Interfaces Java. Overview. n Java interfaces. q Introduction. q Sintaxe. q UML notation. q Multi-inheritance of interfaces

Transcription

1 Interfaces Java José Valente de Oliveira 11-1 Overview n Java interfaces q Introduction q Sintaxe q UML notation q Multi-inheritance of interfaces q (Some) Java pre-defined interfaces q Design pattern: Strategy jvo@ualg.pt José Valente de Oliveira

2 Recall: Data type definition n A data type is a set of values and a collection of operations on the set of values. jvo@ualg.pt José Valente de Oliveira 4-3 Recall: Abstract Data Type definition n An ADT (abstract data type) is a data type whose values are only available to clients through the operations in the ADT interface jvo@ualg.pt José Valente de Oliveira 4-4 2

3 Recall: Some advantages of ADT 1. Clients of an ADT do not know the representation therefore do not mess up the representation 2. Even if implementation changes, as long as the interface remains stable, no changes are required in the client. 3. It is easy to ensure invariants, 4. The us er of an ADT can focus on the problem domain and forget about representation and implementation details. 5. We have now ADT that can be easily reused in other problems 6. An ADT based program is (much) more stable than another based solely on procedures/functions. José Valente de Oliveira 4-5 Even in problems as simple as the distance between 2 points ADT can be useful. jvo@ualg.pt José Valente de Oliveira

4 The interface IPoint interface IPoint { int getx(); int gety(); void setx(int x); void sety(int y); double dist(ipoint p); jvo@ualg.pt 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) { if (a<0) System.exit(1); x=a; public void sety(int b) { if (b<0) System.exit(1); y=b; public double dist(ipoint p) { int dx = x p.getx(); int dy = y p.gety(); return Math.sqrt(dx*dx+dy*dy); jvo@ualg.pt José Valente de Oliveira

5 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)); jvo@ualg.pt José Valente de Oliveira 4-9 Recall n Type relates to behaviour while class relates to implementation. n Two objects from the same class have necessarily the same type. n Two objects from the same type (e.g. IPoint) can be instances from different classes (e.g., CartesianPoint, PolarPoint) jvo@ualg.pt José Valente de Oliveira

6 Revisiting the notion of class n 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: q Properties (data structure); q Behaviour (functions), and q Relationships jvo@ualg.pt José Valente de Oliveira 3-11 Classes, again A class is a (possibly partial) implementation of a data type. jvo@ualg.pt José Valente de Oliveira

7 The Stack ADT n A stack is a last-in-first-out (LIFO) data structure n Adding an item q Referred to as pushing it onto the stack n Removing an item q Referred to as popping it from the stack 13 The Stack ADT n An ordered collection (list) of data items, that can be accessed at only one end (the top) of the collection n Basic operations, informally: qcreate a stack (usually empty) qcheck if it is empty qpush: add an element to the top (if it is not full) qtop: retrieve the top element (if it is not empty) qpop: remove the top element (if is not empty) 14 7

8 @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); 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); jvo@ualg.pt José Valente de Oliveira 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(); jvo@ualg.pt José Valente de Oliveira

9 AStack implements IStack class AStack implements IStack { // // IStack s; s = new IStack (); // ERROR! s = new AStack(); // OK jvo@ualg.pt José Valente de Oliveira Sintaxe Qualif* interface Ident [extends Ident] [, Ident]*] { [Qualif* Type Ident = expression;]* [Qualif* Type Ident] ([Type Ident [, Type Ident]*); ]* n Qualf n Ident qualificador Identifier (name) jvo@ualg.pt José Valente de Oliveira

10 UML notation José Valente de Oliveira University people class diagram, again José Valente de Oliveira

11 University people class diagram, again José Valente de Oliveira A poor solution for multi-inheritance jvo@ualg.pt José Valente de Oliveira

12 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!!! jvo@ualg.pt José Valente de Oliveira Multi-inheritance of interfaces jvo@ualg.pt José Valente de Oliveira

13 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 { jvo@ualg.pt José Valente de Oliveira Implementing the classes class Pessoa { // class Estudante extends Pessoa implements IEstudante { // class Trabalhador extends Pessoa implements ITrabalhador { // class EstudanteComSalário extends Estudante implements ITrabalhadorEstudante { // jvo@ualg.pt José Valente de Oliveira

14 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!!! jvo@ualg.pt José Valente de Oliveira 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!!! jvo@ualg.pt José Valente de Oliveira

15 Java pre-defined interfaces José Valente de Oliveira Previously on POO: The Quicksort abstract unchecked ) abstract class Quicksort { private Comparable[] data; public final void quicksort(comparable[] d) { data = d; quicksort(0, data.length-1); 15

16 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 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); jvo@ualg.pt José Valente de Oliveira */ 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)); jvo@ualg.pt José Valente de Oliveira

17 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]); jvo@ualg.pt José Valente de Oliveira To take away today n Java interfaces q An introduction q Sintaxe q UML notation q Multi-inheritance of interfaces q Comparable: a Java pre-defined interface q Coming soon: the Strategy design pattern jvo@ualg.pt José Valente de Oliveira