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Java

Java. Pila/coda polimorfa. La Pila in Java - 1. package strutture; public abstract class Stack { protected int size; protected int defaultGrowthSize=5; protected int marker; protected Object contenuto[]; protected final int initialSize=3; public Stack() {

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Java

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  1. Java Pila/coda polimorfa

  2. La Pila in Java - 1 • package strutture; • public abstract class Stack { • protected int size; • protected int defaultGrowthSize=5; • protected int marker; • protected Object contenuto[]; • protected final int initialSize=3; • public Stack() { • size=initialSize; • marker=0; • contenuto=new Object[size]; • }

  3. La Pila in Java - 2 Abilita lo static binding • public final void inserisci(Object k) { • if (marker==size) • {cresci(defaultGrowthSize);} • contenuto[marker]=k; • marker++; • } • public abstract Object estrai() ;

  4. La Pila in Java - 3 • private void cresci(int dim){ • Object temp[ ]=new Object[size]; • for (int k=0;k<size;k++) • temp[k]=contenuto[k]; • contenuto=new Object[size+defaultGrowthSize]; • for (int k=0;k<size;k++) • contenuto[k]=temp[k]; • size+=defaultGrowthSize; • }

  5. La Pila in Java - 4 • package strutture; • public class Pila extends Stack { • public Object estrai() { • assert(marker>0):"Estrazione da Pila vuota"; • return contenuto[--marker]; • } • }

  6. La Coda in Java - 5 • package strutture; • public class Coda extends Stack { • Object estrai() { • assert(marker>0):"Estrazione da Coda vuota"; • Object retval=contenuto[0]; • for (int k=1; k<marker; k++ ) • contenuto[k-1]=contenuto[k]; • marker--; • return retval; • } • }

  7. La Pila in Java – 6a • public static void main(String args[]) { • int dim=10; • Stack s=new Pila(); // s= new Coda(); • for (int k=0;k<dim;k++){ • Integer o=new Integer(k); • s.inserisci(o); • } • for (int k=0;k<3*dim;k++) { • Integer i = s.estrai(); • int w=i.intValue(); • System.out.println(w); • } • }

  8. La Pila in Java – 6b • public static void main(String args[]) { • int dim=10; • Pila s=new Pila(); • for (int k=0;k<dim;k++){ • Integer o=new Integer(k); • s.inserisci(o); • } • for (int k=0;k<3*dim;k++) { • Integer i = s.estrai(); • int w=i.intValue(); • System.out.println(w); • } • } ERRORE! Non posso mettere un Object in un Integer!

  9. La Pila in Java – 6c • public static void main(String args[]) { • int dim=10; • Pila s=new Pila(); • for (int k=0;k<dim;k++){ • Integer o=new Integer(k); • s.inserisci(o); • } • for (int k=0;k<3*dim;k++) { • Integer i = (Integer)s.estrai(); • int w=i.intValue(); • System.out.println(w); • } • }

  10. La Pila in Java – 7a • public static void main(String args[]) { • int dim=10; • Pila s=new Pila(); • //INSERIMENTO • for (int k=0;k<dim;k++){ • Object o; • if (Math.random()<0.5) • o=new Integer(k); • else • o=new Float(k*Math.PI); • s.inserisci(o); • }

  11. La Pila in Java – 7b • // ESTRAZIONE • for (int k=0;k<dim;k++) { • Object o = s.estrai(); • if (o instanceof Integer) { • Integer i = (Integer) o; • int w = i.intValue(); • System.out.println("an int:"+w); • } else if (o instanceof Float) { • Float i = (Float) o; • float w = i.floatValue(); • System.out.println("a float:"+w); • } else • System.out.println("Unknown class!"); • } • }

  12. La Pila in Java – 7c • OUTPUT: • a float:28.274334 • an int:8 • an int:7 • a float:18.849556 • an int:5 • an int:4 • a float:9.424778 • a float:6.2831855 • a float:3.1415927 • a float:0.0

  13. Java Generics

  14. Definizione • A generic type is a reference type that has one or more type parameters. In the definition of the generic type, the type parameter section follows the type name. It is a comma separated list of identifiers and is delimited by angle brackets.  • class Pair<X,Y>  {    private X first;   private Y second;   public Pair(X a1, Y a2) {     first  = a1;     second = a2;   }   public X getFirst()  { return first; }   public Y getSecond() { return second; }   public void setFirst(X arg)  { first = arg; }   public void setSecond(Y arg) { second = arg; } }

  15. Definizione - continua • The class Pair has two type parameters X and Y .  • They are replaced by type arguments when the generic type Pair is instantiated. • For instance, in the declaration Pair<String, Date> the type parameter X is replaced by the type argument String and Y is replaced by  Date .  • The scope of the identifiers X and Y is the entire definition of the class.  In this scope the two type parameters X and Y are used like they were types (with some restrictions).

  16. Esempio • public void printPair( Pair<String,Long> pair) { • System.out.println("("+pair.getFirst()+",“ • +pair.getSecond()+")"); • } • Pair<String,Long> limit = • new Pair<String,Long> ("maximum",1024L); • printPair(limit);

  17. Wildcard instantiation • public void printPair( Pair<?,?> pair) { • System.out.println("("+pair.getFirst()+",“ • +pair.getSecond()+")"); • } • Pair<?,?> limit = • new Pair<String,Long> ("maximum",1024L); • printPair(limit);

  18. Generics are not usable… • for creation of arrays • in an instanceof expression

  19. Referenze su generics: • Il meglio: • http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html

  20. Uso di Generics nelle API di Java • Here is a simple example taken from the existing Collections tutorial: • // Removes 4-letter words from c. Elements must be strings • static void expurgate(Collection c) { • for (Iterator i = c.iterator(); i.hasNext(); ) • if (((String) i.next()).length() == 4) • i.remove(); • } • Here is the same example modified to use generics: • // Removes the 4-letter words from c • static void expurgate(Collection<String> c) { • for (Iterator<String> i = c.iterator(); i.hasNext(); ) • if (i.next().length() == 4) • i.remove(); • } In Java 5 molte classi sono state riscritte usando i generics

  21. Generic Pila • public class Pila <T> { • … • public Pila () {… } • private void cresci(int dim) {…} • public final void inserisci(T k) { • if (marker == size) { • cresci(defaultGrowthSize); • } • contenuto[marker] = k; • marker++; • } • public T estrai() { • assert(marker > 0):"Estrazione da Pila vuota"; • return (T) contenuto[--marker]; • }

  22. Generic Pila - compilazione • javac Pila.java –source 1.5

  23. Generic Pila • public static void main(String args[]) { • int dim = 10; • Pila<Integer> s = new Pila<Integer>(); • for (int k = 0; k < dim; k++) { • s.inserisci(new Integer(k)); • } • for (int k = 0; k < 3 * dim; k++) { • Integer w = s.estrai(); • // Integer w =(Integer)s.estrai(); • System.out.println(w); • } • } • }

  24. Generic Pila • public class Pila <T> { • … • public Pila () {… } • private void cresci(int dim) {…} • public final void inserisci(T k) { • if (marker == size) { • cresci(defaultGrowthSize); • } • contenuto[marker] = k; • marker++; • } • public T estrai() { • assert(marker > 0):"Estrazione da Pila vuota"; • return (T) contenuto[--marker]; • } Note: Pila.java uses unchecked or unsafe operations.

  25. Generic Pila • public static void main(String args[]) { • int dim = 10; • Pila<Integer> s = new Pila<Integer>(); • for (int k = 0; k < dim; k++) { • s.inserisci(new String("pippo"))); • } • for (int k = 0; k < 3 * dim; k++) { • Integer w = s.estrai(); • // Integer w =(Integer)s.estrai(); • System.out.println(w); • } • } • } Pila.java:43: inserisci(java.lang.Integer) in Pila<java.lang.Integer> cannot be applied to (java.lang.String) s.inserisci(new String("pippo")); ^

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