int getY();
void setX(int x);
void setY(int y);
double dist(IPoint p);
}
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José Valente de Oliveira
11-7
Class CartPoint implements IPoint
class CartPoint implements IPoint {
private int x, y;
public CartPoint (int a, int b) {setX(a); setY(b); }
public int getX() {return x;}
public int getY() {return y;}
public void setX(int a) { assert a>=0; x=a; }
public void setY(int b) { assert b>=0; x=b; }
public double dist(IPoint p) {
int dx = x – p.getX();
int dy = y – p.getY();
return Math.sqrt(dx*dx+dy*dy);
}
}
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José Valente de Oliveira
11-8
4
A client
public class Client {
public static void main(String[] args) {
IPoint A, B;
A = new CartPoint(1, 1);
B = new PolarPoint(4, 2);
System.out.println(A.dist(B));
}
}
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José Valente de Oliveira
4-9
Recall
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Type relates to behaviour while class relates
to implementation.
„
Two objects from the same class have
necessarily the same type.
„
Two objects from the same type (e.g. IPoint)
can be instances from different classes (e.g.,
CartesianPoint, PolarPoint)
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José Valente de Oliveira
11-10
5
Revisiting the notion of class
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From the modeling perspective, a class
describes a set of objects with the same
semantics (the same role in the program),
i.e., with the same:
‰
‰
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Properties (data structure);
Behaviour (functions), and
Relationships
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3-11
Classes, again
A class is a (possibly partial)
implementation
of a data type.
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José Valente de Oliveira
11-12
6
The Stack ADT
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A stack is a last-in-first-out (LIFO) data
structure
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Adding an item
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Referred to as pushing it onto the stack
Removing an item
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Referred to as popping
it from the stack
13
The Stack ADT
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An ordered collection (list) of data items, that
can be accessed at only one end (the top) of
the collection
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Basic operations, informally:
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create a stack (usually empty)
check if it is empty
Push: add an element to the top (if it is not full)
Top: retrieve the top element (if it is not empty)
Pop: remove the top element (if is not empty)
14
7
@SuppressWarnings(“unchecked”)
interface IStack {
int DEFAULT_SIZE=12;
/**
* PRE: isEmpty() == false
* POS: this == old this */
Object top();
/**
* PRE: isEmpty() == false
* POS: isFull() == false */
Object pop();
/**
* PRE: isFull() == false
* POS: isEmpty() == false &&
obj == top() */
void push(Object obj);
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Example: interface IStack
interface IStack<T> {
int DEFAULT_SIZE=12;
/**
* PRE: isEmpty() == false
* POS: this == old this */
T top();
/**
* PRE: isEmpty() == false
* POS: isFull() == false */
T pop();
/**
* PRE: isFull() == false
* POS: isEmpty() == false &&
o == top() */
void push(T obj);
José Valente de Oliveira
11-15
Example: interface IStack
/**
* PRE: true
* POS: responde um inteiro com o tamanho actual do stack
*/
int size();
/**
* PRE: true
* POS: responde true se o stack está vazio; responde false caso
contrário
*/
boolean isEmpty();
/**
* PRE: true
* POS: responde true se o stack está cheio; responde false caso
contrário
*/
boolean isFull();
}
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José Valente de Oliveira
11-16
8
AStack implements IStack
class AStack implements IStack {
//…
}
//…
IStack s;
s = new IStack (); // ERROR !
s = new AStack(); // OK
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José Valente de Oliveira
11-17
Sintaxe
Qualif* interface Ident [extends Ident] [, Ident]*] {
[Qualif* Type Ident = expression;]*
[Qualif* Type Ident] ([Type Ident [, Type Ident]*); ]*
„
„
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Qualf
Ident
qualificador
Identifier (name)
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11-18
9
UML notation
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José Valente de Oliveira
11-19
University people class diagram, again
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José Valente de Oliveira
11-20
10
A poor solution for multi-inheritance
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José Valente de Oliveira
11-21
A poor solution for multi-inheritance
class Test {
private static void f(Trabalhador t) {
System.out.println (t.obtemSalário());
}
public static int main(String [] args) {
EstudanteComSalário ecs = new EstudanteComSalário();
f(ecs); // ERRO !!!
}
}
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José Valente de Oliveira
11-22
11
Multi-inheritance of interfaces
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11-23
Declaring the interfaces
interface IPessoa {
int nbi();
String nome();
String morada();
}
interface IEstudante extends IPessoa {
int número();
}
interface ITrabalhador extends IPessoa {
int salário();
}
interface ITrabalhadorEstudante extends IEstudante, ITrabalhador {
}
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José Valente de Oliveira
11-24
12
Implementing the classes
class Pessoa {
// …
}
class Estudante extends Pessoa implements IEstudante {
//…
}
class Trabalhador extends Pessoa implements ITrabalhador {
//…
}
class EstudanteComSalário extends Estudante implements
ITrabalhadorEstudante {
// …
}
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José Valente de Oliveira
11-25
Required changes in the previous code
class Test {
private static void f(Trabalhador t) {
System.out.println (t.obtemSalário());
}
public static int main(String [] args) {
EstudanteComSalário ecs = new EstudanteComSalário();
f(ecs); // ERRO !!!
}
}
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José Valente de Oliveira
11-26
13
Required changes in the previous code
class Test {
private static void f(ITrabalhador t) {
System.out.println (t.obtemSalário());
}
public static int main(String [] args) {
ITrabalhorEstudante ecs = new EstudanteComSalário();
f(ecs); // OK !!!
}
}
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José Valente de Oliveira
11-27
Java pre-defined interfaces
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José Valente de Oliveira
11-28
14
Previously on POO: The Quicksort abstract class
@SuppressWarnings(“unchecked”)
abstract class Quicksort {
private Comparable[] data;
public final void quicksort(Comparable[] d) {
data = d;
quicksort(0, data.length-1);
}
/
Java pre-defined interfaces
Interface Comparable<T> {
/** Answer a negative integer if the receiver is lower than the argument obj;
answer 0 if both receiver and obj are equal;
answer a positive integer is receiver is greater than obj
*/
int compareTo(T obj);
}
@Deprecated
interface Comparable {
/** Answer a negative integer if the receiver is
lower than the argument obj; answer 0 if both
receiver and obj are equal; answer a positive
integer is receiver is greater than obj
*/
int compareTo(Object obj);
}
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José Valente de Oliveira
11-30
15
Example
/** @version 3.0
*/
class Fraction implements Comparable<Fraction> {
private int num, den;
// …
public int compareTo (Fraction f) {
double q = (double) num / (double) den ;
double fq = (double) f.num /(double) f. den ;
return ((int) Math.signum(q – fq));
}
}
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José Valente de Oliveira
11-31
Example
public class Main {
public static void main(String[] args) {
Fraction [] a = {new Fraction(1, 7),
new Fraction(5, 2),
new Fraction(1, 2) };
ProbQuickSort<Fraction> pqs;
pqs = new ProbQuickSort<Fraction>();
pqs.quicksort(a);
for(int i=0; i< a.length; i++ )
System.out.println (a[i]);
}
}
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José Valente de Oliveira
11-32
16
Pre-defined Java interfaces
interface Cloneable {}
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José Valente de Oliveira
11-33
Main methods of abstract superclass Object
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„
protected Object clone()
throws CloneNotSupportedException
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public boolean equals(Object obj)
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protected void finalize() throws Throwable
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public final Class<?> getClass()
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public String toString()
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11-34
17
Example
Interface IStack {
int DEFAUL_SIZE = 10;
//…
}
class AStack implements IStack, Cloneable {
public AStack() {
this(DEFAULT_SIZE);
}
public Object clone() {
// TODO
}
// …
}
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José Valente de Oliveira
11-35
Check point: where is the error in the class Client?
class Point implements IPoint {
private int x, y;
public Point (int a, int b) {setX(a); setY(b); }
public int getX() {return x;}
public int getY() {return y;}
public void setX(int a) { assert a>=0; x=a; }
public
class
Client
{ }
public void setY(int b)
{ assert
b>=0;
x=b;
public int dist(Point p) { public static void main(String[] args) {
int dx = x – p.getX(); IPoint A, B;
int dy = y – p.getY(); A = new IPoint(1, 1);
B = new IPoint(4, 2);
return (int) Math.sqrt(dx*dx+dy*dy);
System.out.println(A.dist(B));
}
}
}
}
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José Valente de Oliveira
11-36
18
To take away today
„
Java interfaces
‰
An introduction
Sintaxe
UML notation
Multi-inheritance of interfaces
(Some) Java pre-defined interfaces
‰
Coming soon: Design pattern: Strategy
‰
‰
‰
‰
[email protected]
José Valente de Oliveira
11-37
19